Service Handbook
VF Boiler
Variable Fire Boiler
Domestic Hot Water/Hydronic Heating
M O D E L S C O V E R E D
VW and VB 500, 750, and 1000 Models
Series 100 and 101
Part No. TC-099 • Printed in the U.S.A. • 0407
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A. O. SMITH WATER PRODUCTS COMPANY
VF BOILER SERVICE MANUAL
Technical Training Department ©2007
CONTENTS
Introduction....................................................................... 2
61
67
68
71
Troubleshooting - Error Messages..................................
Display Is Blank................................................................
Error Message “Display Fail”..........................................
Error Message “No Config Key”......................................
Error Message “Config Key CRC”...................................
Error Message “Config Key Part”....................................
Error Message “Low AC Voltage”....................................
Error Message “Low 24 VAC”..........................................
Error Message “Low Water”............................................
Error Message “Low Gas”...............................................
Error Message “No Flow”................................................
Error Message “Sequence Err”.......................................
Error Message “Ignition Speed”......................................
Error Message “Mtr Spd < Min Spd”................................
Error Message “Blower Prov Stg1”.................................
Error Message “Blocked Flue”........................................
Error Message “Flame Stg1.............................................
Error Message “Igniter Stg1.............................................
Boiler Controls Information.............................................
Operating Temperatures..................................................
Bypass Lines.....................................................................
Technical Specifications..................................................
Glossary............................................................................
Qualifications.................................................................... 2
Service Warning................................................................ 2
EMC 5000 Modulation Control System........................... 4
UIM - User Interface Module - Overview.......................... 4
UIM - Status Lights............................................................ 6
UIM - Input Buttons........................................................... 7
UIM - Changing Settings.................................................. 8
UIM - Menu Screens.......................................................... 9
Venturi Operation - Gas Train.......................................... 12
Config Key - Blower Speed Programming...................... 14
Sequence Of Operation.................................................... 15
Adjusting Manifold Gas.................................................... 19
Poor Combustion - Ignition Problems............................ 20
Control Panel Layout........................................................ 24
MCB - Modulation Control Board.................................... 25
PDB - Power Distribution Board...................................... 35
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VF BOILER SERVICE MANUAL
INTRODUCTION
This Service Manual is designed to aid in servicing and troubleshooting A. O. Smith VF
boilers. Models VW/VB 500 - 1000 - series 100/101 boilers are covered in this manual.
The instructions and illustrations contained in this service manual will provide you with
troubleshooting procedures to verify proper operation and diagnose and repair common
service problems.
Important Service Reminder
When performing any troubleshooting step outlined in this service manual always consider
the wiring and connectors between components. Perform a close visual inspection of all
wiring and connectors to and from a given component before replacement. Ensure wires
were stripped before being crimped in a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets and plugs are not damaged or worn,
ensure plugs and sockets are mating properly and providing good contact.
Failure to perform this critical step or failing to perform this step thoroughly often results in
needless down time, unnecessary parts replacement, and customer dissatisfaction.
Instruction Manual
Have a copy of the Instruction Manual that came with the boiler on hand for the model and
series number before servicing.
Installation information given in this service manual is not a complete installation instruction.
Installation information covered in this service manual has a limited focus as it applies to
servicing the boiler. This Service Manual does not replace or supersede the Instruction
Manual that came with the boiler. Always refer to the Instruction Manual that came with the
boiler for complete installation instructions.
Qualifications
Service of A. O. Smith VF boilers requires ability equivalent to that of a Qualified Service
Agent (defined by ANSI below) in the field involved. Installation skills such as plumbing, air
supply, venting, gas supply, electrical supply are required in addition to electrical testing
skills. Start up and servicing of VF boilers requires combustion analysis test equipment.
ANSI Z223.1 2006 Sec. 3.3.83: “Qualified Agency” - “Any individual, firm, corporation or
company that either in person or through a representative is engaged in and is responsible
for (a) the installation, testing or replacement of gas piping or (b) the connection, installation,
testing, repair or servicing of appliances and equipment; that is experienced in such work;
that is familiar with all precautions required; and that has complied with all the requirements
of the authority having jurisdiction.”
Service Warning
If you are not licensed or certified to perform a given task do not attempt to perform any of
the service or installation procedures outlined in this manual. If you do not understand the
instructions given in this manual or do not feel confident in your abilities to perform a given
task do not attempt to perform any procedures outlined in this manual.
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VF BOILER SERVICE MANUAL
Tools Required
• Instruction Manual that came with the boiler.
• Hand tools common to installation and service of commercial water heaters and boilers.
• TORX® T40 or 5mm hex wrench - for setting gas mixture at gas valve.
• 3mm or 7/64in hex (allen) wrench - for setting gas mixture at gas valve.
• Two - U tube manometers (preferably) or gas pressure gauges. U tube manometers can
be used to accurately measure supply gas pressures. However, their resolution is
insufficient to be used for measuring draft, manifold gas pressure on VF boilers, or
perform air pressure switch testing. U tube manometers or gas pressure gauges may not
be substituted for Digital Manometers listed below.
• Digital Manometer (s) range -20.00 to +20.00" W.C., resolution 0.01" W.C. Recommend
UEI model EM200 or equivalent. Used to test performance of air pressure switches,
measure draft, and measure manifold gas pressure.
• True RMS Digital Multi Meter DMM, recommend UEI model DL289 or Fluke equivalent.
Capable of measuring:
AC/DC Voltage
VAC Frequency (Hz)
Ohms
DC micro amps µA
• AC amp meter- recommend UEI model DL289 or equivalent.
• Combustion analyzer
Capable of measuring:
CO2
CO
Draft Pressure
Exhaust Temperature
Efficiency
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VF BOILER SERVICE MANUAL
EMC 5000 MODULATION CONTROL SYSTEM
This portion of the service manual will cover the EMC 5000 Modulation Control system
(EMC - Energy Management Control). The EMC 5000 control system includes several
components: a UIM (User Interface Module), a MCB (Modulation Control Board), and a PDB
(Power Distribution Board). The EMC 5000 Control System can control single stage,
multiple stage, and modulating boilers and water heaters. This service manual should be
used as a reference for A. O. Smith VW/VB 500-1000 Series 100-101 boilers only.
Features Include:
• EMI / RFI filtering - built into all circuit boards. (EMI = Electro Magnetic Interference, RFI
= Radio Frequency Interference) Helps prevent or eliminate erratic operation caused by
EMI/RFI.
• Help screens - text based operational information to help the user understand how to
change settings and navigate the menu screens.
• Self diagnostics - text based diagnostic information (error and fault messages) on board
to help service technicians quickly and accurately service the boiler.
• Error message log - will retain a 9 event history (plus the current event) of error
messages with a time stamp. This will help diagnose load and/or environmental conditions
that may be contributing to a problem with operation or a lock-out.
• Short cycling protection - if any stage logs more than 30 cycles in one hour the control
enters a short cycle prevention condition. The boiler will continue to operate in this mode.
The UIM will display and log a “Short Cycle Cond” error message with the yellow Standby
system status LED flashing. The MCB will add a 180 second delay before activating any
stage’s call for heat after the last call for heat during this operating mode.
The short cycle protection mode can be ended (reset) by touching the Select button on the
UIM while the error message is displayed.
• Temperature probe filtering - The inlet, outlet, and remote Tank/Loop temperature
probes are read twice per second by the MCB and are filtered for 4 seconds. This filtering
will help prevent rapid short cycling caused by momentary fluctuations in temperature.
• Pressure/flow switch filtering - input switches such as air pressure switches are read
every second by the MCB and are filtered (de-bounced) for 6 seconds. This desensitizes
the input signal and will help prevent nuisance error/fault conditions due to momentary
fluctuations caused by wind gusts or blower speed changes. The flow switch is filtered for
4 seconds.
• Network capability - multiple boilers can be networked together (daisy chained) with
standard Category 5/6 network cable and given individual network addresses.
• Access/monitoring with future A. O. Smith EMS (Energy Management System)
controller.
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UIM - USER INTERFACE MODULE - OVERVIEW
The UIM is an assembly that consists of several electronic components. The main circuit
board in the assembly is the UIB (User Interface Board) which houses the communications
port. The UIB relays user input and data to and from the MCB, controls the LCD, and
activates the LEDs. Mounted to the UIB is a TSB (Touch Sensor Board) containing the touch
sensor pads that are the user input buttons. There is a LCD module mounted to the UIB that
displays operational information and diagnostic messages in plain English.
LEDs
• Three “System Status” LED lights: Service, Standby, Running.
Located to the right of the LCD.
(Light Emitting Diode)
• Firing Rate Status indicator - located in the lower right portion of
the UIM. Four LED lights behind a green (gradient) translucent
cover. This display indicates the approximate firing rate
between 25% and 100%.
LCD
• LCD display - 4 lines, up to 20 characters per line.
(Liquid Crystal Display)
• 10 different screens - Menus, Temperatures, System Status,
Control States, User Settings, Configuration Settings, Log &
System Information, Current Error, Error History, and Reload
Defaults (see page 9).
• Text based operational and diagnostic information.
User Input Buttons
Settings / Memory
• Select - Menu - Help - Up - Down.
• 5 touch sensitive buttons for user input. Located on the lower
left portion of the UIM.
• No moving parts - no pressure is required; these buttons
activate on finger presence.
• Non volatile memory; once new settings are confirmed
(touching the Select button) they remain in memory.
LCD display
4 line x 20 character
System Status
LEDs
Firing Rate Status
5 Touch Sensitive
User Input Buttons
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UIM - USER INTERFACE MODULE - STATUS LIGHTS
System Status
LEDs
LCD display
4 line x 20 character
Firing Rate
Status
5 Touch Sensitive
User Input Buttons
System Status LED Lights
The three “System Status” LED lights on the upper
right portion of the UIM convey current operational
and diagnostic information.
The red Service LED will be continuously illuminated
for soft lock-outs and flashing on and off for hard lock-
outs.
Soft lock-outs will automatically reset after the condition that caused the error has been
corrected or a factory programmed time period of one hour. Soft lock-outs can also be reset
by touching the Select button while the error screen is displayed.
Hard lock-outs can only be reset by touching the Select button while the error screen is
displayed. Power interruptions or cycling the power on and off will not reset hard or
soft lock-outs.
The yellow Standby LED is illuminated whenever the system set point has been satisfied.
The green Running LED is illuminated continuously whenever a call for heat is active.
Firing Rate Status
The Firing Rate Status indicator shows the current
firing rate of the boiler between; 25 and 100%.
The Firing Rate Status indicator will not illuminate
unless flame is proven by one of the two flame
sensors; high fire sensor/low fire sensor.
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UIM - USER INTERFACE MODULE - USER INPUT BUTTONS
System Status
LEDs
LCD display
4 line x 20 character
Firing Rate
Status
5 Touch Sensitive
User Input Buttons
Select Button is used to:
• Enter menu screens.
• Activate adjustment mode for various settings. The [ > ] will be flashing
slowly to the left of the menu item when the adjustment mode is active.
• Confirm and store new values and settings in memory.
• Reset the control when in a lock-out condition.
Menu Button is used to:
• Display a list of available menu screens.
• Abort or cancel new values and settings, returning to last saved value.
Help Button is used to:
• Touch once to display helpful information about the current menu screen.
Touch a second time to return to previous screen.
• Touched from the main menu screen and the UIM displays general
information on how to use the EMC 5000 user interface.
Up and Down Buttons are used to:
• Scroll or navigate up and down lists of menu screens and menu items.
When the current menu contains more than four lines of text, Up and
Down arrows will appear on the right side of the LCD screen indicating
more information is available off screen.
• Change values for user settings when adjustment mode has been
activated by pressing the Select button. (Select button must be touched
once more to confirm and store the new value in memory)
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UIM - USER INTERFACE MODULE - CHANGING SETTINGS
The sequence of UIM screen displays below is an example of how to navigate the menus to change
the Operating Set Point.
Screen Display 1: shows the temperatures screen. This is the default display screen the UIM comes
to rest at after approximately 60 seconds without any user input. The Menu button is touched once
to enter the Menus screen containing 9 different sub menus.
Screen Displays 2 - 4: show the first four lines of the Menus screen. The Down button is touched
several times to navigate to the User Settings menu on line four. The [ > ] sign on the left moves
down one line each time the down button is touched. Also notice the down arrow that appears on
the lower right corner, this indicates there are more menu items below the four lines currently being
displayed. Up arrows appear on the top right corner when there are more menu items above.
Screen Display 5: the Select button is touched once to enter the User Settings menu.
Screen Display 6: shows the first four lines of the User Settings menu. Touching the Select button
while the [ > ] sign is lined up with the “Oper Setpnt” menu item activates the adjustment mode for
this menu item. (Oper Setpnt = Operating Set Point)
1
2
3
4
5
6
The Operating Set Point can now be changed. The [ > ] sign begins to flash on and off slowly
indicating the adjustment mode is active for this setting. Touching the Up button once at this point
would change the Operating Set Point 1 degree higher. Touching the Down
button once would lower the setting by 1 degree. Touch the Up or Down
button continuously and the setting will start to ramp up or down rapidly.
Release the Up or Down button when the desired setting is reached.
Touch the Select button once to confirm and store the new value in memory.
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UIM - USER INTERFACE MODULE - MENU SCREENS
At the top center of the UIM panel is the display LCD. This LCD is used to provide
information to the user through menu activated screens. Within each of the screens, helpful
context sensitive information can be displayed at any time by touching the “Help” button.
Touching the help button once more returns the user to the previous screen.
The 10 available screens are:
Menu Screen:
Displayed when the “Menu” button is touched. This screen is the selection point for the other
menu screens.
Temperatures Screen (Default Screen):
Displays the temperatures sensed from the Outlet, Inlet, and optional remote Tank/Loop
temperature probes. This screen also displays the calculated temperature rise (Outlet minus
Inlet) through the boiler, sometimes referred to as the Delta T (ΔT). Shorted and
disconnected probes will have “Short” and “----” displayed to the right. The Temperatures
Screen is the default screen the boiler will come to rest at without any user input for
approximately 60 seconds.
There are no adjustable user inputs available from this screen.
System Status Screen:
This screen is used to view the status of switch inputs and output states. An asterisk (*) is
displayed next to the label when the status is “True” (the description is fulfilled). For
example; if water is flowing, as detected by the flow switch, an asterisk (*) will appear in front
of the Flow label (IE: *Flow).
There are no adjustable user inputs available from this screen.
Control States Screen:
The CCB/FCB operating states of the boiler are displayed in this menu screen along with the
end cap of the blower motor. Blower rpm are displayed in real time. CCB and MCB are the
same component on VF Boilers. See explanation for this and the FCB term on page 25.
There are no adjustable user inputs available from this screen.
User Settings Screen:
This screen is used to enter values for various user settings such as; the operating set point
abbreviated Oper Setpnt, the Hi Limit (automatic high limit), pump post circulate time etc.
The Select button must be touched once to activate the adjustment mode for a user setting
and again to confirm and store the new setting into memory.
Configuration Settings Screen
There are no adjustable user inputs available from this screen.
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UIM - USER INTERFACE MODULE - MENUS
Log & System Info Screen:
This screen displays the following:
• Elapsed hours of operation (Total time system has been powered up).
• Number of running minutes (Number of minutes system has been in the run mode).
• Number of heating cycles.
• kBtu rating of the boiler.
• Software revision of the CCB (MCB), FCB, and Config Key (see explanation on page 25).
There are no adjustable user inputs available from this screen.
Current Error Screen:
Displays the current error the system has detected, plus a timestamp of when the error
occurred. (The timestamp is based on the elapsed hours value at the time the error
occurred. It is displayed in hours and minutes). This error remains displayed as long as it is
still valid. When cleared it is moved to the Error History Screen. The system will
automatically jump to this screen when an error is detected. It will also go to this screen
upon power-up if an error was still valid when power was turned off.
Errors are cleared (control system is reset) from this screen by touching the Select button.
There are no adjustable user inputs available from this screen.
Error History Screen:
This screen displays a list of the last 9 errors (with timestamps) that have occurred. The last
error to occur is displayed first. The actual date and time of the event can be calculated by
subtracting the error time stamp from the elapsed hours of operation in the Log & System
Info Screen. This can be helpful when determining if the error is related to environmental or
load conditions.
There are no adjustable user inputs available from this screen.
Reload Defaults Screen:
From this screen the user can restore the factory default values for screen adjustable
configurations by touching the Select button. See page 70 for default values.
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PRINCIPLES OF OPERATION
VF boilers do not have a gas orifice. The blower “pulls” gas from the gas valve into a Venturi
attached to the suction (inlet) side of the blower (page 12). As the speed of the blower is
increased the vacuum in the Venturi is also increased and more fuel gas and combustion air
is supplied to the burner. This is the basic principle of how a VF boiler modulates firing rate.
VFD and Blower Operation
VF boilers have a small circuit board (Daughter Board) attached to the MCB circuit board
(page 25). The MCB controls a VFD (variable frequency drive) through the J24 socket
directly. The VFD controls blower speed precisely by modulating the frequency (Hz) of the
power supplied to the blower motor. VFD output to the blower is a 3 Phase power supply.
Three conditions must be met before the VFD will start the blower:
1. 120 VAC is supplied to the VFD 120 VAC input.
2. The MCB closes an enable/disable circuit that enables VFD operation.
3. The MCB sends a 1-10 VDC instruction to modulate blower speed.
The DC voltage instruction sent by the MCB is directly proportional to blower speed - higher
DC voltage = higher (faster) blower speed.
The blower power frequency (Hz) is also directly proportional to blower speed - higher
frequency = higher (faster) blower speed. The output frequency is displayed on the VFD.
Ground Hot Neutral
120 VAC input
Output Frequency
Display
VFD
R/L1
N
3 Phase Power
Variable Frequency
Output To Blower
Blower Motor Wires
Black White Red
Low Voltage Terminal Strip
U/T1 V/T2 W/T3
RC RA
1-10 VDC Instruction
Enable/Disable
+15
0V AI1 +5V DO LI1 LI2 LI3 LI4
V
1-10 VDC Speed Instruction
Enable/Disable Circuit
See the important service notes on page 34
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VF BOILER SERVICE MANUAL
PRINCIPLE OF OPERATION (CONT)
Venturi Operation - Gas Train
A Venturi is connected to the inlet side of the combustion blower on VF Boilers. The gas line
from the outlet of the gas valve connects to the Venturi. The Venturi contains a cone shaped
restrictor that constricts the air passage to the blower. As air enters the constriction point it’s
velocity increases. A pressure drop occurs at this point and creates a negative (vacuum)
pressure in the cavity between the cone shaped restrictor and the Venturi housing. This
negative pressure “pulls” gas from the outlet of the gas valve into the blower where it is
mixed with combustion air and then supplied to the burner.
As the blower speed is increased by the MCB (page 25) working in conjunction with the VFD
increases the vacuum created by the Venturi and more fuel gas is pulled from the gas valve.
This increases the firing rate (input Btu/hr) of the VF boiler. As the blower speed is
decreased less fuel gas is supplied and the firing rate is reduced.
The MCB controls blower speed according to temperature demand. When system
temperature sensed at the Controlling Probe (pages 62 and 63) is well below the VF Boiler’s
Operating Set Point (pages 8 and 63) the MCB will run the blower at maximum speed =
100% firing rate. As system temperature approaches the VF Boiler’s Operating Set Point the
MCB will modulate the blower at slower speeds which subsequently reduces the firing rate.
VF boilers are designed to modulate between 25% and 100%; a 4 to 1 turn down rate. See
the illustration below.
Venturi
Blower
Negative Pressure Area
Burner
Velocity Increases
At Constriction Point
Pressure Drops
Combustion Air
Blower Inlet
Negative Pressure Area
VF Boiler Gas Train
Venturi
Combustion
Air Inlet
Blower
Gas Valve
Gas Valve
Burner Flange
Blower Outlet
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VF BOILER SERVICE MANUAL
PRINCIPLE OF OPERATION (CONT)
Blower Speed Verification - Hall Effect Sensor
The MCB (page 25) initiates blower operation by closing the VFD enable/disable circuit AND
modulate blower speed.
The MCB monitors actual blower speed (rpm) to verify the speed dictated by the instruction
has been achieved and to change the instruction as necessary. Actual blower speed
information is relayed to the MCB by a “Hall Effect” sensor built into the end cap of the
blower motor. Three wires connect between the J24 socket on the MCB Daughter Board
Example: If the MCB has sent an 4.3 VDC instruction to the VFD to run the blower at 2500
rpm and the actual rpm relayed back from the Hall Effect sensor indicates the blower is
running at 2400 rpm, the MCB will adjust (increase) the DC volt instruction slightly until the
actual rpm information matches the instruction sent by the MCB. See VFD and Blower
The actual blower speed can be viewed in the Control States menu in real time through the
A round magnet is attached to the end of the blower shaft. The Hall Effect sensor is located
in the magnetic field. The rotation of the magnet generates a signal in the sensor that is
sent to the MCB. The MCB interprets this signal as blower speed (rpm) information.
Blower Motor
Hall Effect Sensor
With End Cap
Removed
Magnet
Press Menu
Navigate to Control States
Press Select
Control States Menu
See the important service notes on page 34
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VF BOILER SERVICE MANUAL
PRINCIPLE OF OPERATION (CONT)
Config Key - Blower Speed Programming
The Config Key is an 18” long cable with a plug on one end and an EEPROM memory chip
sealed at the other. The Config Key plugs into the J23 socket on the MCB Daughter Board
to the input Btu/hr and fuel type for each VF boiler. VF boilers are available in natural and
propane gas and in three input Btu/hr sizes; 500,000 - 750,000 - 1,000,000. There are 6
different Config Keys. High altitude Config keys may be produced in the future.
Current production Config Keys (VF boilers) are certified up to 7700 foot elevation.
Each time the boiler is powered up the MCB reads and then stores the blower speed
programming data from the Config Key into it’s own internal memory. The MCB then uses
Modulating blower speed also modulates the firing rate on VF boilers (page 12).
There is a label at the end of the Config Key cable near the memory chip. This end of the
cable is threaded into the wiring chase during production. The quickest way to verify the
correct Config Key is installed is to navigate to the Log & System Info menu in the UIM
Current Config Key Software Revisions
Config Key
Software
VW/VB 500
VW/VB 750
VW/VB 1000
Natural Gas Propane Gas Natural Gas Propane Gas Natural Gas Propane Gas
1.00 1.01 1.10 1.11 1.20 1.21
Revisions
Rev #
Even revision numbers are for natural gas models, odd numbers are for propane.
1.00 - 1.09 range for VW/VB 500 models
1.10 - 1.19 range for VW/VB 750 models
1.20 - 1.29 range for VW/VB 1000 models
Press Menu
Navigate to Log & System Info
Press Select
Log & System Info menu
Navigate to ConfigKey Rev
Config Key Label
Config Key
Memory Chip
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VF BOILER SERVICE MANUAL
SEQUENCE OF OPERATION
Call For Heat is Activated
EMC 5000 Control System
compares the temperature read
from controlling probe (Inlet or
remote Tank/Loop) to the
Operating Set Point.
MCB board applies power to pump relay
Pump Is Energized
Boiler enters Service Mode
and locks out. UIM displays
Flow Switch Closes Contacts
YES
NO
No Flow
Error message
If the temperature read is less than
the Operating Set Point minus
Stage 1 Differential AND the
Enable/Disable (thermostat) circuit
is closed a call for heat is activated
Low Blower Prover Switch
Contacts Verified Open
NO
NO
High Blower Prover Switch
Contacts Verified Open
YES
Boiler enters Service Mode
and locks out. UIM displays
Boiler enters Service Mode
and locks out. UIM displays
MCB Board
Closes VFD Enable/Disable Circuit
Blower Prov Stg1
NO
Error message
Sequence Err
Error message
Sends 1-10 VDC Speed Instruction to VFD
YES
Blower Is Energized By VFD
Pre Purge State (approx 5000 rpm)
10-30 Seconds
Low Blower Prover Switch
Contacts Verified Closed
NO
NO
NOTE
The events shown in this flow chart
are in sequential order. The EMC
5000 is a multi-task control that
performs some functions
YES
High Blower Prover Switch
Contacts Verified Closed
simultaneously. Only key events are
shown in order to provide a general
understanding of how the control
operates. Event timings can vary
depending on actual conditions.
YES
MCB Instructs VFD (1-10 VDC)
To Reduce Blower Speed
Ignition State (approx 2100 rpm)
High Blower Prover Switch
Contacts Verified Open
AND
NO
Low Blower Prover Switch
Contacts Remain Closed
YES
Next Page
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VF BOILER SERVICE MANUAL
SEQUENCE OF OPERATION (CONT)
Previous Page
MCB Energizes Ignitor
Ignition State – 18 Seconds
NOTE
The events shown in this flow chart
are in sequential order. The EMC
5000 is a multi-task control that
performs some functions
Boiler enters Service Mode
and locks out. UIM displays
MCB Senses Minimum
NO
Igniter Stg1
Error message
simultaneously. Only key events are
shown in order to provide a general
understanding of how the control
operates. Event timings can vary
depending on actual conditions.
2.7 AC Amps Through Ignitor
YES
Gas Valve Is Energized
Fuel Gas Is Drawn Into
Blower Inlet By Venturi Assembly
Mixed With Combustion Air
After 1 or 3 trials
(SW1 dip switch 2 setting)
Control enters Service Mode
and locks out. UIM displays
Minimum Flame Sensing Current
2.5 µA (DC micro amps)
NO
Detected From Flame Sensors
Flame Stg1
Error message
YES
SERVICE NOTE
VF boilers utilize two flame sensors.
One is positioned closer to the buner
than the second to enable flame
detection during low and high fire
conditions.
MCB Instructs VFD (1-10 VDC)
To Modulate Blower Speed
According To Load Conditions
Throughout Heating Cycle
The wiring from the two flame
sensors is joined and connects to a
single connection point on the MCB
circuit board.
Water Is Heated To
Operating Set Point
When the “Flame Stg1” error
message is displayed remove,
inspect, and clean both flame
sensors.
Gas Valve Is De-Energized
Blower Runs 25 Seconds
Post Purge State – Shuts Off.
Pump Runs Continuous Or
Programmed Post Circulate
Period Shuts Off.
Boiler Goes Into Standby
SERVICE NOTE:
In standby and running modes the system constantly monitors the signals and the internal
operation for faults.
Any detected fault will halt the heating sequence and shift the system to the service mode
where the detected fault will be displayed.
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START UP PROCEDURE
Prior To Start Up
In addition to normal supplies and hand tools necessary for installing and servicing water
heaters and boilers the following tools and test equipment should be on hand. See the tool
requirements page 3.
• A combustion analyzer capable of measuring draft pressure, CO, and CO2 or O2.
• True RMS Digital Multi Meter DMM capable of reading AC volts, DC volts, ohms,
DC micro amps µA, and frequency Hz.
• AC amp meter.
• TORX® T40 or 5mm hex wrench - for setting gas mixture at gas valve.
• 3mm or 7/64in hex (allen) wrench - for setting gas mixture at gas valve.
• A U-tube manometer or pressure gauge for measuring supply gas pressure.
• A digital manometer for measuring (negative/vacuum) manifold gas pressure.
Firing Modes - Min/Max/Mod
When performing a Start Up on a VF boiler, the boiler’s firing mode must be set to the Min
Mode (minimum firing - 25%) and the Max Mode (maximum firing - 100%). While the boiler
down until the > cursor to the left of the display is lined up with the User Settings menu and
press the Select button. Scroll down to Mod Mode menu item and press the Select button
again. The > cursor starts flashing on and off slowly indicating adjustment of this menu item
is now possible. Use the up and down buttons to select between these three options:
• Min (forced minimum firing rate - 25%)
• Max (forced maximum firing rate - 100%)
• Mod (MCB automatic controlled firing rate - modulation mode)
Press the Select button for the desired option to confirm. The > cursor stops flashing and the
boiler enters the firing mode selected.
The Min Mode and Max Mode are used for checking combustion during start up. Return the
boiler to the Mod Mode after checking combustion. The boiler will automatically return to the
Mod Mode after 10 minutes. Review the UIM and Menus information on pages 4 - 10.
Turning The Boiler Off
NEVER TURN THE BOILER OFF BY SECURING POWER WHILE IT IS FIRING. Repeated
sudden stops while firing can damage the boiler. To shut down the boiler safely do one of the
following so the boiler can go through a normal shut down sequence with post purge cycles:
• Lower the Operating Set Point to it’s lowest setting.
• Lower the set point of any external control in use to it’s lowest setting.
• Open the boiler’s 24 VAC Enable/Disable circuit manually (wires in the junction box on the
back of the boiler). Be careful not to let the bare wire ends touch any grounded surface.
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START UP PROCEDURE (CONT)
Start Up Procedure
1. Before starting the boiler, please review the boilers Instruction Manual supplied with the
boiler. Ensure the water piping, gas line, controls, and venting have been installed per
the instruction manual. Further information on water piping and controls is on pages 61 -
open.
3. Purge all air from the gas line to the boiler and ensure there are no gas leaks. Ensure
the main supply gas valve is open.
4. Be certain that the system is full of water, that all air has been purged from the boiler,
storage tank (s), and the water lines. Open both the supply and return water valves to
the boiler. Ensure there are no water leaks.
5. Ensure the power supply meets the minimum requirements on page 68 in this manual.
Ensure all control wiring (remote Tank/Loop probe - external Enable/Disable control) is
run in a dedicated conduit per requirements on page 68.
7. Prior to turning on the gas, proper sequence of most of the system can be verified.
through a heating cycle. It should stop when it checks for the flame and declare a fault.
This will verify that the pump, flow switch, igniter, VFD (variable frequency drive), gas
valve, blower and low/ high blower prover switches are all functioning.
8. Drill a 7/16” hole in the side of the boot tee approximately 8" from back panel to insert
the gas analyzer probe. The hole must be sealed upon completion of the start-up.
9. Open the main supply gas valve to the boiler.
10. Turn the boiler’s on/off switch on. Ensure the Operating Set Point is set high enough to
activate a call for heat (page 8). If an external control is using the boiler’s Enable/Disable
circuit (Aquastat, Boiler Sequencing Controller, EMS) ensure the external controls
Disable circuit ensure the two wires provided for this circuit are wire nutted together in
the junction box on the back of the boiler. Review the controls section beginning on page
61 and refer to the wiring diagram on the boiler.
11. Allow the unit to run for at least 15 minutes before proceeding with the combustion
analysis. Take a combustion sample and record CO and CO2 or O2 readings.
12. Attach a U tube manometer to supply gas pressure source at the boiler. Measure the
supply gas pressure with a U tube manometer with the boiler firing in Max Mode. Set the
4.0 " W.C. for natural gas or 11.0 " W.C. for propane is present with the boiler firing at
100%. Maximum supply gas pressures are 11.0 " W.C. natural gas and 13.8 " W.C.
propane gas. Adjust supply gas pressure at the supply gas regulator as needed to
maintain these supply gas pressure requirements.
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START UP PROCEDURE (CONT)
Adjusting Manifold Gas
combustion analyzer. Compare CO2 readings taken to the high fire CO2 table below. If
combustion readings are not in accordance with the high fire CO2 table adjust as
follows:
Remove the flat, round, blue plastic cap from the top of the boiler’s 24 VAC gas valve.
Using a 3mm (7/64”) hex wrench, turn the high fire adjustment screw under the cap
counterclockwise to increase or clockwise to decrease gas flow and achieve the desired
CO2 level per high fire table below. When desired adjustments are complete, reinstall
the cap on the gas valve. CO readings should be less than 200 ppm.
combustion analyzer. Compare CO2 readings taken to the low fire CO2 table below. If
combustion readings are not in accordance with the low fire CO2 table adjust as follows:
Remove the small metal slotted cap near the outlet of the boiler’s 24 VAC gas valve.
Using a TORX® T40 or a 5mm hex wrench, carefully turn the low fire adjustment screw
under the cap clockwise to increase or counterclockwise to decrease gas flow and
achieve the desired CO2 level per low fire table below. When desired adjustments are
complete, reinstall the cap on the gas valve. CO readings should be less than 200 ppm.
Start Up Note: There will be a time delay between adjustments on the gas valve and the
response of the CO2 measuring instrument. Adjust the settings in small increments and
allow the combustion readings to stabilize for at least 3 minutes before readjusting.
High Fire CO2 Settings
NATURAL GAS
PROPANE GAS
8.5 - 9.0% CO2
9.5 - 10.5% CO2
Low Fire CO2 Settings
NATURAL GAS
PROPANE GAS
6.5 - 7.5% CO2
7.5 - 8.5% CO2
15. When start up procedure is complete return to the User Settings Menu and place the
boiler back into Mod Mode firing mode (Firing Modes - page 17). Adjust the Operating
Set Point in the User Settings Menu (page 8) to desired system temperature. Remove
all test instruments and replace all plugs and caps.
should have a bypass line with a throttling valve. Adjust the outlet valve to achieve a 20
- 40°F temperature rise through the boiler and the bypass valve to achieve a 120°F
minimum inlet water temperature. Make these adjustments at 100% firing rate.
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POOR COMBUSTION - IGNITION PROBLEMS
If the high and/or low fire CO2 combustion readings are not in accordance with the tables
shown on page 19, the CO readings are high, or if the boiler is experiencing ignition failure
or rough starting perform the following procedures:
Adequate Combustion - Proper Venting
1. Ensure there is an adequate supply of fresh air for combustion and the boiler is vented
properly. DO NOT OVERLOOK THIS STEP. Lack of combustion air and improper
venting is often the root cause for poor combustion.
Direct Vent Installations: If the boiler is direct vented ensure the intake air or the vent
pipe is not restricted and neither has exceeded the maximum 70 equivalent feet. Ensure
the vent and intake air terminations are positioned correctly so that vent gases are not
recirculating to the intake air termination. Installation note: each 90° elbow is equivalent
to 10 linear feet of pipe on VF boilers. If the boiler is in an equipment room with a door to
the outdoors, temporarily disconnect the intake air pipe and prop open the equipment
room door. Take combustion readings again to see if this corrects the problem.
Conventional Vent Installations: If the boiler is using room air for combustion ensure
the vent pipe has not exceeded the 100’ maximum equivalent feet. Installation note:
each 90° elbow is equivalent to 10 linear feet of pipe on VF boilers. Ensure the fresh air
openings in the equipment room are adequately sized for the combined input Btu/hr
rating of all conventionally vented gas fired appliances in the room. If the boiler is in an
equipment room with a door to the outdoors - prop open the equipment room door. Take
combustion readings again to see if this corrects the problem.
General venting guidelines are shown here. Refer to the Instruction Manual that came with
the boiler for complete combustion air and vent installation information.
Direct Vent
Configurations
Installation Note:
VF Boilers are classified as Category IV
appliances. AL29-4C vent material must
be used for the exhaust vent on all
installations.
Conventional Vent
Configurations
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POOR COMBUSTION - IGNITION PROBLEMS (CONT)
Burner Inspection
2. Remove the burner and inspect the burner for any signs of damage or debris inside. If
the burner is damaged or contaminated with debris - replace the burner. Check all
gaskets in the burner/blower assembly for wear or damage. Replace any worn or
damaged gaskets.
Firing Rate - Modulation Performance
rating and fuel type of the boiler you are working on.
4. Verify the blower speed is acceptable during the following three operating states:
• Igniter warm up period - (AC amps can be detected through the igniter).
Start the boiler and check the three parameters listed below during the three operating
states given above.
Procedure
• Measure the DC volt instruction from the MCB at the VFD to ensure all wiring and
connections between the J24 socket on the MCB Daughter Board and the VFD
• Check the actual blower rpm from the Control States Menu via the UIM display
(page 13),
Compare these values to those shown in the tables on page 22.
The actual blower rpm displayed by the UIM should be within 20% of the values given in
1953.
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POOR COMBUSTION - IGNITION PROBLEMS (CONT)
VFD Operating Parameters
Approximate MCB/VFD operating parameters for natural gas models are shown in the first
table below. Parameters for propane models are shown in the second table.
When combustion is poor, the firing rate of the boiler or blower speed is in question, or the
boiler is experiencing rough starts or ignition failure compare the values given here to actual
readings taken during the various operating states shown. Actual readings will vary
somewhat but should be relatively close to what is shown here when the MCB and the VFD
are operating properly.
VW/VB VW/VB VW/VB
Approximate Values Natural Gas
Blower Speed Control VFD
MCB Instruction Pre/Post Purge Mode Approx - VDC
MCB Instruction Ignitor Warm Up Period Approx - VDC
MCB Instruction Min Mode 25% Approx - VDC
MCB Instruction Max Mode 100% Approx - VDC
500
750
1000
8.6
3.7
1.3
4.3
8.6
3.7
2.1
7.2
8.9
3.7
2.1
7.7
Blower Speed Pre/Post Purge Mode - Approx RPM
Blower Speed Ignitor Warm Up Period - Approx RPM
Blower Speed Min Mode 25% - Approx RPM
Blower Speed Max Mode 100% - Approx RPM
5000
2200
800
5000
2200
1200
4100
5000
2200
1250
4500
2500
Blower Power Freq Pre/Post Purge Mode - Approx Hz
Blower Power Frequency Ignitor Warm Up Period - Approx Hz
Blower Power Frequency Min Mode 25% - Approx Hz
Blower Power Frequency Max Mode 100% - Approx Hz
172 Hz
75 Hz
26 Hz
85 Hz
172 Hz
75 Hz
41 Hz
179 Hz
75 Hz
43 Hz
144 Hz
155 Hz
VW/VB VW/VB VW/VB
Approximate Values Propane Gas
Blower Speed Control VFD
MCB Instruction Pre/Post Purge Mode Approx - VDC
MCB Instruction Ignitor Warm Up Period Approx - VDC
MCB Instruction Min Mode 25% Approx - VDC
MCB Instruction Max Mode 100% Approx - VDC
500
750
1000
8.6
3.7
1.4
4.3
8.6
3.7
2.1
7.7
8.9
3.7
2.4
7.7
Blower Speed Pre/Post Purge Mode - Approx RPM
Blower Speed Ignitor Warm Up Period - Approx RPM
Blower Speed Min Mode 25% - Approx RPM
Blower Speed Max Mode 100% - Approx RPM
5000
2200
1000
2500
5000
2200
1300
4500
5000
2200
1400
4500
Blower Power Freq Pre/Post Purge Mode - Approx Hz
Blower Power Frequency Ignitor Warm Up Period - Approx Hz
Blower Power Frequency Min Mode 25% - Approx Hz
Blower Power Frequency Max Mode 100% - Approx Hz
172 Hz
74 Hz
27 Hz
85 Hz
171 Hz
75 Hz
43 Hz
179 Hz
75 Hz
47 Hz
155 Hz
155 Hz
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POOR COMBUSTION - IGNITION PROBLEMS (CONT)
Manifold Gas Pressure Check
5. Attach Digital manometer to one of the manifold gas pressure taps on gas line between
the boiler’s 24 VAC gas valve outlet and the gas inlet on the Venturi as shown in the
images below. Recommend UEI model EM200 or equivalent. Manifold gas pressure on
VF boilers is a negative (vacuum) pressure. The lower the pressure (deeper the
vacuum) the higher the firing rate will be. Measure the manifold gas pressure with the
the ranges listed in the table below.
Gas Valve
Venturi
Gas Train
Shut Off Valve
Blower
MANIFOLD PRESSURE
MAX MODE - 100% FIRING
-1.5" W.C. to -4.0" W.C.
-1.5" W.C. to -4.0" W.C.
MIN MODE - 25% FIRING
-0.15" W.C. to -1.0" W.C.
-0.15" W.C. to -1.0" W.C.
NATURAL GAS
PROPANE GAS
Service Note: If the actual pressures observed during the test above are considerably
different (higher pressures - closer to a positive pressure) closely inspect all fittings and
flanges in the gas train between the outlet of the boiler's 24 VAC gas valve and the Venturi
assembly. See page 12. Reseal/repair any leaking connections - replace any worn or
damaged gaskets.
Disassemble the Venturi from the inlet connection at the blower (round flange) and the
intake air connection (square flange) - ensure the large “O” ring gaskets are not damaged
and seating properly.
Closely inspect the cone shaped restrictor inside the Venturi for any signs of damage or
wear. If the restrictor is missing, damaged, or visibly worn - replace the Venturi.
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CONTROL PANEL LAYOUT
The illustration below shows the control panel on VF boilers.
Blower Prover Switches
VF boilers are equipped with High and Low blower prover switches or BPS. Both are
normally open air pressure switches. Both close on a fall in pressure (a negative/vacuum
pressure). The contacts of both must be open at the beginning of each heating cycle. The
High BPS is only used when the boiler is in the Pre/Post Purge operating states (blower at
5000 rpm). The High BPS contacts must close during these two states only and must remain
open at all other times. The Low BPS must be closed whenever the blower is running. See
the Sequence of Operation on page 15.
Note which sensing ports are used. The two sensing ports on each pressure switch are
differentiated with the letters “L” and “H” embossed on the switch body. The sensing tubes
must be connected to the ports marked “L.” The other port remains open. The barbed fitting
for the “H” port is typically cut off at the factory to ensure proper connection.
The two sensing tubes that connect to the blower prover switches are T fitted together and a
single tube from the T fitting then connects to a port on the gas train.
If the EMC 5000 control system is locked out and displays the “Blower Prov Stg1” error
message closely inspect the sensing tubes and wiring to the two blower prover switches.
• Ensure the sensing tubes are connected to the right ports.
• Ensure the sensing tubes are not kinked and securely connected at both ends.
• Ensure the two wires to each switch are connected to the wiring terminals marked C and NO.
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VF BOILER SERVICE MANUAL
MCB - MODULATING CONTROL BOARD - OVERVIEW
MCB / CCB / FCB Terminology Explanation
MCB and CCB are separate terms that refer to the same circuit board which is the main
controller for the EMC 5000 control system on VF boilers. MCB stands for “modulating
control board,” CCB stands for “central control board.” In VF boiler product literature MCB is
is referred to as the CCB. Some UIM menu screens show the term FCB (flame control
board). The FCB is a portion of the MCB that controls ignition. It is not a separate part.
The EMC 5000 control system is used on other products such as Genesis boilers that are
staged boilers and do not have modulating burners. However, this MCB circuit board is not
an interchangeable part with Genesis boilers.
VF boilers have a small circuit board (Daughter Board) attached to the MCB that houses the
J23 and J24 sockets. The VFD, Hall Effect sensor, and the Config Key are connected to
Daughter Board
Side View
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VF BOILER SERVICE MANUAL
MCB - MODULATING CONTROL BOARD - OVERVIEW (CONT)
The MCB circuit board is the main controller. All instructions for burner modulation and
temperature control originate from this circuit board. Diagnostic and operational messages
are generated by the MCB and sent to the UIM. Most of the boiler’s components, such as
the igniter, blower, gas valve, and temperature probes are directly connected to one of the
MCB’s 16 socket connectors.
Wiring and connection details given here reference 100-101 Series VF boilers.
The wiring and connection information shown in this service manual are in reference to how
the EMC 5000 controls and circuit boards connect on VF boilers. This service manual
should be used as a reference for A. O. Smith VF boilers only.
The MCB circuit board measures approximately 13” x 7” and has 16 Molex/AMP socket
connectors. The MCB circuit board has been divided into sections in the illustration below,
each section will be covered in the pages that follow.
IMPORTANT NOTE - MCB CIRCUIT BOARD ORIENTATION
The MCB circuit board is mounted upside down on VF boilers compared to the illustration
below. Keep this in mind when referencing the illustrations in this manual.
The pages that follow reference the MCB circuit board as oriented in this illustration.
The MCB board is physically mounted upside down inside the boiler cabinet compared to this illustration.
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MCB - SECTION A
The upper left corner of the MCB contains the following sockets/components:
Red LED (illuminated when the F1 fuse is open or missing)
F1 Fuse (7.5 amp automotive fuse - 24 VAC transformer protection)
J3 Socket (24 VAC power supply from transformer)
• Pin 1 - 24 VAC line
• Pin 2 - 24 VAC line
• Pin 3 - Ground
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
Red LED
F1 Fuse J3 Socket
Illuminated when 24 VAC 24 VAC power supply
F1 fuse is blown
7.5 amp from transformer
A
3
2
1
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MCB - SECTION B
The upper middle section of the MCB contains the following sockets/components:
† J4 Socket (Outputs/Inputs)
J5 Socket (Inputs)
• Pin 1 - 24 VAC Flow switch
• Pin 1 - Spare - not used
• Pin 2 - 24 VAC Flow switch
• Pin 2 - Spare - not used
• Pin 3 - 24 VAC Low gas press switch
• Pin 4 - 24 VAC Low gas press switch
• Pin 5 - 24 VAC Blocked flue (exhaust) switch
• Pin 6 - 24 VAC Blocked flue (exhaust) switch
• Pin 7 - 24 VAC IRI prover switch - N/A on VF boilers
• Pin 8 - 24 VAC IRI prover switch - N/A on VF boilers
• Pin 9 - Spare - Not used
• Pin 3 - Spare - not used
• Pin 4 - 24 VAC line low water cut off (optional)
• Pin 5 - Low water cut off proving signal
• Pin 6 - 24 VAC line low water cut off (optional)
• Pin 7 - 24 VAC line alarm bell circuit (optional)
• Pin 8 - 24 VAC line alarm bell circuit (optional)
• Pin 9 - Spare - not used
• Pin 10 - Spare - Not used
• Pin 10 - Spare - not used
J17 Socket (Blower Prover / Hi Gas Press Switch)
J11 Socket (Enable/Disable or Tstat circuit)
• Pin 1 - 24 VAC to dry control contacts
• Pin 2 - 24 VAC to dry control contacts
• Pin 3 - Spare - not used
• Pin 1 - 24 VAC High Blower Prover switch
• Pin 2 - 24 VAC High Blower Prover switch
• Pin 3 - 24 VAC High gas press switch (optional)
• Pin 4 - 24 VAC High gas press switch (optional)
• Pin 5 - 24 VAC Low Blower Prover switch
• Pin 6 - 24 VAC Low Blower Prover switch
J19 Socket
• Pin 1 - 24 VAC - Gas valve solenoid
• Pin 2 - 24 VAC - Gas valve solenoid
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
J4 Outputs
J11 Tstat J5 Inputs
1 2 3
6
7
8
9
10
1
2
3
4
5
2
1
3
2
1
1
2
3
4
5
6
7
8
9
4 5 6
J19 Gas Valve
J17 BlwPrv/Hi Gas
10
B
†. J4 Socket Pins 1-8 provide 24 VAC circuits for optional components and equipment. These are switched
24 VAC control circuits with a maximum amp rating of 1 amp. When these optional components are used the
MCB must be configured to recognize and enable the components by setting the SW1 dip switches
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MCB - SECTION C
The upper right corner of the MCB contains the following sockets/components:
J2 Socket (Pump relay coil; IRI gas valve)
• Pin 1 - 120 VAC switched hot wire to pump relay coil
• Pin 2 - 120 VAC neutral wire to pump relay coil
• Pin 3 - 120 VAC hot wire to IRI gas valve - N/A on VF boilers.
• Pin 4 - 120 VAC neutral wire to IRI gas valve - N/A on VF boilers.
J15 Socket Not used on VF boilers
J1 Socket (MCB power supply from PDB)
• Pin 1 - 120 VAC hot wire
• Pin 2 - 120 VAC neutral wire
• Pin 3 - Ground
J18 Socket (Silicon Carbide igniter power)
• Pin 1 - Igniter 120 VAC hot wire
• Pin 2 - Igniter 120 VAC neutral wire
F2 Fuse - N/A on VF boilers - if fuse is removed or blown it will not effect operation of VF boiler
JP4 Jumper - Should be on - removed during manufacturing only
J16 Flame - Flame sensor connection (single wire connect on MCB - split wire serves two flame sensors)
JP2 Jumper - Set jumper on pins 1 & 2 for 2.7 amp igniter
JP3 Jumper - Should be on - removed during manufacturing only
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
J2 Pump / IRI Gas J15 Not Used J1 Line Pwr
J18 SiCar
4
3
2
1
4
3
2
1
3
2
1
2
1
F2 Fuse
not used on
VF boilers
JP4
J16 Flame
C
1
2
3
JP2
JP3
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MCB - SECTION D
MCB Master Control Dip Switches
System configurations are made on the MCB circuit boards with dip switches. This page
explains how the Master Control dip switches (SW1) are configured on the MCB. These dip
switch/configuration settings can be viewed from the “Configuration Settings” menu screen
on the UIM (see page 9).
Switch 1 Type of boiler application
On = VB
Off = VW
235° max high limit
70° to 220° Oper Set Point
210° max high limit
70° to 190° Oper Set Point
Switch 2 Number of trials for ignition
Switch 3 IRI gas valve - N/A set to “off”
Switch 4 Controlling probe
On = 3
Off = 1
On = IRI gas valve present
On = Tank/Loop (Remote)
On = Yes
Off = IRI not present
Off = Inlet
Off = No
Switch 5 Power vent kit - N/A set to “off”
Switch 6 Low water cut off present
On = Yes
Off = No
Switch 7 Low gas pressure switch present On = Yes
Off = No
†
Firing Mode
On = Single Modulating Burner Off = Multi-Stage Burners
Switch 8
Switch 9
Off
Switch 10
Off
‡
Number of stages
1 stage
2 stage
3 stage
4 stage
Switch 9
Switch 10
Off
On
On
Off
On
On
†. The control system will not initiate blower operation on VF boilers if dip switch 8 IS NOT set to “on”
for modulating burner operation. The pump will start but the blower will not. After approximately 5
minutes the control system will lock out and display the “Blower Prover Stg 1” error message.
‡. Stage configuration dip switches 9 & 10 are disregarded by the MCB when dip switch 8 is set to “on”
for single modulating burner operation. With dip switch 8 in the “on” position the number of stages is
internally set to 1.
10
9
D
Firing Mode
8
7
6
5
4
3
2
1
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MCB - SECTION E
The lower left corner of the MCB contains the following sockets/components:
J8 Socket (Inlet probe)
J6 Socket (Gas valve relay - N/A on VF boilers)
• Pin 1 - Not used
• Pin 1 - 24 VDC gas valve relay circuit
• Pin 2 - N/A - plug is filled
• Pin 3 - 24 VDC gas valve relay circuit
• Pin 4 - Ground
• Pin 2 - 0 - 5 VDC inlet temperature probe
• Pin 3 - 0 - 5 VDC inlet temperature probe
• Pin 4 - N/A - plug is filled
† J9 Socket (Remote Tank/Loop probe)
J7 Socket (Outlet probe)
• Pin 1 - N/A - plug is filled
• Pin 2 - Not used
• Pin 3 - 0 - 5 VDC Tank/Loop probe
• Pin 4 - 0 - 5 VDC Tank/Loop probe
• Pin 1 - 24 VDC manual reset ECO
• Pin 2 - 24 VDC manual reset ECO
• Pin 3 - Not used
• Pin 4 - 0 - 5 VDC outlet temperature probe
• Pin 5 - 0 - 5 VDC outlet temperature probe
• Pin 6 - Not used
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
1
2
3
4
E
J6 Gas Pwr
1
2
3
4
5
6
J7 Outlet Prb
1
2
3
4
J8 Inlet Prb
J9 Tank Prb
3
1
4
2
† J9 Socket Pins 3 & 4 - the remote Tank/Loop temperature probe connects to this socket. Two wires from this
socket are routed to the junction box on the back of the boiler. Dedicated field wiring and conduit must be
routed between the Tank/Loop probe and the junction box. The remote Tank/Loop temperature probe is
factory supplied with VF boilers. This temperature probe is used as the “Controlling Probe” (pages 62 and 63)
on VF boilers. SW1 dip switch #4 must be turned on to activate the this probe - see page 30.
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MCB - SECTION F
This section of the MCB contains the communication or “Comm” ports:
ICSP Port
The ICSP port (In Circuit Serial Programming) is used for programming the MCB board. DO NOT plug any
device or cable into this port. This port is used by A. O. Smith engineering personnel only. Damage
caused by plugging cables/devices into this port would not be covered under warranty.
External Comm Ports (2)
These ports are used to connect external communication devices such as a modem or PC. These can also
be used to network boilers together using standard Category 5/6 network cable. One port would receive a
cable from the previous boiler and the second port would be connected to the next boiler in the network. Up
to 31 boilers can be networked (daisy chained) together in this way. For a boiler to be “seen” on the network
it’s default network address of 0 must be changed to a number between 1 and 31 in the User Settings menu
from the UIM. Each boiler on the network would need to have a different network address.
Internal Comm Ports (2)
These are parallel ports used to connect internal components that communicate with the MCB. One is
reserved for the UIM and the other would be used to connect to remote FCB circuit boards on products with
two or more stages of heating. Because these ports are parallel it does not matter which one connects to the
UIM.
JP1 Jumper
A jumper is installed between the two pins of JP1 on the MCB whenever an external communications
network/cable is longer than 2000 feet. On multiple boiler networks the jumper would be installed on the last
boiler only (furthest away) on the network. This jumper would not be installed on any other boilers in the
network.
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
JP1
ICSP
External Comm
Internal Comm
F
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MCB - SECTION G
Along the bottom edge of the MCB there is a second dip switch (SW2). This page details
and explains the SW2 dip switch. There are 3 switches on SW2: switch 1 is a spare, switch
2 must be configured correctly, dip switch 3 does not affect operation on VF boilers.
Switch 1 Spare
On
Off
†
High gas pressure switch present On = Yes
Off = No
Switch 2
Switch 3
‡
Number of blower speeds
On = 1 speed
Off = 2 speed
†. High gas pressure switch is optional; this switch must be set to “Off” if there is not a high gas
pressure switch installed on the boiler.
‡. The position of this dip switch on VF boilers has no effect on operation.
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
1 2 3
SW2
G
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MCB SECTION H
This section of the MCB contains a small “Daughter Board” that contains the J23 and J24
Service Notes: J24 Pins 1 and 2 (1-10 VDC VFD instruction) are polarity sensitive. If these
two wires are not connected as shown. The VFD will not start the blower. The pump on the
boiler will start but the blower will not. After 5 minutes the MCB will lock out and display the
“Sequence Err” error message.
connections in the wiring chase for these three; red, blue, and black wires. The wire colors
must be connected as shown here. If these wires are not connected or cross connected (in
the wring chase) the MCB will read 0 rpm even though the blower is running. The control
system would then lock out and display the “Ignition Speed” error message.
The MCB contains a resistor for the VFD enable/disable circuit to limit current. There is
approximately 1100 ohms resistance when this circuit has been closed by the MCB; use a
2000 to 20,000 ohm scale on the ohm meter being used. The audible continuity test feature
on most ohm meters is a max 200 ohms scale; this would incorrectly show an open circuit.
J23 Socket (Config Key)
• Pin 1 - Config Key
• Pin 2 - Config Key
J24 Socket (VFD & Hall Effect Sensor)
• Pin 1 - VFD - Negative 1-10 VDC instruction - Black/White Stripe Wire
• Pin 2 - VFD - Positive 1-10 VDC instruction - Yellow /Red Stripe Wire
• Pin 3 - VFD - Enable/Disable Circuit - Red/White Stripe Wire
• Pin 4 - VFD - Enable/Disable Circuit - Blue/Black Stripe Wire
• Pin 5 - Not used
• Pin 6 - Hall Effect Sensor - Red Wire
• Pin 7 - Hall Effect Sensor - Blue Wire
• Pin 8 - Hall Effect Sensor - Black Wire
The MCB circuit board is mounted upside down on VF boilers compared to the illustrations below.
J24 Variable Frequency Drive (VFD)
and Hall Effect Sensor (blower rpm)
J23 Config Key
(modulationkey
socket)
5
6
7
8
1
2
3
4
1
2
H
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PDB - POWER DISTRIBUTION BOARD - OVERVIEW
The PDB provides connection points for line-input (120 VAC) power, the pump, and the
transformer. It distributes the line-input power and the transformer output (24 VAC) power to
necessary locations. It also contains fuses for the pump and the MCB circuit board. The
Wiring and connection details given here reference 100-101 Series VF boilers.
The wiring and connection information given in this document is in reference to how the
EMC 5000 controls and circuit boards connect on 100-101 Series VF boilers. This control
system may be used on other products such as Genesis boilers and while some wiring and
connection information will be the same; most things will differ.
The PDB circuit board has been divided into sections in the illustration below, each section
will be covered in the pages that follow.
The pages that follow reference the PDB circuit board as oriented in this illustration.
The terminal strip (Section A) should be on the left.
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PDB - SECTION A
The left side of the PDB circuit board contains a terminal strip (TB1) used to connect the 120
VAC power supply to the boiler, the pump relay, and the pump motor. VW models will have a
factory mounted pump powered by the pump relay; VB models are not equipped with factory
Power supply must be dedicated/isolated: 120 VAC, single phase, with a grounded neutral
line, per NEMA standards using a 30 amp dedicated circuit breaker. The power supply must
be in a dedicated conduit. Hot wires, neutral wires, and ground wires must not be shared
with any other appliances including other VF boilers. External control or Tank/Loop Probe
wiring must also be in dedicated conduits. See the power requirements on page 68.
TB1
A
120 VAC Hot From F2 Fuse
To Pump Relay Com Contact
1
2
F2 Fuse
20 AMP
(See page 37)
120 VAC Hot To F2 Fuse
120 VAC Hot Wire From
Pump Relay N.O. Contact
3
120 VAC Hot Wire From Pump
120 VAC Neutral Wire From Pump
120 VAC Neutral Wire
4
Internal
Connections
5
6
JP2 Jumper
Should be on -
removed during
manufacturing
only
Ground Wire From Pump
7
120 VAC Hot Wire
(From Boiler On/Off Switch)
120 VAC
8
120 VAC Neutral Wire
9
Power
Supply
Ground To Boiler
10
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PDB - SECTION B
The mid section of the PDB contains the fusing for the following:
F1 Fuse - 10 amp 240 VAC - European Models only
†F2 Fuse - 20 amp 120 VAC - Pump
F3 Fuse - 3 amp 120 VAC - Transformer Primary Winding
F4 Fuse - 10 amp 120 VAC - FCB 4 Power Supply - N/A on VF boilers
F5 Fuse - 10 amp 120 VAC - FCB 3 Power Supply - N/A on VF boilers
F6 Fuse - 10 amp 120 VAC - FCB 2 Power Supply - N/A on VF boilers
F7 Fuse - 15 amp 120 VAC - MCB Power Supply
F1 Fuse - 10 Amp - 240 VAC
European Models Only
F2 Fuse - 20 Amp - 120 VAC
Pump Power Supply
B
F3 Fuse - 3 Amp - 120 VAC
Primary Winding Transformer
F4 Fuse - 10 Amp - 120 VAC
FCB 4 Power Supply
(FCB 4 not used on VF Boilers)
F5 Fuse - 10 Amp - 120 VAC
FCB 3 Power Supply
(FCB 3 not used on VF Boilers)
F6 Fuse - 10 Amp - 120 VAC
FCB 2 Power Supply
(FCB 2 not used on VF Boilers
F7 Fuse - 15 Amp - 120 VAC
MCB Power Supply
†. Field installed boiler circulation pumps (VB models): For outputs up to 120 VAC 5 FLA use on board
remote pump relay connected to TB1 terminal board on the PDB (page 36). For 120 VAC outputs between 5
and 20 FLA, use an external relay/starter and supply 120 VAC pump power through the 20 amp F2 pump
fuse on the PDB shown above. For outputs above 120 VAC or 20 FLA use an external relay/starter and a
separate dedicated power supply for the pump, use the on board pump relay Com and N.O. contacts to
initiate the field supplied starter/relay coil only.
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PDB - SECTION C
The upper right corner of the PDB contains the following:
J2 Socket (24 VAC Outputs)
• Pin 1 - 24 VAC Hot to MCB J3 Socket
• Pin 2 - 24 VAC Return to MCB J3 Socket
• Pin 3 - Ground to MCB J3 Socket
• Pin 4 - 24 VAC Hot to Stage 2 FCB - N/A on VF boilers.
• Pin 5 - 24 VAC Return to Stage 2 FCB - N/A on VF boilers.
• Pin 6 - N/A
• Pin 7 - N/A
• Pin 8 - 24 VAC Hot to Stage 3 FCB - N/A on VF boilers.
• Pin 9 - 24 VAC Return to Stage 3 FCB - N/A on VF boilers.
• Pin 10 - 24 VAC Hot to Stage 4 FCB - N/A on VF boilers.
• Pin 11 - 24 VAC Return to Stage 4 FCB - N/A on VF boilers.
• Pin 12 - N/A
J1 Socket (100VA Transformer 120 VAC x 24 VAC)
• Pin 1 - 24 VAC Common
• Pin 2 - 24 VAC Hot
• Pin 3 - 120 VAC Hot to Transformer
• Pin 4 - 24 VAC Common
• Pin 5 - 24 VAC Hot
• Pin 6 - 120 VAC Neutral to Transformer
C
J2 Socket
24 VAC Outputs
10 11 12
7
4
1
8
5
2
9
6
3
J2 Pin Numbers
1
2
3
4
5
6
J1 Socket
Transformer
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PDB - SECTION D
The lower right corner of the PDB contains the J3 Socket and the JP1 jumper. The J3 Socket
is described here the JP1 jumper is described on page 40.
J3 Socket (120 VAC Outputs)
• Pin 1 - 120 VAC Hot to MCB J1 Socket
• Pin 2 - 120 VAC Neutral to MCB J1 Socket
• Pin 3 - Earth Ground to MCB J1 Socket
• Pin 7 - Spare 120 VAC Hot
• Pin 8 - Spare 120 VAC Neutral
• Pin 9 - Spare Earth Ground
• Pin 10 - Spare 120 VAC Hot
• Pin 11 - Spare 120 VAC Neutral
• Pin 12 - Spare Earth Ground
• Pin 13 - Spare 120 VAC Hot
• Pin 14 - Spare 120 VAC Neutral
• Pin 15 - Spare Earth Ground
J3 Socket
120 VAC Outputs
13 14 15
10 11 12
D
7
4
1
8
5
2
9
6
3
J3 Pin Numbers
JP3
JP1
JP3 Jumper
JP1 Jumper
Power Test Jumper
Should be on - removed
during manufacturing only
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PDB - POWER SUPPLY TEST
The lower right corner of the PDB also contains two jumpers and three power supply status
LED lights. With power applied the green DS2 LED should always be lit. The JP1 jumper is
used to activate a power supply test function. The JP2 and JP3 jumpers are used for
manufacturing purposes only.
Power Test Procedure:
Turn power off to the boiler at the breaker or disconnect switch. Disconnect the wiring plugs
at J1, J2, and J3 Sockets on the PDB. Relocate jumper JP1 from the “Run” pins to the “Test”
power supply is properly connected the Yellow and Green LEDs should be illuminated and
the red LED should be off. If any other combination of LEDs are illuminated refer to the table
below for the problem indicated and what corrective action must be taken.
Note:
The JP1 jumper should be in the run position during normal operation. Leaving the jumper in the test mode
when operating the system may cause trouble with Ground Fault Interrupters.
The wire harnesses that normally connect to J1, J2, & J3 should be disconnected while performing this test.
Leaving them connected will not cause damage but the status indicated by the LED's will be incorrect.
LINE CONNECTION STATUS
Proper Connection
DS1 YELLOW LED
DS2 GREEN LED
DS3 RED LED
ON
OFF
OFF
OFF
ON
ON
ON
OFF
OFF
ON
Open Ground
Reverse Polarity
Open Hot
ON
OFF
ON
OFF
ON
Open Neutral
Reverse Hot & Ground
ON
OFF
OFF
ON
Hot wire on Neutral connect & Open
Neutral wire
OFF
ON
DS1
DS2 DS3
Yellow Green Red
Test
Run
D
JP1
JP3 Jumper
Should be on - removed
JP1 Jumper
Power Test Jumper
during manufacturing only
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ERROR MESSAGES - TROUBLESHOOTING
The EMC 5000 control system performs exhaustive self diagnostics and displays detected
fault conditions on the UIM (page 6). There are approximately 80 different error messages.
Troubleshooting procedures for the most common error messages are covered in this
service manual. A more complete list of error messages is in the boiler’s Instruction Manual.
The troubleshooting procedures shown here relate to the EMC 5000 control system
on VF boilers VW/VB 500 - 1000 Series 100/101 only.
The first column in the tables that follow show the actual error message as displayed by the
UIM along with an explanation. The second column details things to check or repair.
Use the System Status menu when troubleshooting:
This menu is used to view the status of various components/switches, gas valve, flame
sensors etc. An asterisk (*) is displayed next to a menu item when the status is true or on,
the asterisk is not displayed when the status of a menu item is false or off. To access this
menu press the Menu button on the UIM, navigate to the “System Status” menu and press
Select. Using the information here along with the Sequence of Operation on page 15 is very
helpful when troubleshooting. IE; if water flow is detected by the flow switch (flow switch
contacts are closed) an asterisk (*) will appear in front of the Flow menu item; *Flow.
Things to check before servicing:
• Using the Instruction Manual that came with the boiler as reference, verify the water
piping, gas line, venting, electrical, and controls are all properly installed.
• Ensure 120 VAC is supplied to the boiler and that the polarity is correct per the electrical
requirements on page 68. 120 VAC hot wire to black, neutral to white in the junction box.
• Ensure minimum supply gas pressure is supplied to the boiler.
Resetting the EMC 5000 control:
Reset the EMC 5000 control by touching the Select button while the “Current Error” menu
screen (page 8) is displayed by the UIM.
IMORTANT SERVICE REMINDER:
When performing any troubleshooting step outlined in this service manual always consider
the wiring and connectors between components. Perform a close visual inspection of all
wiring and connectors to and from a given component before replacement.
Ensure wires were stripped before being crimped in a wire connector, ensure wires are
crimped tightly in their connectors, ensure connection pins in sockets and plugs are not
damaged or worn, ensure plugs and sockets are mating properly and providing good
contact.
Failure to perform this critical step or to perform this step thoroughly often results in
needless down time, unnecessary parts replacement, and customer dissatisfaction.
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ERROR MESSAGES
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
•
Ensure the on/off switch is on and working -
replace switch if defective.
UIM Display Is Blank
Boiler is turned off
Ensure 7.5 amp fuse on MCB is not blown/miss-
120 VAC power supply problems
24 VAC power supply problems
Blown Fuses
Defective transformer
Wiring or plug/socket connection problems
UIM communication cable problems
Check comm cable connections at UIM and the
MCB’s Internal Comm Ports. Secure power and
install a new comm cable. (standard Cat 5 net-
work cable). Plug UIM comm cable into other
Important Service Reminder:
•
•
•
Closely inspect communication ports on MCB
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Ensure 120 VAC power is properly connected in
the junction box on the back of the boiler.
Ensure 120 VAC is supplied to TB1 terminals 8
and 9 on the PDB. Hot wire to terminal 8, neu-
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
•
•
Ensure F7 fuse on PDB is not blown/missing
(page 37) - replace fuse.
Check F3 transformer fuse on PDB (page 37) if
fuse is blown check wiring for shorts - then
replace fuse.
•
•
Check 120 VAC to transformer at the primary
winding terminals on the transformer. If not
present check wiring from PDB J1 socket.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
Check for 120 VAC between pins 3 & 6 of the J1
with the wiring harness plugged into the J1
Socket. Ensure the J1 socket and plug are mak-
ing good contact.
•
Check for 24 VAC output at the transformer sec-
ondary winding terminals - if 24 VAC is not
present AND the two checks above were per-
formed and the results were successful -
replace the transformer. Check all wiring for
shorts before powering up the new transformer.
•
•
Check for 24VAC between pins 1 & 2 and
between pins 4 & 5 of the J1 Socket on the PDB
ness plugged into the J1 Socket.
Ensure the pins in the J1 socket on the PDB
and the pins in the J1 plug and wiring harness
are in good condition and making good contact.
Repair or replace any parts that are worn, dam-
aged, or failing to make a good connection.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
Plug UIM comm cable into the other “internal”
“Display Fail”
Communication between the MCB and UIM has
been interrupted - the MCB is having trouble
communicating with the UIM.
Check comm cable to the UIM. Turn off power
and Install a new/different comm cable
(standard Cat 5 network cable).
•
Inspect both internal communication ports on
MCB and UIM for damage or wear. Repair or
replace any components that are worn or dam-
aged.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
•
•
Ensure the Config Key is present and securely
plugged into the J23 socket on the MCB circuit
“No Config Key”
MCB does not recognize the presence of a Config
Key at MCB socket J23.
Closely inspect Config Key plug pins - if dam-
aged Install a new Config Key for the correct
Btu/hr model and fuel type.
The Config Key contains model specific (Btu/hr &
fuel type) blower speed programing data. This data
is written to the MCB’s internal memory at power up.
The MCB then uses this data to instruct the VFD
(variable frequency drive) to modulate blower speed
during various operating states.
•
•
Install a new Config Key for the correct Btu/hr
model and fuel type.
Closely inspect the pins in the J23 socket on the
MCB for signs of wear or damage.
If this key is not plugged into the J23 socket on the
MCB circuit board the EMC 5000 control system will
declare a fault and display this error message on the
UIM.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
Ensure the Config Key is securely plugged into
the J23 socket on the MCB circuit board (see
“Config Key CRC”
Data read from the Config Key by the MCB was not
properly confirmed at power up.
Turn power off to the boiler for 1 minute and
then turn power back on. Press the Select but-
ton on the UIM to clear any error messages.
This will provide the MCB a second attempt to
read and store data from the Config Key.
The Config Key contains model specific (Btu/hr &
fuel type) blower speed programing data. This data
is written to the MCB’s internal memory at power up.
The MCB then uses this data to instruct the VFD
(variable frequency drive) to modulate blower speed
during various operating states.
•
•
Closely inspect Config Key plug pins - if dam-
aged replace the config key.
Install a new Config Key for the correct Btu/hr
model and fuel type if this error message contin-
ues.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
•
Closely inspect the pins in the J23 socket on the
MCB for signs of wear or damage.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
•
•
Turn power off to the boiler for 1 minute and
then turn power back on. Press the Select but-
ton on the UIM to clear any error messages.
This will provide the MCB a second attempt to
read and store data from the Config Key.
“Config Key Part”
MCB has detected the Config Key is defective.
The Config Key contains model specific (Btu/hr &
fuel type) blower speed programing data. This data
is written to the MCB’s internal memory at power up.
The MCB then uses this data to instruct the VFD
(variable frequency drive) to modulate blower speed
during various operating states.
Install a new Config Key for the correct Btu/hr
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
•
Ensure power supply polarity is not reversed
and boiler is properly grounded.
“Low AC Voltage”
Power supply polarity is reversed. Line voltage to
boiler is less than 90 VAC.
Perform Power Supply Test Procedure (page
40) - correct any problems indicated by test.
Check incoming power supply, wiring, and all
line voltage connections on the boiler and at the
breaker or disconnect switch - repair/restore
115 - 120 VAC power supply to boiler.
•
Check 120 VAC to transformer. Check for 115 -
120 VAC between pins 3 & 6 of the J1 socket on
wiring harness plugged into the J1 Socket.
“Low 24 VAC”
Voltage from transformer is less than 18 VAC.
Important Service Reminder:
•
•
Ensure transformer is 100 VA rated - replace
transformer if rating is less than 100 VA.
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Secure power to boiler; temporarily disconnect
load wiring from secondary coil on transformer.
Turn power back on and check voltage at sec-
ondary coil - if secondary (24 VAC) voltage
remains low - replace transformer.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
•
•
Check all 24 VAC wiring for worn/damaged con-
nections or wires - replace/repair as necessary.
Replace transformer.
•
•
•
Ensure there is water in the lines/boiler.
Remove and clean LWCO sensor.
“Low Water”
Water is not being sensed by optional LWCO (low
water cutoff) device’s sensor.
If the SW1 #6 dip switch is configured for LWCO
present when the optional control is not installed
on the boiler. This will return a “Low Water” error
message. Check “Config Settings” menu screen
switch setting. Set SW1 #6 dip switch on the
•
•
Check all wiring, plugs, and sockets (J4 socket
nections - repair/replace damaged/worn parts.
Check supply gas pressure with boiler firing at
100%. Ensure supply pressure is maintained
above 4.0" W.C. for natural gas and 6.4" W.C.
for propane - replace low gas switch if supply
pressures are maintained above activation point
at 100% firing rate and error message persists.
“Low Gas”
Supply gas pressure has dropped below pressure
activation point of the Low Gas Pressure switch.
Low gas pressure switch is standard on VF boilers.
SW1 dip switch #7 is set to “on” to configure the
•
Ensure gas line is properly sized. - adjust gas
supply pressure at regulator serving the boiler.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
“No Flow”
VB Models - No Factory Pump
Pump IS NOT Running
•
•
Check for power at the pump motor - if the
pump has power but will not start - replace/
repair pump.
(If pump is running proceed to the next page)
Normally open contacts on factory mounted flow
switch are not closing or are not remaining closed
after the pump is energized.
If power to the field installed pump is not present
- ensure all field installed pump controls, wiring,
connectors, and relays or starters are function-
ing correctly. Make necessary repairs to restore
power to pump.
Pump Relay Note:
The style/manufacturer of the pump relay used may
change over time. To determine which terminals are
designated as the 120 VAC relay coil terminals and
which are the Common and Normally Open contact
terminals - inspect/remove the relay and refer to the
relay label or diagram.
•
If field installed pump is being powered from the
the boiler follow pump power, fuse, and pump
relay steps for VW models below.
VW Models - Factory Pump Installed
•
•
Ensure factory installed pump is running. Check
for 120 VAC at the pump motor - if the pump has
power but will not start - replace/repair pump.
VB Field Installed Pump Note:
Check F2 fuse on PDB - replace fuse if blown
(page 37). Read the VB Field Installed Pump
Note to the left if this fuse is blown on a VB
model using the boiler power supply and/or
pump relay.
For field installed pumps rated up to 120 VAC 5 FLA
use on board remote pump relay connected to TB1
outputs between 5 and 20 FLA, use an external
relay/starter and supply 120 VAC pump power
through on-board 20 amp fuse (page 37). For
outputs above 120 VAC or 20 FLA use an external
relay/starter and feed the power through discrete
wires from a separate branch circuit. Pump power is
not passed through the on-board 20 amp fuse; use
external breaker. See output specifications for pump
•
•
With a call for heat present check for 120 VAC
at pump relay coil from MCB J2 socket pins 1 &
nector pins and wiring to pump relay coil for
damage or wear - repair/replace.
Ensure pump relay N.O. (normally open) con-
tacts are closing with 120 VAC supplied to the
pump relay coil. If contacts do not close -
replace pump relay. See Pump Relay Note to
the left.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
•
•
Check for 120 VAC hot to the Common contact
terminal of the pump relay at TB1 terminal 1 on
Check for 120 VAC hot return from the N. O.
contact terminal of the pump relay at TB1 termi-
neutral.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
•
•
Check for 120 VAC to the pump motor between
Ensure hot/neutral wires from pump are in good
condition and properly connected to TB1 termi-
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
New boiler installations in older systems;
Check for restrictions in the water lines (accu-
mulated lime) to and from the boiler. System
lines can become restricted with lime and sedi-
ment accumulation over time and sometimes
must be replaced on older systems.
“No Flow”
Pump is Running
(If pump is not running go to the preceding page)
Normally open contacts on factory mounted flow
switch are not closing or are not remaining closed
after the pump is energized.
New boiler installations in older systems;
Ensure existing water lines to and from the
boiler are properly sized. Supply and return
lines to the boiler must not be smaller than the
inlet and outlet connections on the boiler. Exist-
ing water lines may be undersized (restrictive)
and need to be replaced.
Water flow rate at the outlet of boiler is below the
activation point for the flow switch. Factory mounted
flow switch is a normally open and closes at
approximately 22 GPM. (GPM = gallons per minute).
Important Installation Note:
•
•
•
Ensure flow switch is wired correctly using the
Common and Normally Open contact terminals.
The combined total of supply and return water
piping must not exceed 50 equivalent feet on VF
boilers equipped with a “factory installed” pump.
De-lime boiler if necessary - lime accumulation
within the boiler (over time) can reduce flow.
Ensure the boiler, supply/return lines, storage
tank, building loop, and all water system compo-
nents are purged of air.
Exceeding this limitation can and will cause “No
Flow” lock outs. Be certain all elbows and fittings in
the supply and return lines between the boiler and
the storage tank or heating loop are considered in
the equivalent feet calculations.
•
•
Check condition of flow switch and paddle -
replace worn or missing paddle - replace flow
switch if damaged or defective.
Installations that exceed the 50 equivalent foot
limitation described above require field supplied and
installed pumps that are properly sized.
Check temperature rise through the boiler at
100% firing. Ensure ΔT is not above 40°F.
Check for partially closed valves that could lead
to reduced flow rate.
Operational Note:
•
•
If ΔT at 100% firing is 20 - 40°F, adjust setting
screw on flow switch to close contacts while the
pump is running. If flow switch cannot be
adjusted to close contacts when temperature
rise/flow rate is correct - replace the flow switch.
Boiler outlet valves are often partially closed to
regulate flow rate and temperature rise through the
boiler. A bypass line with a throttling valve should be
installed between the inlet and outlet lines of the
boiler. Bypass valves are often partially open to
maintain inlet water temperature at or above 120°F.
Turn off power to the boiler and TEMPORARILY
INSTALL JUMPER WIRE across the flow
switch wiring terminals. Turn power back on and
reset the control - see page 41.
Throttle outlet valve to achieve a 20 - 40°F
temperature rise through the boiler.
If error message persists with jumper wire on -
call the A. O. Smith technical information center
for further assistance at 800 527-1953
Throttle bypass valve to maintain minimum inlet
water temperature of 120°F before performing the
check/repair procedures here.
REMOVE JUMPER WIRE FROM FLOW
SWITCH IMMEDIATELY AFTER PERFORM-
ING THIS TEST. FAILURE TO DO SO CAN
RESULT IN PROPERTY DAMAGE AND/OR
PERSONAL INJURY.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
•
Ensure all wiring to VFD is connected properly.
“Sequence Err”
Enable/disable circuit wires to VFD disconnected.
1-10 VDC instruction wires to VFD disconnected.
1-10 VDC instruction wires to VFD reversed polarity.
MCB failing to send 1-10 VDC instruction to VFD.
Check for 1-10 VDC instruction on VFD low volt-
Close visual inspection of J24 plug/socket con-
nection on MCB Daughter Board. Ensure plug/
socket connect is in good condition and making
•
Ensure all wring between the J24 socket on the
MCB and the Hall Effect sensor are properly
connected and the red, blue, and black wires
have not been cross connected in the wiring
“Ignition Speed”
Wiring between MCB J24 socket and the Hall Effect
Sensor in the blower motor end cap is open or cross
connected. Fault occurred at beginning of heating
cycle during the pre purge operating state.
•
•
Close visual inspection of J24 plug/socket con-
nection on MCB Daughter Board. Ensure plug/
socket connect is in good condition and making
OR
Hall Effect sensor is failing to send blower rpm
information to MCB during the pre purge state.
Remove end cap from blower motor and inspect
Hall Effect sensor for any sign of damage or
wear. Ensure Hall Effect sensor is properly
mounted.
•
Ensure all wring between the J24 socket on the
MCB and the Hall Effect sensor are properly
connected and the red, blue, and black wires
have not been cross connected in the wiring
“Mtr Spd < Min Spd”
One or more wiring connections between MCB J24
socket and the Hall Effect Sensor in the blower
motor end cap is open. Fault condition occurred
during the heating cycle operating state.
•
•
•
Close visual inspection of J24 plug/socket con-
nection on MCB Daughter Board. Ensure plug/
socket connect is in good condition and making
OR
Blower motor is running slow or failing during the
heating cycle.
Remove end cap from blower motor and inspect
Hall Effect sensor for any sign of damage or
wear. Ensure Hall Effect sensor is properly
mounted.
OR
Ensure the blower motor shaft and blower wheel
are not obstructed. Ensure blower motor bear-
ings are not worn - motor cycling on thermal
overload etc. Ensure blower motor is running at
proper speed throughout the heating cycle. See
Hall Effect sensor is failing to send blower rpm
information to MCB during the heating state.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
SW1 dip switch #8 is in the off position, this dip
switch must be in the “on” position (page 30).
“Blower Prov Stg1”
Blower IS NOT Starting
(If the blower is starting proceed to the “Blower Prov
Stg1” Blower Is Starting tables)
Turn off power to boiler and disconnect the
wires to both BPS (BPS = Blower Prover
Switches) - check for continuity between the
“Com” and N.O. terminals on both BPS. If there
is continuity - replace the switch (s) showing
continuity. Ensure you ARE NOT checking
between Com and N.C. terminals or the N.C
and N.O terminals.
IMPORTANT SERVICE NOTE:
BEFORE performing any of the check/repair
procedures in the right column of this table; review
the VFD and Blower Operation section in this
connections and service notes contained on page
34. Understanding how the MCB and VFD work
together to operate the blower is necessary.
•
•
Check both BPS for wiring errors - wires from
MCB must be connected to the Com and N.O.
terminals on both. Ensure the terminals have
not been jumpered - remove any jumper wires.
Turn off power to boiler and disconnect one wire
from both BPS. Turn power back on and acti-
vate a call for heat. MCB should initiate blower
operation. If blower still does not start reconnect
wiring to both BPS and continue to the next test.
Operational Note:
If the Normally Open Low Blower Prover or High
Blower Prover pressure switch contacts (circuits) are
closed at the beginning of the heat sequence prior to
the MCB initiating blower operation for the pre purge
operating state; the MCB will not initiate blower
operation in this condition - the MCB will declare a
fault condition and lock out. The “Blower Prov Stg1”
error message will be displayed on the UIM.
•
With power on - check for 120 VAC at the input
is not present check for 120 VAC between pins
wiring to VFD input and PDB J3 socket/plugs
are in good condition and making good contact -
repair/replace anything worn or damaged
restore 120 VAC to VFD input.
See blower prover switch information on page 24
and the Sequence of Operation on page 15
•
•
Ensure ground wire from PDB J3 pin 6 to VFD
input is providing earth ground to VFD.
Important Service Reminder:
Turn off power to boiler - disconnect the two
enable/disable circuit wires from the VFD (page
11). Turn power back on and with a call for heat
activated check for resistance between these
two wires with an ohm meter set to a range
between 2000 and 20,000 ohms. See service
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement. Ensure
wires were stripped before being crimped in a wire
connector, ensure wires are crimped tightly in their
connectors, ensure connection pins in sockets and
plugs are not damaged or worn, ensure plugs and
sockets are mating properly and providing good
contact.
If the MCB is closing the VFD enable/disable
circuit a reading of approximately 1100 ohms
should be measured. If the resistance mea-
sured is infinite - check/repair all wiring, sockets,
and plugs between the VFD enable/disable ter-
minals and pins 3 & 4 of the J24 socket on the
thing worn or damaged.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
Ensure all wiring is reconnected to the VFD and
all plugs are securely plugged into the MCB/
PDB. With power on and a call for heat acti-
vated check for a 1-10 VDC instruction from the
MCB at VFD low voltage terminals “0V” and
also the Firing Rate - Modulation Performance
a “DC” volt meter for this test. At the beginning
of a heat sequence (during pre purge) there
should be 8-9 VDC measured at these two ter-
minals. If there is not any DC voltage measured
- check/repair all wiring, sockets, and plugs
between the VFD and pins 1 & 2 of the J24
“Blower Prov Stg1”
Blower IS NOT Starting
(If the blower is starting proceed to the “Blower Prov
Stg1” Blower Is Starting tables)
IMPORTANT SERVICE NOTE:
BEFORE performing any of the check/repair
procedures in the right column of this table; review
the VFD and Blower Operation section in this
connections and service notes contained on page
34. Understanding how the MCB and VFD work
together to operate the blower is necessary.
Operational Note:
•
Check blower motor output voltage from the
VFD. This is a 3 phase power supply see page
11. With power on and a call for heat activated
perform three AC voltage checks at the begin-
ning of the heat sequence (ensure meter is set
for AC voltage). Check between VFD Blower
Output terminals; U/T1 & V/T2, U/T1 & W/T3,
and V/T2 & W/T3. Actual AC voltages measured
may be between 90 and 240 VAC. The actual
amount of voltage is not important within this
range. However, all three readings should be
within 10% of each other. If no voltage is mea-
sured during any of the three voltage checks or
if any one or two measurements were signifi-
cantly lower AND all other MCB/VFD test results
above were successful - replace the VFD.
If the Normally Open Low Blower Prover or High
Blower Prover pressure switch contacts (circuits) are
closed at the beginning of the heat sequence prior to
the MCB initiating blower operation for the pre purge
operating state; the MCB will not initiate blower
operation in this condition - the MCB will declare a
fault condition and lock out. The “Blower Prov Stg1”
error message will be displayed on the UIM.
See blower prover switch information on page 24
and the Sequence of Operation on page 15
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement. Ensure
wires were stripped before being crimped in a wire
connector, ensure wires are crimped tightly in their
connectors, ensure connection pins in sockets and
plugs are not damaged or worn, ensure plugs and
sockets are mating properly and providing good
contact.
•
•
If the VFD output to blower motor test above
showed consistent voltage between 90 and 240
AC volts during all three checks and the blower
motor still does not start - check all wiring
between the VFD blower output terminals and
the blower motor. Repair/replace any worn or
damaged wiring.
If all wiring to blower motor from the VFD is in
good condition and the blower will not start with
correct AC voltage applied: Ensure the blower
motor IS NOT hot to the touch. If the blower
motor is cold to the touch at this point - replace
the blower.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
If the motor is hot to the touch at this point pro-
ceed to the next check/repair procedure.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
If the MCB and VFD passed all tests above and
the blower motor will not start and is hot to the
touch - let the blower motor cool off and try
starting the boiler again later after the blower
motor has had time to cool down. If the blower
motor hums or tries to start but the shaft will not
turn - disassemble the blower housing - check
for/remove any debris that could be preventing
the blower wheel from spinning free.
“Blower Prov Stg1”
Blower IS NOT Starting
(If the blower is starting proceed to the “Blower Prov
Stg1” Blower Is Starting tables)
IMPORTANT SERVICE NOTE:
BEFORE performing any of the check/repair
procedures in the right column of this table; review
the VFD and Blower Operation section in this
connections and service notes contained on page
34. Understanding how the MCB and VFD work
together to operate the blower is necessary.
Check the blower wheel for damage or binding/
catching against the blower housing. Adjust/cor-
rect any blower wheel alignment problems if
possible.
If the blower wheel is damaged or binding and
adjustment is not possible - replace the blower.
Operational Note:
•
•
Check to see if the blower motor shaft will spin
free - if the blower motor shaft is seized -
replace the blower.
If the Normally Open Low Blower Prover or High
Blower Prover pressure switch contacts (circuits) are
closed at the beginning of the heat sequence prior to
the MCB initiating blower operation for the pre purge
operating state; the MCB will not initiate blower
operation in this condition - the MCB will declare a
fault condition and lock out. The “Blower Prov Stg1”
error message will be displayed on the UIM.
If the MCB and VFD passed all tests above and
the blower comes on and runs yet overheats
and is cutting out on thermal overload - replace
the blower.
See blower prover switch information on page 24
and the Sequence of Operation on page 15
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
Ensure the sensing tubes are connected to the
“L” sensing ports on both BPS (BPS = Blower
Prover Switches) see the control panel layout
and pressure switch information on page 24.
“Blower Prov Stg1”
Blower Is Starting
(If the blower is not starting go to the “Blower Prov
Stg1” Blower Is Not Starting tables)
Ensure all BPS sensing tubes are not kinked
and are properly connected. The two tubes from
the BPS are T fitted together and a single sens-
ing tube from this T fitting connects to a sensing
port on the gas train (the BPS sense manifold
gas pressure) - check/repair all connections.
Three conditions can cause this error message
when the blower is starting:
Normally Open Low Blower Prover or High Blower
Prover pressure switch contacts are not closing
during the pre or post purge operating states - or
within 5 minutes of call for heat being activated.
•
Ensure the blower motor wiring at the VFD
blower output terminals is wired correctly. The
blower motor wires are color coded; Black,
White, and Red. The proper connection points
motor is 3 phase - incorrect wiring at the VFD
will cause the blower motor to run backwards
and cause this lock out condition.
OR
Normally Open High Blower Prover pressure switch
contacts did not re-open after pre purge operating
state was completed. The High Blower Prover switch
is used to prove adequate purging of the combustion
chamber during pre and post purge cycles only. The
blower runs at approximately 5000 rpm during pre
and post purge operating states and at lower speeds
during all other operating states. High blower prover
switch contacts must close during the pre and post
purge states ONLY and open during all other
operating states.
•
Ensure the gas train shut off valve between the
boiler’s 24 VAC gas valve and the Venturi is
•
•
Ensure the wiring to both BPS is properly con-
nected to the Com and N.O. terminals.
OR
Check all wiring between both BPS and the J17
socket connectors are making good contact.
Replace/repair anything worn or damaged.
Normally Open Low Blower Prover pressure switch
contacts have opened during any operating state
when the blower should be running. IE: This can be
caused due to blower failure during the heating cycle
or if the pressure sensed by the switch rises above
the switches activation pressure for any reason. The
normally open contacts on both Blower Prover
switches close on a fall in pressure - the pressure
sensed must be in a vacuum.
•
•
Ensure the wiring to the Low BPS is coming
from MCB J17 pins 5 & 6 and the wiring to the
High BPS is coming from MCB J17 pins 1 & 2
Ensure blower is operating at correct speed dur-
ing Igniter Warm Up period, Min Fire mode, and
Max Fire mode.
Operational Note:
Perform Firing Rate - Modulation Performance
See blower prover switch information on page 24
and the Sequence of Operation on page 15.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
Determine if Low BPS is causing the lock out.
“Blower Prov Stg1”
Blower Is Starting
(If the blower is not starting go to the “Blower Prov
Stg1” Blower Is Not Starting tables)
•
The normally open Low BPS (BPS = Blower
Prover Switch) switch must close its contacts
after the blower starts. The Low BPS contacts
must remain closed whenever the blower is run-
ning to avoid the “Blower Prov Stg1” lock out.
Helpful Troubleshooting Note:
The state of the contacts for the Low BPS can be
viewed in real time using the System Status Menu.
An asterisk will appear next to the “BlwLo Prv” menu
item when the switch contacts are closed. The
asterisk is not present when the switch contacts are
open.
Turn power off; disconnect the two wires to the
Low BPS. See panel layout on page 24 to deter-
mine which is the Low BPS.
Prepare a jumper wire to jumper between the
two wires just disconnected. Turn the boiler
back on, ensure a call for heat is present.
AFTER the blower motor starts jumper the two
(24 VAC) wires together carefully. Do not let the
wire ends touch any grounded surface.
Three conditions can cause this error message
when the blower is starting:
Normally Open Low Blower Prover or High Blower
Prover pressure switch contacts are not closing
during the pre or post purge operating states - or
within 5 minutes of call for heat being activated.
Notice whether or not the boiler will continue
operation to the igniter warm up period when
the igniter is energized or locks out again on
“Blower Prov Stg1.” Clip an amp meter around
one of the igniter wires to see when the igniter
warm up period begins.
OR
Normally Open High Blower Prover pressure switch
contacts did not re-open after pre purge operating
state was completed. The High Blower Prover switch
is used to prove adequate purging of the combustion
chamber during pre and post purge cycles only. The
blower runs at approximately 5000 rpm during pre
and post purge operating states and at lower speeds
during all other operating states. High blower prover
switch contacts must close during the pre and post
purge states ONLY and open during all other
operating states.
If the boiler will not continue on to the igniter
warm up period and locks out again on “Blower
Prov Stg1” before the igniter is energized; the
Low BPS is not causing the lock out. Turn off
power to the boiler and reconnect the wires to
the Low BPS. Continue on to: “Determine if
High BPS is causing the lock out” in the table on
OR
If the boiler operation continues on to the igniter
warm up period without lock out. The Low BPS
is causing the lock out condition. Continue on to
Normally Open Low Blower Prover pressure switch
contacts have opened during any operating state
when the blower should be running. IE: This can be
caused due to blower failure during the heating cycle
or if the pressure sensed by the switch rises above
the switches activation pressure for any reason. The
normally open contacts on both Blower Prover
switches close on a fall in pressure - the pressure
sensed must be in a vacuum.
Operational Note:
See blower prover switch information on page 24
and the Sequence of Operation on page 15.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
Determine if High BPS is causing the lock out.
“Blower Prov Stg1”
Blower Is Starting
(If the blower is not starting go to the “Blower Prov
Stg1” Blower Is Not Starting tables)
•
The normally open High BPS (BPS = Blower
Prover Switch) switch must close its contacts
during the Pre and Post purge operating states
only. This is when the blower is running at
approximately 5000 rpm. When blower speed is
reduced for ignition and throughout the heating
mode the contacts of the High BPS must re-
open and remain open. The contacts of the High
BPS switch must close and open as described
here during normal operation to avoid the
“Blower Prov Stg1” lock out. See the Sequence
of Operation on page 15.
Helpful Troubleshooting Note:
The state of the contacts for the High BPS can be
viewed in real time using the System Status Menu.
An asterisk will appear next to the “BlwHi Prv” menu
item when the switch contacts are closed. The
asterisk is not present when the switch contacts are
open.
Three conditions can cause this error message
when the blower is starting:
Turn power off; disconnect the two wires to the
High BPS. See panel layout on page 24 to
determine which is the High BPS.
Normally Open Low Blower Prover or High Blower
Prover pressure switch contacts are not closing
during the pre or post purge operating states - or
within 5 minutes of call for heat being activated.
Prepare a jumper wire to jumper between the
two wires just disconnected. Turn the boiler
back on, ensure a call for heat is present.
AFTER the blower motor starts jumper the two
(24 VAC) wires together carefully. Do not let the
wire ends touch any grounded surface.
OR
Normally Open High Blower Prover pressure switch
contacts did not re-open after pre purge operating
state was completed. The High Blower Prover switch
is used to prove adequate purging of the combustion
chamber during pre and post purge cycles only. The
blower runs at approximately 5000 rpm during pre
and post purge operating states and at lower speeds
during all other operating states. High blower prover
switch contacts must close during the pre and post
purge states ONLY and open during all other
operating states.
When the blower reduces speed at the begin-
ning of the ignition period disconnect the jumper
wire and leave the two wires from the High BPS
apart. Do not let the wire ends touch any
grounded surface.
Notice whether or not the boiler will continue
operation to the igniter warm up period when
the igniter is energized or locks out again on
“Blower Prov Stg1.” Clip an amp meter around
one of the igniter wires to see when the igniter
warm up period begins.
OR
Normally Open Low Blower Prover pressure switch
contacts have opened during any operating state
when the blower should be running. IE: This can be
caused due to blower failure during the heating cycle
or if the pressure sensed by the switch rises above
the switches activation pressure for any reason. The
normally open contacts on both Blower Prover
switches close on a fall in pressure - the pressure
sensed must be in a vacuum.
If the boiler will not continue on to the igniter
warm up period and locks out again on “Blower
Prov Stg1” before the igniter is energized; the
High BPS is not causing the lock out. Go to:
“Determine if Low BPS is causing the lock out”
Operational Note:
If the boiler operation continues on to the igniter
warm up period without lock out, the High BPS
is causing the lock out condition. Continue on to
See blower prover switch information on page 24
and the Sequence of Operation on page 15.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
Pressure Reading Test
“Blower Prov Stg1”
Blower Is Starting
(If the blower is not starting go to the “Blower Prov
Stg1” Blower Is Not Starting tables)
•
Take a manifold gas pressure reading. See
boiler to operate until the Pre purge cycle com-
pletes and the blower speed reduces for igni-
tion. Record the highest and lowest pressures
observed. This may require jumpering the BPS
Three conditions can cause this error message
when the blower is starting:
Normally Open Low Blower Prover or High Blower
Prover pressure switch contacts are not closing
during the pre or post purge operating states - or
within 5 minutes of call for heat being activated.
This pressure must be between -0.12" W.C. and
-4.8" W.C. (negative pressures - in a vacuum)
during all operating states or one of the two BPS
switches will trigger a the “Blower Prov Stg1”
fault condition and lock out will occur.
OR
Normally Open High Blower Prover pressure switch
contacts did not re-open after pre purge operating
state was completed. The High Blower Prover switch
is used to prove adequate purging of the combustion
chamber during pre and post purge cycles only. The
blower runs at approximately 5000 rpm during pre
and post purge operating states and at lower speeds
during all other operating states. High blower prover
switch contacts must close during the pre and post
purge states ONLY and open during all other
operating states.
If the actual pressure readings maintain within
this range - at least one BPS is defective as
determined from the pressure readings here
sensing tubes are T fitted together it is not con-
clusive which BPS is at fault. IE: a leaking dia-
phragm on the Low BPS could prevent the High
BPS from closing its contacts. Therefore -
replace both the High and Low BPS.
OR
If the actual pressure observed in the pressure
test above did not drop to at least -4.8" W.C.
during the pre purge period when the blower is
drop to at least -0.12" W.C.or lower at any time
while the blower was running during the test the
BPS switches are not defective. Continue:
Normally Open Low Blower Prover pressure switch
contacts have opened during any operating state
when the blower should be running. IE: This can be
caused due to blower failure during the heating cycle
or if the pressure sensed by the switch rises above
the switches activation pressure for any reason. The
normally open contacts on both Blower Prover
switches close on a fall in pressure - the pressure
sensed must be in a vacuum.
•
•
Closely inspect all fittings and flanges in the gas
train between the outlet of the boiler’s 24 VAC
gas valve and the Venturi assembly. See page
12. Reseal/repair any leaking connections -
ensure all gaskets are properly seating.
Operational Note:
See blower prover switch information on page 24
and the Sequence of Operation on page 15.
Disassemble the Venturi from the inlet connec-
tion at the blower and the intake air connection -
ensure the large “O” ring gaskets are not dam-
aged and seating properly.
Closely inspect the cone shaped restrictor
inside the Venturi for any signs of damage or
wear. If the restrictor is damaged or worn -
replace the Venturi.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
Turn off power and disconnect the wires to the
Blocked Flue (Exhaust) Switch, see page 24 for
location. Check for continuity between the termi-
nals of switch with an ohm meter. If there IS
NOT continuity through the switch contacts -
replace the switch (this is a normally closed
switch).
“Blocked Flue”
MCB senses the normally closed Blocked Flue
(Exhaust) pressure switch contacts are open.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
•
•
Check all wiring between the Blocked Flue
Switch and the MCB J5 socket pins 5 & 6 (page
28). Closely inspect the J5 socket/plug - repair/
replace anything worn or damaged.
Disconnect the sensing tube from the Blocked
Flue Switch. Take a pressure reading with a dig-
ital manometer from this end of the sensing tube
with the boiler firing at 100%.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
Turn the boiler on and put the boiler in the Max
If the pressure reading taken is at or above
+1.0” W.C. - check for excessive equivalent feet
or excessive number of elbows in the exhaust
vent, (see the boiler’s Instruction Manual for
venting requirements), check for/eliminate any
restrictions in the exhaust vent. If the vent sys-
tem is installed per requirements in the Instruc-
tion Manual that came with the boiler - call the
A. O. Smith technical information center for fur-
ther assistance at 800 527-1953
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
•
If the pressure reading taken is below +1.0”
W.C. - turn power off to the boiler.
TEMPORARILY DISCONNECT THE TWO
WIRES TO THE BLOCKED FLUE SWITCH
Turn the boiler on and put the boiler in the Max
Check for continuity between the terminals of
switch with an ohm meter while firing at 100%. If
contacts are opening during operation with
pressure below +1.0" W.C. - replace the switch.
RECONNECT THE WIRES TO THE BLOCKED
FLUE SWITCH IMMEDIATELY AFTER PER-
FORMING THIS TEST. FAILURE TO DO SO
CAN RESULT IN PROPERTY DAMAGE AND/
OR PERSONAL INJURY.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
Ensure the supply gas shut off valve to the
boiler and the gas train shut off valve (see
“Flame Stg1”
Flame or ignition failure.
Error message displayed after 1 or 3 losses of flame
or after 1 or 3 failed trials for ignition. EMC 5000
control system will lock-out after 1 or 3 trials
depending on the SW1 #2 dip switch setting on the
•
•
Perform the Poor Combustion/Ignition Problems
procedures on pages 20-23.
•
View the burner through the view port near the
igniter on the top of the combustion chamber
during the ignition trial period. If the burner is
igniting but the flame goes out in a few seconds
ensure the flame sensor wiring is connected
properly and in good condition.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Note: There are two flame sensors on VF boil-
ers. A single wire is connected to the J16 con-
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
bifurcated (divided) into two wires and each end
then connects to one of the two flame sensors.
Repair/replace any worn or damaged wiring.
•
If the flame sensor wiring is in good condition -
perform a flame sensing test. Using a DC micro
amp meter; place the meter test probes in
series with the flame sensor wire. Disconnect
the wire at the J16 connect on the MCB (page
29) insert the tip of one test probe into the wire
connector securely - touch and hold the other
test probe to the J16 connect.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
Measure the flame sensing current during igni-
tion. If the flame sensing current is less than 2.5
µA - remove, inspect, and clean both flame sen-
sors. 5.0 µA flame sensing current is typical.
If either flame sensor shows signs of damage or
the ceramic insulator is cracked - replace the
flame sensor (s). If the flame sensors are in
good condition clean both sensors with fine
steel wool and reinstall both sensors. Check
flame sensing current again to see if the current
is higher and burner flame is established.
If the burner flame is still going out after a few
seconds and all the above checks and proce-
dures have been performed - call the A. O.
Smith technical information center for further
assistance at 800 527-1953.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
Check igniter amperage during igniter warm up
period. If amp draw is less than 3.0 amps
replace the igniter.
“Flame Stg1” (cont)
Flame or ignition failure.
Error message displayed after 1 or 3 losses of flame
or after 1 or 3 failed trials for ignition. EMC 5000
control system will lock-out after 1 or 3 trials
depending on the SW1 #2 dip switch setting on the
Ensure the 1/8” steel sensing tube that connects
between the gas valve (near the low fire adjust-
ment screw cap) and the Venturi assembly is
not clogged or damaged and properly con-
nected at both ends. If the tube is kinked or
damaged - replace the tube. Remove the steel
tube and blow through it to ensure it is not
clogged. Reinstall and tighten the compression
fittings tightly at both ends.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
•
Ensure the 24 VAC gas valve is being energized
during ignition:
If the burner is failing to ignite when viewed
through the view port on the top of the heat
exchanger - check for 24 VAC between pins 1 &
end of the igniter warm up period. If 24 VAC is
present - check all wiring between the J19
socket and the gas valve. Check the J19 socket/
plug pins for wear or damage. Check the ground
(green) wire from gas valve coil to the ground
connections in the junction box on the boiler -
repair/replace anything worn or damaged.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
If 24 VAC is not present between pins 1 & 2 of
the J19 socket on the MCB - call the A. O. Smith
technical information center for further assis-
tance at 800 527-1953.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
Ensure the 24 VAC gas valve is opening and
gas is flowing through the valve:
“Flame Stg1” (cont)
Flame or ignition failure.
Secure power to the boiler. Remove the gas
pressure test plug at the outlet of the boiler’s 24
VAC gas valve. Install a barbed hose adapter fit-
ting in this test port. Connect a U-tube or digital
manometer to the barbed fitting.
Error message displayed after 1 or 3 losses of flame
or after 1 or 3 failed trials for ignition. EMC 5000
control system will lock-out after 1 or 3 trials
depending on the SW1 #2 dip switch setting on the
Close the gas train shut off valve (see image on
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Turn power to the boiler back on - ensure there
is a call for heat present.
The blower will reduce speed as the igniter
warm up period begins. The gas valve is ener-
gized at the end of the igniter warm up period.
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
When the gas valve is energized a positive
pressure should be measured from the outlet
test port on the boiler’s 24 VAC gas valve (with
the gas train shut off valve closed). The mea-
sured pressure should be approximately the
same as supply gas pressure to the inlet of the
valve.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
If there is no pressure increase when perform-
ing this test. Turn power to the boiler off. Close
the supply gas shut off valve to the boiler and
remove the boiler’s 24 VAC gas valve - clean/
remove any obstructions or debris from the inlet
and outlet of the gas valve. Ensure the supply
gas line to the boiler and the gas train inside the
boiler are not obstructed - clean/remove any
obstructions or debris.
If the gas valve is being energized (previous
page) and all the above tests have been per-
formed and there is no gas pressure increase at
the gas valve outlet during this test - call the A.
O. Smith technical information center for further
assistance at 800 527-1953.
REMEMBER TO RE-OPEN THE GAS TRAIN
SHUT OFF VALVE.
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ERROR MESSAGES (CONT)
DISPLAYED MESSAGE
CONDITION/INDICATES
CHECK/REPAIR
•
•
•
Check all wiring and connections between pins
1 & 2 of the J18 socket on the MCB (see page
29) and the igniter. Repair/replace any worn or
damaged wiring or connectors.
“Igniter Stg1”
MCB is not sensing minimum igniter current of 2.7
AC amps during the igniter warm up period.
Closely inspect the pins in the J18 socket/plug
for wear or damage. Ensure the plug and socket
are properly connected and making good con-
tact.
Important Service Reminder:
When performing any troubleshooting steps outlined
in this service manual always consider the wiring
and connectors between components. Perform a
close visual inspection of all wiring and connectors
to a given component before replacement.
Ensure there is 120 VAC between pins 1 & 2 of
the J18 socket on the MCB during the igniter
warm up period (blower speed reduces after pre
purge cycle is completed).
Ensure wires were stripped before being crimped in
a wire connector, ensure wires are crimped tightly in
their connectors, ensure connection pins in sockets
and plugs are not damaged or worn, ensure plugs
and sockets are mating properly and providing good
contact.
If 120 VAC is not present at the J18 socket dur-
ing the igniter warm up period - call the A. O.
Smith technical information center for further
assistance at 800 527-1953.
Check igniter amps during igniter warm up
period (blower speed reduces after pre purge
cycle is completed). If amp draw is less than 3.0
amps replace the igniter.
Failure to perform this critical step or failing to
perform this step thoroughly often results in
needless down time, unnecessary parts
replacement, and customer dissatisfaction.
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BOILER CONTROLS
Boiler controls that are improperly installed or set up can cause serious operational and service
related problems such as short cycling. This section provides information for how various
controls can work together or independently to provide proper system and boiler control.
Two conditions must be met for the VF boiler to start a heating cycle:
1. The Operating Set Point (page 8 and 63) which responds to temperature sensed by the
Controlling Probe (page 63) must be “calling for heat.” The Operating Set Point (page
63) is always active and must be calling for heat or the boiler will not fire.
The EMC 5000 control system, using it’s own Operating Set Point and a remote Tank/
Loop probe, can provide reliable system temperature control as a Primary System
the Enable/Disable circuit.
wiring junction box on the back of the boiler for this circuit. The Enable/Disable circuit is
provided to allow the VF boiler to be started or stopped by different forms of external
supervisory controls (page 65) including: Aquastats, Boiler Sequencing Controllers, and
EMS - Energy Management Systems.
The Enable/Disable circuit is always active, if an external control is not using this circuit
for supervisory control, the two Enable/Disable wires in the junction box must be wire
nutted together or the boiler will not fire. See the illustration below.
The “Operating Set Point” is an
internal User Setting adjusted at
the boiler. It may be used as a
stand alone “Primary System
Control.”
External controls, using the
provided Enable/Disable circuit,
can be used in place of the
Operating Set Point as a Primary
System Control and to provide
additional supervisory control such
MCB CIRCUIT BOARD
as night set back, freeze
protection, and outdoor air reset
control.
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BOILER CONTROLS (CONT)
Primary System Control
A Primary System Control is a top level “system” temperature control. A Primary System
Control activates the call for heat or enables boiler operation when system temperature falls
below the control’s set point and deactivates a call for heat or disables the boiler when
system temperature reaches set point. The “system” is typically a storage tank (domestic hot
water supply) or a hydronic loop (building heat system).
The Primary System Control must be able to sense temperature conditions in the system to
work properly. It must not be adversely affected by false heat such as heat added by multiple
boilers upstream or a bypass line.
The Primary System Control can be the Operating Set Point on the boiler acting on
temperature sensed by the factory supplied remote Tank/Loop probe; the “Controlling
Probe.” The Primary System Control can also be an External Temperature Control; a
separate mechanical or electronic device that will have it’s own system temperature sensor
(s) and a set of dry contacts to close or open the boiler’s Enable/Disable circuit.
Temperature Probes
The temperature probes (3) used by the EMC 5000 control system are thermistors.
Thermistors are thermally sensitive resistors. A change in temperature causes a
corresponding change in electrical resistance (ohms). The EMC 5000 control system
interprets the probe resistance as temperature.
VF boilers have two factory installed temperature probes; an Inlet Probe and an Outlet
Probe to monitor the inlet and outlet water temperatures.
VF boilers are shipped from the factory with a third probe; the remote Tank/Loop Probe.
Remote Tank/Loop Probe
Installation/Configuration: Remote Tank/Loop probes are furnished with VF boilers and
should be installed in the storage tank on domestic hot water systems or in a T fitting in the
return line before the boiler’s inlet on heating systems. On multiple boiler systems all Tank/
Loop probes must be installed in T fittings before the first boiler’s inlet.
Two wires are provided for the Tank/Loop Probe in the junction box on the back of the boiler.
Dedicated conduits must be provided for the field installed wiring to each Tank/Loop probe
(page 68).
The SW1 dip switch #4 on the MCB (page 30) must be turned “on” to configure the remote
Tank/Loop probe as the Controlling Probe (page 63).
Application Note: If the MCB SW1 dip switch #4 is turned off the control system will use the
Inlet probe as the Controlling Probe; reliable system temperature control will not be possible.
Service Note: If the MCB SW1 dip switch #4 is turned on and the Tank/Loop Probe is not
connected to the designated wires in the junction box on the back of the boiler the EMC
5000 control will declare a Tank Probe failure/fault condition and lock out. The “Tank Probe”
error message will be displayed.
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BOILER CONTROLS (CONT)
Controlling Probe
The Operating Set Point on the VF boiler’s EMC 5000 control system reacts to temperature
sensed by the Controlling Probe.
The EMC 5000 control system can be configured to use either the Inlet probe or the remote
Tank/Loop probe as the “Controlling Probe.” VF boilers are designed to use the remote
Tank/Loop probe as the Controlling Probe. Review the installation and configuration
information on page 62.
INTERNAL TEMPERATURE CONTROLS
Operating Set Point - User Setting “Oper Setpnt”
when there is no external primary system control in use. The Operating Set Point has a
Operating Set Point is a local EMC 5000 User Setting that can be adjusted through the UIM
(page 8). See the dead-band operational note on page 64.
The Operating Set Point reacts to temperature sensed by the Controlling Probe. When
the temperature sensed at the Controlling Probe drops below the Operating Set Point minus
activated.
When an external control is used as the Primary System Control set the Operating Set
Point at least 20°F higher than the Primary System Control setting to ensure reliable system
temperature control and avoid conflict and short cycling between the two set points.
Stage 1 Differential Setting - User Setting “Stage1 Diff”
This is a “local” EMC 5000 User Setting that can be adjusted through the UIM (page 8).
When the temperature sensed at the Controlling Probe drops below the Operating Set Point
minus the Stage 1 Differential setting a call for heat is activated. IE: with the settings shown
in the screen shot below the call for heat would not be activated until the temperature
sensed at the Controlling Probe dropped to 157° F (160 - 3 = 157). The call for heat is
deactivated when the temperature sensed by the Controlling Probe rises to the Operating
Set Point.
Stage 1 Differential setting has a range of 1°F to 50°F. The higher the setting the wider the
system temperature swing will be. The default setting is 3°F which is a good initial setting.
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BOILER CONTROLS (CONT)
INTERNAL TEMPERATURE CONTROLS (CONT)
Automatic High Limit - User Setting “Hi Limit”
The Automatic High Limit is a “local” EMC 5000 User Setting that can be adjusted through
the UIM (page 8).
The Automatic High Limit is a temperature limiting control, it reacts to temperatures sensed
programmed set point. If the temperature sensed at the Outlet Probe rises to the Automatic
High Limit set point the call for heat is deactivated. The boiler will go through a normal shut
down sequence with post purge cycling of the blower and pump. This IS NOT a Primary
System Control. The Automatic High Limit has a range of 90°F to 210°F for VW models and
90°F to 235°F for VB models (page 70).
Dead-band Operational Note: There is a fixed minimum dead-band between the
Automatic High Limit and the Operating Set Point of 20°F to prevent short cycling the boiler.
The Automatic High Limit can be programmed for more than 20°F above Operating Set
Point but not for less.
The Operating Set Point cannot be raised higher than 20°F below the Automatic High Limit
(Hi Limit) set point. Keep this in mind if the Operating Set Point seems stuck or will not allow
higher settings within it’s range. You may have to raise the Hi Limit set point first.
Automatic High Limit Differential - User Setting “Hi Limit Dif”
This Automatic High Limit Differential is a “local” EMC 5000 User Setting that can be
adjusted through the UIM (page 8). The Automatic High Limit Differential has a range of 1°F
to 50°F.
point, the EMC 5000 control system will deactivate the call for heat. The boiler will go
through a normal shut down sequence with post purge cycling of the blower and pump. The
call for heat will not be activated again until the temperature sensed at the Outlet Probe
drops below Automatic High Limit set point minus the Automatic High Limit Differential
setting.
IE: water temperature at the outlet of the boiler reaches 210°F, with an Automatic High Limit
setting of 210°F the boiler shuts down. With an Automatic High Limit Differential setting of
20°F the call for heat will not be activated again until the outlet temperature drops to 190°F.
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BOILER CONTROLS (CONT)
EXTERNAL CONTROLS
Enable/Disable Control Circuit
The Enable/Disable Control Circuit is always active. This control circuit must be “closed” to
enable boiler operation. Two wires located in the junction box on the back of the boiler are
provided for this circuit. From the factory these two wires are wire nutted together.
If some form of external control is desired/required, that external control’s “dry” (no power or
load) switch contacts would be connected to this circuit through field supplied wiring run in
dedicated conduit (page 68). The external control can then enable/disable the boiler by
closing or opening its contacts. When the external control opens it’s contacts the boiler is
disabled and goes through a normal shut down sequence; the gas valve (s) close, blower (s)
run for a programmed post purge period, and the boiler goes into standby mode.
External controls, using the
provided Enable/Disable circuit,
can be used in place of the
Operating Set Point as a Primary
System Control and to provide
additional supervisory control such
as night set back, freeze
protection, and outdoor air reset
control.
MCB CIRCUIT BOARD
If no external control is used/connected to this circuit, the two wires must be wire nutted
together to enable boiler operation. System temperature and boiler operation is then
controlled solely by the EMC 5000 Operating Set Point working with the Controlling Probe
(page 63).
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BOILER CONTROLS (CONT)
EXTERNAL CONTROLS (CONT)
Types of External Controls
An External Control can be a Primary System Control or a Supervisory Control or both. The
contacts. Field control wiring must be installed in a dedicated conduit.
When an external control is used as a Primary System Control the Operating Set Point on
the boiler (pages 8 and 63) must be set at least 20°F higher than the external control’s
temperature setting. This prevents the boiler’s Operating Set Point from cycling the boiler off
before the Primary System Control’s Set Point has been reached (short-cycling the boiler).
Aquastat is one form of an external Primary System Control. A simple mechanical
thermostat installed in a storage tank (domestic water) or in a T fitting on a heating loop
(heating system).
EMS (energy management system) is another type of external Primary System Control.
EMS controls can also provide supervisory enable/disable functionality. System temperature
can be controlled by an EMS control equipped with it’s own Tank /Loop temperature sensor.
Supervisory functionality such as occupied/unoccupied and/or freeze protection can be
programmed in EMS systems to enable or disable boiler operation based on temperature,
time, and event programing.
Outdoor Air Reset Control is another functionality EMS controllers often provide. The EMS
system will have an outdoor temperature sensor or external load sensor in addition to a
system temperature sensor. Typical Hydronic Reset Control operation; as the outdoor
temperature drops, the system set point is automatically raised to provide more heating
capacity as the load increases. As the outdoor temperature rises, the system set point is
automatically lowered to provide energy savings during low load conditions.
Boiler Sequencing Controllers are another type of external control that can sequence
multiple boilers on and off as the system load fluctuates using their own Tank or Loop
temperature sensing device. These types of controls offer lead/lag operation of multiple
boilers to provide equal run time rotation (also available in many EMS controllers) which
evens the wear on all boilers over time. Many boiler sequence controllers are also available
with Outdoor Air Reset Control functionality described above.
Multiple Boiler Control
EMS or Boiler Sequencing Controllers are strongly recommend in multiple boiler
applications.
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OPERATING TEMPERATURES
Temperature rise through the boiler: 20 - 40°F.
The temperature rise (ΔT = Delta T) through the boiler should be maintained between 20 -
40°F while firing at 100%. This flow rate maintains a balance between the harmful effects of
calcium/lime accumulation (low flow rate) and velocity erosion (excessive flow rate). The ΔT
will be lower when the firing rate is less than 100%. Install and throttle a flow control valve in
the outlet line to achieve 20 - 40°F ΔT. DO NOT throttle/adjust the outlet valve unless the
boiler is firing at 100%. Never throttle the inlet valve to the boiler it must remain completely
open at all times.
Minimum Inlet Water Temperature
The inlet water temperature should be a minimum of 120°F during normal operation to
prevent excessive condensate from forming on the heat exchanger. If this minimum inlet
water temperature cannot be maintained the boiler may be undersized. On domestic water
systems DO NOT connect the cold water make-up line to the boiler return (inlet) piping as
this will make maintaining 120°F minimum inlet water temperature more difficult.
Bypass Lines
Bypass lines should be installed on all VF boilers to maintain a minimum inlet water
temperature of 120°F. Bypass lines feed a portion of the outlet water back to the inlet of the
boiler which raises the inlet water temperature and helps prevent condensation from forming
on the heat exchanger. A bypass line with a flow control valve should be installed between
the inlet and outlet lines of the boiler as shown in the illustration below. Bypass lines must be
installed on the “system side” of the boiler circulation pump. The bypass line should be the
same size as the inlet and outlet piping on the boiler. As the boiler reaches operating
temperature throttle the bypass valve to achieve minimum inlet water temperature of 120°F.
VW models are shipped with a factory installed pump. VB models are not. VB models will
require a properly sized field installed pump. Pump must be installed on the inlet side of the
boiler as shown in the illustration below. See footnotes for field installed pumps on page 37.
Domestic Hot Water Piping
Single Temperature
With building recirculation
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TECHNICAL SPECIFICATIONS
Input Power and Circuit Protection:
†
Power Supply Voltage
120 VAC
108 VAC RMS minimum; 132 VAC RMS maximum.
Below 90 VAC will result in lock out. “Low AC Voltage” error
message will be displayed.
Power Supply Requirements
Dedicated 30 amp circuit breaker should be provided.
Voltage across the neutral and ground wires should not
exceed 1 VAC. Dedicated wiring – no shared hot, neutral or
ground wires.
Line Protection (MCB)
Line Output (Pump)
24VAC Power supply
Fused on PDB to 15 amp @ 120 VAC
Fused on PDB to 20 amp @ 120 VAC
100 VA transformer. Below 18 VAC will result in lockout. “Low 24
VAC” error message will be displayed
24VAC Protection
Fused on MCB to 7.5 amp
Input Specifications:
†
Temperature Sensors (Probes)
Outlet, Inlet, and remote Tank/Loop temperature sensors
(probes). (Display accuracy = ± 5.4° F (3°C)
Wiring to Tank/Loop probe must be in dedicated conduit.
Flame Sensors (2)
High and Low fire
5µA (DC micro amps) current typical. Flame detection failure
occurs under 2.5 µA.
Igniter - Silicon Carbide
40 to 70 ohms cold; 3.0 to 4.0 AC amps typical - minimum
proving current of 2.7 AC amps.
AC Line Polarity Sensor
Detects reversed power connections and unconnected ground
leads on power lines referenced to earth ground.
ECO Switch
24 VDC
Normally Closed bimetal thermostat built into outlet probe,
opens on temperature rise @ 244° F ± 5.4° F (118°C ± 3° C)
Enable/Disable Circuit
(Tstat MCB J11 Socket - see
External control wiring must be in dedicated conduit.
This circuit must be closed or the boiler is disabled. This circuit
can be used to enable/disable boiler with a supervisory control
for: night setback, occupied/unoccupied, freeze protection, lead/
lag of multiple boilers.
Application Warning:
This is a switching circuit;
DO NOT apply any external
voltage or connect any load
(IE: relay coil) to this circuit.
This circuit can also be used with a standard aquastat.
If this circuit is used with an external system temperature control
the on board “Oper Setpnt” should be set a minimum of 20°F
higher than external control set point.
When an external control is not used this circuit must be
physically wired together or jumpered to enable operation.
†. Control System Temperature Accuracy: ± 5.4° F (3° C)
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TECHNICAL SPECIFICATIONS
Input Specifications (cont):
†
High Blower Prover Pressure
Normally Open, closes on a pressure fall at -4.8" W.C. ± 0.01
Normally Open, closes on a pressure fall at -0.12" W.C.± 0.01
Normally Closed, opens on a pressure rise at 1.0" W.C. ± 0.1
Switch
Low Blower Prover Pressure
Switch
Blocked Flue (Exhaust)
Pressure Switch
Low Gas Pressure Switch
Normally Open, closes on a pressure rise.
Natural Gas Activation Pressure: 4.0" W.C. ± 0.4
Propane Gas Activation Pressure: 6.4 " W.C. ± 0.5
Water Flow Switch
Normally Open, closes when flow rate exceeds an adjustable
setting (approximately 22 GPM)
Low Water Cut Off Switch
Normally Closed, opens when water level is low
†. Signal level passed through switches: 24 VAC @ >7ma. (Does not require low-level con-
tacts).
†Output Specifications:
High Speed Combustion Blower
Low Speed Combustion Blower
120 VAC, 50/60 Hz, 3A (MCB J15 Socket)
120 VAC, 50/60 Hz, 3A (MCB J15 Socket)
Hot surface igniter - Silicon Carbide 120 VAC, 50/60 Hz must prove 2.7 AC amp minimum.
Gas Valve
Low Water Cut Off
Alarm
24 VAC, 50/60 Hz, Switched 3.5 A
24 VAC, 50/60 Hz, Not Switched, 1 A
24 VAC, 50/60 Hz, Switched 1 A
24 VAC, 50/60 Hz, Switched 1 A
N/A on VF boilers.
Spare
IRI Gas Valve
†
Pump
120 VAC, 50/60 Hz, maximum 20 FLA
†. Field installed boiler circulation pumps (VB models): For outputs up to 120 VAC 5 FLA
For 120 VAC outputs between 5 and 20 FLA, use an external relay/starter and supply 120
VAC pump power through the 20 amp F2 pump fuse on the PDB (page 37). For outputs
above 120 VAC or 20 FLA use an external relay/starter and a separate dedicated power
supply for the pump, use the on board pump relay Com and N.O. contacts to initiate the
field supplied starter/relay coil only.
†. Total currents are limited by the input circuit breaker and fuses on the PDB and MCB. See above
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TECHNICAL SPECIFICATIONS
Ignition Timing:
Pre-Purge - cold start
32 ± 2 seconds
Pre-Purge – not cold start
Shutter Adjust
10 ± 2 seconds
5 minutes
Pre-Circulate
34 to 45 seconds or until flow is proven
18 ± 1 seconds
Igniter proving time
Inter-Purge
15 ± 2 seconds
Post-Purge
25 ± 2 seconds
Pump post-circulate time
Selectable 45, 90, 180 seconds or continuous
Default User Settings/Ranges:
As shipped from the factory the SW1 dip switches (page 30) on the MCB are preset to the
appropriate values according to the system that was ordered, either VW or VB. The user
settings are also preset to the following default values:
VW MODELS
DEFAULT SETTING ADJUSTABLE RANGE
Oper Setpnt (Operating Setpoint)
Stage1 Diff (Operating Setpoint Differential)
Hi Limit (Automatic High Limit Setpoint)
Hi Limit Dif (Automatic High Limit Differential)
145° F (63° C)
3° F (2° C)
70ºF to 190ºF
1° to 50°F
210° F (99° C)
20° F (11° C)
90ºF to 210ºF
1° to 50°F
VB MODELS
DEFAULT SETTING ADJUSTABLE RANGE
Oper Setpnt (Operating Setpoint)
Stage1 Diff (Operating Setpoint Differential)
Hi Limit (Automatic High Limit Setpoint)
Hi Limit Dif (Automatic High Limit Differential)
190° F (88° C)
3° F (2° C)
70ºF to 220ºF
1° to 50°F
230° F (110° C)
20° F (11° C)
90ºF to 235ºF
1° to 50°F
BOTH MODELS
DEFAULT SETTING ADJUSTABLE RANGE
Tempert Units (Temperature Units)
Post Cir Secs (Post Circulation pump delay)
Netwrk Address (Network Address)
°F
°F or °C
45 seconds
0 non-valid address
45, 90, 180, Continuous
0 or 1 to 31 valid address
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GLOSSARY
ΔT:Symbol used for “Delta T.” Engineering term to convey “temperature rise” or “difference”
through a boiler or heat exchanger. IE: If the inlet water temperature to a boiler is 120°F and
the outlet water temperature leaving the boiler is 140°F; the temperature rise through the
boiler is 20°F, which is commonly expressed as “20° ΔT.”
Blocked Flue Switch - Blocked Exhaust Switch: Two terms describing the same device.
This is a normally closed air pressure switch that opens on a rise in pressure. Used to detect
a blocked exhaust vent. Connects to J5 socket on the MCB, see page 28.
EJB - External Junction Box: Abbreviation used in wiring/connection diagrams. This is a
wiring box mounted outside of boiler. Connections for 120 VAC supply power, external
thermostat/enable/disable control, and remote Tank/Loop temperature probe are made here.
Hall Effect Sensor: A small sensor mounted in the end cap of the blower motor that
converts blower rotation motion into electrical data that is interpreted by the MCB as actual
rpm data.
High Blower Prover Switch: This is a normally open air pressure switch that closes on a
fall in pressure. It is used to detect that the blower is running at the proper higher speed
used during the pre/post purge operational states. Connects to J17 socket on the MCB, see
Low Blower Prover Switch: This is a normally open air pressure switch that closes on a
fall in pressure. It is used to detect that the blower is running during the heating cycle.
Connects to J17 socket on MCB, see pages 24 and 28.
Low Gas Pressure Switch: This is a normally open pressure switch that closes on a rise in
pressure used to detect supply gas pressure is maintained above minimum requirement.
This switch is standard on VF boilers. Connects to J5 socket on MCB, see page 28. The
pressure switch is used to activate and monitor this circuit.
Low Water Cut Off:This is an electronic continuity based control used to prove water level
in the boiler. This is an optional component usually used to satisfy local code requirements.
MCB - Modulation Control Board :This is the main control circuit board for the EMC 5000.
It controls all centralized functions including temperature control, diagnostics, automatic high
limit etc. It sends information to/from UIB and implements the External Network Interface.
See pages 25 and 26.
PDB - Power Distribution Board:Distributes 120VAC and 24VAC power and supplies
fusing to the MCB circuit board and the pump. Contains a test circuit for determining if line
power is properly applied to the system. See page 35.
TSB - Touch Sensor Board:Circuit board containing touch switch pads (user input buttons)
which is a portion of the UIM assembly.
UIB - User Interface Board:Part of the UIM assembly. Communicates with the MCB.
Controls the operation of the LCD. Receives inputs from the touch switches. Activates the
LED's according to commands sent from the MCB.
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GLOSSARY (CONT)
UIM - User Interface Module:Plastic housing that holds the UIB, LCD, and TSB. Provides
means to both send and receive information from the user. Only one cable to the MCB is
required for communications and power. Operates on 5 VDC only. Connects to internal
communications port on MCB, see page 32.
VFD:Variable Frequency Drive. A component that varies the power supply’s frequency.
Water Flow Switch:This is a normally open paddle type switch mounted in the outlet of the
boiler that closes when water flow through the boiler exceeds the minimum flow “make point”
of the switch. This switch does not prove required flow rate only that water present and that
it is flowing. Connects to J5 socket on MCB, see page 28.
Author:
Paul Stewart
Training Specialist
A. O. Smith Water Products Company ©2007
Ashland City, Tennessee
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COMMERCIAL
Visit the “Information Central” link of
available Service Handbooks.
For additional information contact:
A. O. Smith Water Products Company
A Division of A. O. Smith Corporation
500 Tennessee Waltz Parkway
Ashland City, TN 37015
1-800-527-1953
©2007 A. O. Smith Corporation
Part No. TC-099
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