A.O. Smith BTH 120 How to use

COMMERCIAL GAS HIGH EFFICIENCY WATER HEATERS
Service Handbook
PRINTED IN THE U.S.A. 0914 329297-000
500 Tennessee Waltz Parkway
Ashland City, TN 37015
FOR MODELS:
BTH 120 THRU 500
SERIES 200/201
INSTALLATION CONSIDERATIONS - PRE SERVICE CHECKS -
CONSTRUCTION - OPERATION & SERVICE - TROUBLESHOOTING
SERVICING SHOULD ONLY BE PERFORMED BY A QUALIFIED SERVICE TECHNICIAN

1
TABLE OF CONTENTS
INTRODUCTION.......................................................................................2
Qualications ........................................................................................2
Service Warning ...................................................................................2
Service Reminder .................................................................................2
Tools Recommended............................................................................2
INSTALLATION CONSIDERATIONS ........................................................3
Instruction Manual ................................................................................3
Closed Water Systems .........................................................................3
Thermal Expansion...............................................................................3
Air Requirements..................................................................................3
Contaminated Air ..................................................................................3
FEATURES AND COMPONENTS ............................................................4
Combustion Blower & Burner Assembly...............................................4
Top View: BTH 120 - 250 Models .........................................................5
Top View: BTH 300 - 500 Models .........................................................5
SEQUENCE OF OPERATION FLOW CHART..........................................6
FAULT AND WARNING CODES ...............................................................7
1. Low Gas Pressure ............................................................................7
High Altitude .................................................................................7
Gas Pressure Specications ........................................................7
Gas Supply Systems....................................................................8
Supply Gas Regulator ..................................................................8
Gas Pressure Test........................................................................ 9
2. Blower Operation Error...................................................................11
Pressure Switches .......................................................................11
Control System Monitoring...........................................................11
Pressure Switch Tests..................................................................12
Continuity Test During Standby....................................................12
Continuity Test During Operation .................................................13
Pressure Test During Operation...................................................13
Pressure Switch Settings .............................................................14
3. Blower Prover Open .......................................................................16
4. Ignition Failure ................................................................................17
5. Blocked Air Intake...........................................................................20
6. Blocked Exhaust.............................................................................21
7. Flame Sensor Error ........................................................................22
Gas Control Valve ........................................................................22
Gas Control Valve Voltage Tests ..................................................23
Flame Sensor...............................................................................24
Flame Sensing Operation ............................................................24
Flame Sensing Current Test......................................................... 25
8. No Water.........................................................................................26
Powered Anodes .......................................................................... 26
Power anode cleaning and removal............................................. 26
9. Anode Shorted................................................................................28
10. Temperature Probe Short (Upper or Lower) .................................29
Temperature Probe ......................................................................29
Temperature Sensor Test .............................................................30
ECO Continuity Test.....................................................................30
11. Temperature Probe Open (Upper or Lower) .................................32
VENTING.................................................................................................33
General Venting Information ........................................................33
INTAKE AIR CONNECTION....................................................................34
ELECTRICAL CONNECTIONS – ALL MODELS.....................................35
Electrical Wiring..................................................................................35
Power Supply Connections ................................................................35
Enable/ Disable Switch.......................................................................36
Power Supply Test..............................................................................36
INSTALLATION CHECK LIST .................................................................37
OPERATION AND SERVICE ..................................................................38
How It Works ......................................................................................38
Combustion Blower ............................................................................38
Modulation ..........................................................................................38
BLOWER SPEED CONTROL – BTH 300 THRU 500 .............................40
VARIABLE FREQUENCY DRIVE – BTH 300 THRU 500........................41
BURNER ASSEMBLY .............................................................................42
COMBUSTION BLOWER AND BURNER REMOVAL/ INSTALLATION
INSTRUCTIONS......................................................................................43
SPARK IGNITER .....................................................................................44
GAS ORIFICE - BTH 300 THRU 500 MODELS......................................46
Orice Chart - BTH 300 thru 500 Models ...........................................46
CCB (CENTRAL CONTROL BOARD).....................................................47
CCB Cover Removal And Replacement.............................................48
CCB Circuit Board Layout ..................................................................49
CCB Connection Identication............................................................50
WIRING DIAGRAM – BTH 120 THRU 250 .............................................52
WIRING DIAGRAM – BTH 300 THRU 500 .............................................53
CONTROL OVERVIEW...........................................................................54

2Servicing should only be performed by a Qualied Service Technician
INTRODUCTION
This Service Handbook covers the water heater Model and Series numbers listed on the front cover only.
The instructions and illustrations contained in this service handbook will provide you with troubleshooting
procedures to verify proper operation and diagnose and repair common service problems.
QUALIFICATIONS
QUALIFIED INSTALLER OR SERVICE AGENCY
Installation and service of this water heater requires ability equivalent to that of a Qualied Agency (as dened
by ANSI below) in the eld involved. Installation skills such as plumbing, air supply, venting, gas supply and
electrical supply are required in addition to electrical testing skills when performing service.
ANSI Z223.1 2006 Sec. 3.3.83: “Qualied Agency” - “Any individual, rm, 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 qualied (as dened by ANSI above) and licensed or certied as required by the authority
having jurisdiction to perform a given task do not attempt to perform any of the procedures described in this
service handbook. If you do not understand the instructions given in this service handbook do not attempt to
perform any procedures outlined in this service handbook.
SERVICE REMINDER
When performing any troubleshooting step outlined in this service handbook 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.
TOOLS RECOMMENDED
• Instruction Manual that came with the water heater.
• All tools common to installation and service of commercial water heaters such as hand tools, screwdrivers,
pipe wrenches etc.
• Long (8-10”) T handle 1/8 inch hex (allen key) wrench for Combustion Blower removal and installation.
• Hex (Allen) wrench sizes: 5/32”, 1/8”, 1/4” and 5/16" - for Burner, and 24 Volt Gas Control Valve removal
and installation.
• Digital manometer: Range -20.00 to +20.00" W.C. Resolution - 0.01" W.C. Required to test pressure
switch performance and supply gas pressure.
• Digital Multi Meter; capable of measuring:
• AC/DC Voltage.
• Ohms.
• DC micro amps (μA) - ame sensing current, see "Flame Sensing Current Test" on Page 25.

3
Servicing should only be performed by a Qualied Service Technician
INSTALLATION CONSIDERATIONS
This section of the Service Handbook covers some of the critical installation requirements that, when
overlooked, often result in operational problems, down time and needless parts replacement. Costs to
correct installation errors are not covered under the limited warranty. Ensure all installation requirements and
instructions contained in the Instruction Manual that came with the water heater have been followed prior to
performing any service procedures.
INSTRUCTION MANUAL
Have a copy of the Instruction Manual that came with the water heater on hand for the model and series
number being serviced. Installation information given in this Service Handbook is not a complete installation
instruction. Installation information given in this Service Handbook has a limited focus as it applies to servicing
the water heater. This Service Handbook does not replace or supersede the Instruction Manual that came
with the water heater. Always refer to the Instruction Manual for complete installation instructions. If the
Instruction Manual is not on hand, copies can be obtained from the manufacturers web site or by calling the
technical support phone number shown on the back cover of this Service Handbook.
CLOSED WATER SYSTEMS
Water supply systems may, because of code requirements or such conditions as high line pressure, among
others, have installed devices such as pressure reducing valves, check valves, and back ow preventers.
Devices such as these cause the water system to be a closed system.
THERMAL EXPANSION
As water is heated, it expands (thermal expansion). In a closed system the volume of water will grow when
it is heated. As the volume of water grows there will be a corresponding increase in water pressure due to
thermal expansion. Thermal expansion can cause premature tank failure (leakage). This type of failure is not
covered under the limited warranty. Thermal expansion can also cause intermittent Temperature-Pressure
Relief Valve operation: water discharged from the valve due to excessive pressure build up. This condition
is not covered under the limited warranty. The Temperature-Pressure Relief Valve is not intended for the
constant relief of thermal expansion.
A properly sized thermal expansion tank must be installed on all closed systems to control the harmful effects
of thermal expansion. Contact a local plumbing service agency to have a thermal expansion tank installed.
AIR REQUIREMENTS
Carefully review the requirements for combustion and ventilation air in the Instruction Manual that came
with the water heater. Failure to meet these requirements when the water heater is installed or overlooking
their importance when servicing the water heater often results in needless down time, unnecessary parts
replacement, and customer dissatisfaction.
An inadequate supply of air for combustion and ventilation often causes operational problems. A lack of
combustion and ventilation air can create a negative ambient air pressure in the installed space which can
lead to improper combustion and operational problems with pressure switches.
CONTAMINATED AIR
Combustion air that is contaminated can greatly diminish the life span of the water heater and water heater
components such as Igniters and Burners. Propellants of aerosol sprays, beauty shop supplies, water softener
chemicals and chemicals used in dry cleaning processes that are present in the combustion, ventilation or
ambient air can cause such damage.
Vapors from volatile compounds such as solvents, cleaners, chlorine based chemicals and refrigerants in
addition to being highly ammable in many cases, can also react to form highly corrosive substances such
as acids inside the combustion chamber. The results can be hazardous and cause product failure.
If the water heater is installed in beauty shops, barber shops or laundries with dry cleaning equipment, it is
imperative the water heater be installed in a Direct Vent conguration so that air for combustion is derived
directly from the outdoor atmosphere through a sealed intake air pipe. See the venting installation section
in the Instruction Manual that came with the water heater for more information on Direct Vent installations.

4Servicing should only be performed by a Qualied Service Technician
FEATURES AND COMPONENTS
COMBUSTION BLOWER & BURNER ASSEMBLY
Air Intake Assembly
Low Gas Pressure Switch
Combustion Blower Gas Control Valve/
Venturi Assembly
Spark Igniter Ground Wire
Blower Gasket
Spark Igniter
Flame Sensor
Spark Igniter Gasket
Blower Adapter
Burner Gasket
Burner
Ignition Cable
Sight Glass
Sight Glass Gasket
Burner Plate
Burner Plate Gasket
(150-250 Models Only)
SS Heat Shield
(150-250 Models Only)
Flue Ring Gasket
Ignition Cable
A
ir Intake Assembly
Spark Igniter
Flame Sensor
Combustion Blower
Blower Gasket
Sight Glass
Sight Glass Gasket
Burner
Venturi
Burner Plate
Burner Plate Gasket
SS Heat Shield Pyrolite Insulation Disk
SS Bolts & Washers
Spark Igniter Gasket
Insulation Spacers
BTH 120 - 250 Models
BTH 300 - 500 Models

5
Servicing should only be performed by a Qualied Service Technician
TOP VIEW: BTH 120 - 250 MODELS
Blocked Exhaust
Switch
Powered Anode
(Short, BTH 150-250
Models Only)
Powered Anode
(Long)
Blower Prover
Switch
Upper Temperature Probe
Blocked Air
Intake Switch
CCB Assembly
Burner/ Blower
Assembly
Exhaust/
Condensate Elbow
Ignition Control
Junction Box
Assembly
UIM
(Label Color: Green)
(Label Color: Blue)
(Label Color: Red)
Water Outlet
1 - 1/2” NPT
TOP VIEW: BTH 300 - 500 MODELS
Water Outlet
1 - 1/2” NPT
CCB
Assembly
Powered Anode
(Short)
Ignition Control
Junction Box
Assembly
Burner/ Blower
Assembly
Blower Prover Switch
(Label Color: Green)
Blocked Air
Intake Switch
(Label Color: Blue)
Blocked Exhaust
Switch
(Label Color: Red)
Powered Anode
(Long)
Upper Temperature
Probe
Exhaust/
Condensate Elbow
VFD
UIM

6Servicing should only be performed by a Qualied Service Technician
SEQUENCE OF OPERATION FLOW CHART
YESNO
Water is heated to
set point
24 Volt Gas Valve
de-energized
Inter-Purge cycle
24 Volt Gas Valve
de-energized
Retries up to
two more times
Post-Purge cycle
Control System
Locks Out
“Ignition Failure”
Fault Msg is
displayed
Water heater goes
into standby mode
If tank temperature drops below Operating Set Point minus
Differential setting a heating cycle is activated
Blower Prover switch contacts confirmed
closed by control system
Spark Ignition Control is energized
24 Volt Gas Valve is energized
gas flows to Main Burner
Is flame sensed at the Main Burner ?
(control system monitors the flame sensor)
Control System Locks Out
Displays Fault Msg
NO
Control System Locks Out
“Blower Prover Open”
Fault Msg is displayed
NO
Combustion Blower is energized
Pre-Purge cycle
Control System performs diagnostic checks
Normal State of all pressure switches and ECO are checked
Blower Prover pressure switch verified open
All other pressure switches and ECO are verified closed
Sequence is shown with Enable/Disable Switch in the Enable position

7
Servicing should only be performed by a Qualied Service Technician
FAULT AND WARNING CODES
1. LOW GAS PRESSURE
CHANGE BACK ADVANCED
Low Gas Pressure
Fault occurred 2 mins ago
Gas pressure is below minimum
required to operate this unit. Check
gas pressure and switch.
Call a service professional:
Your Company Name Here
(press [DOWN] for more....)
CAUSES:
• The Low Gas Pressure Switch is detecting gas
pressure below the value required for proper
operation.
CHECK/ REPAIR:
• Check Low Gas Pressure Switch wiring and connectors. Connectors must be clean and tight.
• Determine proper minimum pressure from rating plate.
• Use manometer or gauge to check gas pressure during all stages of operation with all other
appliances ring.
• If pressure falls below rating plate requirement, supply pipe may be too small for application or supply
regulator may be operating improperly. See Gas Pressure section below.
HIGH ALTITUDE
The water heaters covered in this Service Handbook are certied for elevations up to 10,100 feet (3,078
meters). For higher elevations call the toll free support phone number shown on the back cover of this
Service Handbook for technical assistance.
GAS PRESSURE SPECIFICATIONS
MODELS Natural
120-250
Propane
120-250
Natural
300/400/500
Propane
300/400/500
Maximum Gas Supply
Pressure
14.0" WC
(3.49 kPa)
14.0" WC
(3.49 kPa)
14.0" WC
(3.49 kPa)
14.0" WC
(3.49 kPa)
Nominal Gas Supply
Pressure
7.0" WC
(1.74 kPa)
11.0" WC
(2.74 kPa)
7.0" WC
(1.74 kPa)
11.0" WC
(2.74 kPa)
Minimum Gas Supply
Pressure (Low Gas
Press. Switch Setting)
4.4" WC
(1.10 kPa)
8.5" WC
(2.12 kPa)
4.8" WC
(1.19 kPa)
8.5" WC
(2.12 kPa)
*Manifold Pressure 0" WC
(0 kPa)
0" WC
(0 kPa)
0" WC
(0 kPa)
0" WC
(0 kPa)
*The Manifold Pressure is the factory setting and is not adjustable. A negative pressure will be seen with just the blower running without
the Gas Control Valve open.

8Servicing should only be performed by a Qualied Service Technician
GAS SUPPLY SYSTEMS
Low pressure building gas supply systems are defined as those systems that cannot under any circumstances
exceed 14” W.C. (1/2 PSI Gauge). These systems do not require additional pressure regulation. Measurements
should be taken to insure that gas pressures are stable and fall within the requirements stated on the water
heater rating plate. Readings should be taken with all gas burning equipment off (static pressure) and with all
gas burning equipment running at maximum rate (dynamic pressure). The gas pressure supplied to this unit
must be below the maximum and above the minimum pressures listed on the unit's rating plate and must not
drop more than 1.5" W.C. when unit is operating at the high fire rate along with all additional gas appliances
operating on the same gas supply system. Pressure drops that exceed 1.5” W.C. may cause rough starting,
noisy combustion or nuisance outages. Increases or spikes in static pressure during off cycles may cause
failure to ignite or in severe cases damage to appliance Gas Control Valves. If your low pressure system does
NOT meet these requirements, do not operate this unit. The installer is responsible for ensuring that the gas
supply pressures meet the requirements described above and listed in the table. Failure of the gas supply to
meet these requirements is not covered under the warranty.
High Pressure building supply systems use pressures that exceed 14” W.C. (1/2 PSI Gauge). These systems
must use field supplied regulators to lower the gas pressure to less than 14” W.C. (1/2 PSI Gauge). Water
heaters require gas regulators that are properly sized for the water heater's BTU input and capable of
delivering the rating plate specified pressures. Gas supply systems where pressure exceeds 5 PSI often
require multiple regulators to achieve desired pressures. Systems in excess of 5 PSI building pressure
should be designed by gas delivery professionals for best performance. Water heaters connected to gas
supply systems that exceed 14” W.C. (1/2 PSI Gauge) must be equipped with a gas supply regulator capable
of preventing the gas supplied to the unit from exceeding 14" W.C. under any upstream supply pressure or
condition.
For Natural Gas BTH 120-250 models require a minimum gas supply pressure of 4.4" W.C., BTH 300-500
models require 4.8" W.C., and all models require 8.5" W.C. for propane gas. The minimum supply pressure is
measured while gas is flowing (dynamic pressure) along with all additional gas appliances operating on the
same gas supply system. The supply pressure should be measured with all gas fired appliances connected
to the common main firing at full capacity. If the supply pressure drops more than 1.5” W.C. (0.05 PSI Gauge)
when the water heater is operating at high fire rate, then the supply gas system including the gas line and/or
the gas regulator may be restricted or undersized. See Supply Gas Regulator section below. The Gas Control
Valve on all models has a maximum gas supply pressure limit of 14” W.C. (1/2 PSI Gauge). The maximum
supply pressure is measured while gas is not flowing (static pressure) to any appliances connected to the
same gas supply system.
SUPPLY GAS REGULATOR
The maximum allowable gas supply pressure for the water heaters covered in this Service Handbook is
14.0 inches W.C. (3.49 kPa) for natural and propane gas. Install a positive lock-up gas pressure regulator in
the gas supply line if inlet gas pressure can exceed these pressures at any time. Regulators must be sized
to equal or exceed the BTU input requirements of the heater model and installed, used, and maintained
according to manufacturer's specifications.
If a positive lock-up regulator is installed follow these instructions:
1. Positive lock-up gas pressure regulators must be rated at or above the input Btu/hr rating of all the
gas appliances that are on the same system as the water heater.
2. Supply gas regulators shall have inlet and outlet connections not less than the minimum supply gas
line size for the water heater they supply.
3. Refer to the gas pressure regulator's manufacturers installation instructions to determine its installation
location requirements.
4. After installing the positive lock-up gas pressure regulator(s) an initial nominal supply pressure
setting of 7.0” W.C. (1.74 kPA) for Natural Gas and 11.0" W.C. (2.74 kPA) for Propane (LP) while the
water heater is operating is recommended and will generally provide good water heater operation.
Adjustments to the regulator maybe required to maintain a steady gas supply pressure.

9
Servicing should only be performed by a Qualied Service Technician
5. When installing multiple water heaters in the same gas supply system it is recommended that individual
positive lock-up gas pressure regulators be installed at each unit from the supply gas connection on
the water heater.
Service notes:
• The manifold gas pressure is factory set and cannot be adjusted in the eld.
• There are two pressure test ports on the water heater’s 24 Volt Gas Control Valve, see Figure below. The
Gas Control Valve outlet pressure test port is closest to the Combustion Blower, see Figure below. There
is a needle valve in each test port that is opened/closed with a small slotted screwdriver. Turn the needle
valve counter-clockwise to open the test port valve and clockwise to close it.
• The hose from the Gas Control Valve supply gas pressure test port should be connected to the positive
pressure port of a manometer; the other pressure port of a manometer should be left open.
• There is usually a drop in supply gas pressure noticed when the water heater’s 24 Volt Gas Control
Valve opens during ignition. The manifold pressure will go from a negative value (during pre-purge) to
near zero when gas is owing.
• A sustained drop in supply gas pressure of 1.5" W.C. or more during ignition may indicate the supply
gas line is undersized. If the water heater is experiencing a sustained drop in supply gas pressure of
1.5" W.C. or more and the water heater is experiencing repeated Ignition Failure fault conditions,
intermittent loss of ame or rough starting ensure the supply gas line is sized in accordance with the
current edition of National Fuel Gas Code (ANSI Z223.1/NFPA 54) or the Natural Gas and Propane
Installation Code (CAN/CSA B149.1).
GAS PRESSURE TEST
1. Use the Enable/Disable Switch located on the front of the water heater and put the unit in disabled mode.
2. Turn off the supply gas to the water heater at the main gas shutoff valve serving the water heater.
3. Open the Supply Gas Pressure Test Port on the Gas Control Valve, see Figures below. Turn the needle
valve slotted head 1/2 to 1 full turn counter-clockwise with a small slotted screwdriver to open the valve.
4. A digital manometer (see "Tools Recommended" on Page 2) is needed to check supply gas pressure. To
monitor supply gas pressure, attach a sensing tube from Gas Control Valve Supply Gas Pressure Test
Port to one digital manometer.
5. Open the main gas shutoff valve serving the water heater.
6. The manometer connected to the Supply Gas Pressure Test Port should read the "static" (gas is not
owing) supply gas pressure.
MANIFOLD GAS
PRESSURE
TEST PORT
SUPPLY GAS
PRESSURE
TEST PORT
VENTURI
COMBUSTION
AIR INLET
GAS CONTRO
L
VALVE
COMBUSTION
BLOWER
CONNECTION
MANIFOLD GAS
PRESSURE
TEST PORT
AIR INTAKE PRESSURE
REFERENCE CONNECTION
SUPPLY GAS
PRESSURE
TEST PORT
BTH 120 - 250 Models BTH 300 - 500 Models

10 Servicing should only be performed by a Qualied Service Technician
7. Record the supply gas pressure.
8. Use the Enable/Disable Switch located on the front of the water heater and put the unit in enabled mode.
The settings on the water heater may need to be changed to cause the water heater to operate. Note the
Operating Temperature Set Point so that the unit can be returned to this setting at the conclusion of this
test procedure. Increase the Operating Set Point if necessary to cause the water heater to have a call
for heat.
9. Record the supply gas pressure when the Ignition Activation operating state begins; when the animated
Gas Control valve Status Icon appears on the UIM Screen.
• The supply gas pressure will typically drop as the Gas Control Valve opens.
Putting The Water Heater Back In Service
10. Lower the Operating Set Point to end the heating cycle. Use the Enable/Disable Switch located on the
front of the water heater and put the unit in disabled mode.
11. Close the main gas shutoff valve serving the water heater.
12. Disconnect manometer sensing tube.
13. Close the Supply Gas Pressure Test Port on the Gas Control Valve. Turn the needle valve slotted head
clockwise until tight.
14. Open the main gas shutoff valve serving the water heater and check for leaks at the Gas Control Valve
test ports.
15. Use the Enable/Disable Switch located on the front of the water heater and put the unit in enabled mode.
Increase the Operating Set Point if necessary to cause the water heater to have a call for heat.
16. Run the water heater through a complete heating cycle before leaving to ensure it is operating properly.
Return the Operating Set Point Temperature to its original setting. Draw water from the nearest faucet to
make sure that the water temperature does not pose a scalding hazard due to the increased temperature
set points that were used in this procedure.

11
Servicing should only be performed by a Qualied Service Technician
2. BLOWER OPERATION ERROR
CHANGE BACK ADVANCED
Blower Operation Error
Fault occurred 2 mins ago
Blower Prover Pressure Switch may
have failed closed. Check/replace
pressure switch.
Call a service professional:
Your Company Name Here
(press [DOWN] for more....)
CAUSES:
• The Blower Prover Pressure Switch was detected
closed before blower operation.
CHECK/ REPAIR:
• This switch is normally open and is used to prove blower operation. This switch should be open when
blower is not running. Check continuity of pressure switch. See "Pressure Switch Tests" section on Page
12 thru Page 15.
PRESSURE SWITCHES
The water heaters covered in this Service Handbook are factory equipped with four pressure switches
(including the Low Gas Pressure Switch). "Top View" Figures on Page 5 shows the location of the three
pressures switches on top of the water heater. This section of the Service Handbook covers pressure switch
construction, operation and the test procedures used to diagnose operational problems associated with
pressure switches. The pressure switches are identied with colored labels; Blower Prover Switch is Green,
Blocked Air Intake Switch is Blue, & Blocked Exhaust Switch is Red.
CONTROL SYSTEM MONITORING
The Control System monitors the “state” of the pressure switch contacts individually through three separate
circuits, see "Wiring Diagrams" on Page 52 & Page 53. The state of the switch refers to whether the switch
contacts are open or closed.
At the beginning of a heating sequence, before the Combustion Blower is energized, the Control System
enters the Input Verication operating state. During Input Verication the Control System monitors all three
pressure switches to ensure their contacts are in the correct "normal" state. The Blower Prover switch contacts
must be open. The contacts for the Blocked Air Intake Switch and Blocked Exhaust Switches must be closed.
If any of the pressure switch contacts are not in their correct normal state during Input Verication the Control
System will lock out and display a fault message on the UIM indicating which pressure switch caused the
fault condition. If all system checks pass during the Input Verication operating state the Control System
enters the Pre-Purge operating state and energizes the Combustion Blower. The Heater Status Screen and
functionality of the water heater can be used for troubleshooting.
After the Combustion Blower is energized the Control System must conrm the Blower Prover switch contacts
have closed, see the "Sequence Of Operation Flow Chart" on Page 6. The Blocked Air Intake Switch and
Blocked Exhaust Switch contacts must remain closed at all times.
If the Blower Prover switch contacts are closed during the Input Verication the Control System will lock out
and display Blower Prover Failure on the UIM. If the Blower Prover switch contacts do not close after the
Combustion Blower is energized the Control System will lock out and display Blower Prover Open on the
UIM. If either the Blocked Air Intake Switch or Blocked Exhaust Switch contacts open at any time during a
heating sequence the Control System will lock out and display Blocked Air Intake or Blocked Exhaust
on the UIM. Wires connecting to air switches are labeled near the terminals. Ensure no mis-wiring among
terminals.

12 Servicing should only be performed by a Qualied Service Technician
PRESSURE SWITCH TESTS
Complete pressure switch testing involves three procedures:
• "Continuity Test During Standby" below.
• "Continuity Test During Operation" on Page 13.
• "Pressure Test During Operation" on Page 13 thru Page 15.
CONTINUITY TEST DURING STANDBY
This test is performed while the Combustion Blower is not running with the water heater turned off. This is a
"normal state" continuity test of the contacts inside each pressure switch. Disconnect the two wires to each
pressure switch for this test.
1. If the water heater is in a heating cycle lower the Operating Set Point to end the cycle.
2. Turn off the circuit breaker that serves the water heater or unplug the water heater appliance cord from
the 120 VAC wall outlet if so equipped.
3. Disconnect both wires at each pressure switch.
4. Using an ohm meter set for continuity testing, check for continuity between the two wiring terminals at
each pressure switch, see Figure below.
5. After completing the tests, conrm that all of the pressure switch wires are reconnected including the
replacement part if necessary based on the Results/Actions listed below. Restore the electrical power
to the water heater. Reset the Operating Temperature Set Point if it was adjusted. Run the water heater
through a complete heating cycle before leaving to ensure it is operating properly.
Results/Actions
Passed Results: If the Blower Prover switch contacts are open during this test (innite ohms/open circuit)
the Blower Prover switch has passed the test. If the Blocked Air Intake Switch and Blocked Exhaust Switch
contacts are closed during this test, (zero ohms/direct short), these switches have passed the test.
Failed Results: If the Blower Prover switch contacts are closed (zero ohms/direct short/beep) during this
test, the Blower Prover switch must be replaced. If the Blocked Air Intake Switch and/or Blocked Exhaust
switch contacts are open during this test, the switch(s) must be replaced.
Proceed to the Continuity Test During Operation on next page.

13
Servicing should only be performed by a Qualied Service Technician
CONTINUITY TEST DURING OPERATION
This test is performed while the Combustion Blower is running at high speed during the Pre-Purge operating
state. The test is performed on all three pressure switches. Disconnect the two wires to each pressure switch
- one at a time, check continuity and then reconnect the wires to that switch before testing the next switch.
1. Turn off the circuit breaker that serves the water heater or unplug the water heater appliance cord from
the 120 VAC wall outlet if so equipped.
2. Ensure the sensing tubes from each pressure switch are connected to the correct sensing port on the
water heater.
3. Disconnect the two wires for the switch to be tested.
4. When testing the Blocked Exhaust and Blocked Air Intake Switches, connect a jumper wire between the
two wires disconnected. This will temporarily prevent the Control System from locking out during the test.
When the test for these two switches is complete, remove the jumper wire and reconnect the pressure
switch wiring before moving on to the next switch.
5. Restore power to the water heater, if the water heater does not begin a heating cycle raise the Operating
Set Point to activate a heating cycle.
6. Using an ohm meter set for continuity testing, check for continuity between the two wiring terminals at the
pressure switch after the Combustion Blower has started and ramped up to high speed during the Pre-
Purge operating state. Record the results; open or closed.
7. Repeat this procedure for each pressure switch.
8. Turn off the circuit breaker that serves the water heater or unplug the water heater appliance cord from
the 120 VAC wall outlet if so equipped. Conrm that all of the pressure switch wires are reconnected
including the replacement part if necessary based on the Results/Actions listed below. Restore the
electrical power to the water heater. Reset the Operating Temperature Set Point if it was adjusted. Run
the water heater through a complete heating cycle before leaving to ensure it is operating properly. Draw
water from the nearest faucet to make sure that the water temperature does not pose a scalding hazard
due to the increased temperature set points that were used in this procedure.
Results/Actions
Passed Results: If the Blower Prover switch contacts activate/close (zero ohms/direct short) during this test
the switch has passed this test. If the Blocked Air Intake Switch and Blocked Exhaust Switch contacts remain
closed during this test, (zero ohms/direct short), these switches have passed the test.
Failed Results: If the Blower Prover switch contacts do not close during this test, the Pressure Test During
Operation below must be performed to determine if the switch is defective. If the Blocked Air Intake Switch
or Blocked Exhaust switch contacts open during this test, the Pressure Test During Operation below must be
performed to determine if the switch(s) is defective. Proceed to the Pressure Test During Operation below.
PRESSURE TEST DURING OPERATION
This test will be performed while the Combustion Blower is running at high speed during the Pre-Purge
operating state. This test is necessary when a pressure switch has passed the "Continuity Test During
Standby" shown on Page 12 but has failed the Continuity Test During Operation above to determine if the
pressure switch is defective or not. This test requires a digital manometer, see "Tools Recommended" on
Page 2.
Service Notes:
• Before performing this test examine the sensing tube connections on the water heater’s sensing ports
and on the pressure switch. Check for wear, cracks, leaks, kinks, or any kind of debris or condensate in
the sensing tubes, repair/replace as necessary.
• To determine if a pressure switch is operating properly you must rst know the "switch action" (whether
the switch activates on a rise or a fall in pressure) and the "activation pressure" the switch will activate
at. Activation pressures are provided in "Pressure Switch Settings" Table on Page 14. Switch actions are

14 Servicing should only be performed by a Qualied Service Technician
as follows:
• Blower Prover Switch - normally open contacts, close on a rise in pressure.
• Blocked Air Intake Switch - normally closed contacts, open on a fall in pressure.
• Blocked Exhaust Switch - normally closed contacts, open on a rise in pressure.
Activation Pressures: Pressure Switch Settings Table below shows the activation pressures and tolerance
for each of the three pressure switches. A + sign before the pressure indicates a positive pressure; above
atmospheric pressure. A - sign before the pressure indicates a negative pressure (in a vacuum); below
atmospheric pressure. Tolerances are specied to allow reasonable leeway for imperfections and inherent
variability without compromising performance.
PRESSURE SWITCH SETTINGS
Models Blower Prover
(+/- 0.05" WC)
Blocked Air
Intake
(+/-0.05" WC)
Blocked Exhaust
(+/-0.05" WC)
Low gas (+/-0.04" WC Nat/
+/-0.05" WC LP)
BTH 120 0.75" W.C. -2.50" W.C. 2.00" W.C. 4.4" W.C. Nat/8.5" W.C. LP
BTH 150-250 1.00" W.C. -2.50" W.C. 2.00" W.C. 4.4" W.C. Nat/8.5" W.C. LP
BTH 300-500 1.00" W.C. -2.30" W.C. 2.00" W.C. 4.8" W.C. Nat/8.5" W.C. LP
Pressure Test:
1. Turn off the circuit breaker that serves the water heater.
2. Disconnect the sensing tube from the switch being tested and add a 3/16" tee to the sensing tube. Add a
short piece of sensing tube connecting the tee and the pressure switch being tested.
3. Connect a digital manometer to the open side of the tee with another piece of sensing tube.
4. Restore power to the water heater, if the water heater does not begin a heating cycle raise the Operating
Set Point to activate a heating cycle.
5. When the Combustion Blower comes on and ramps up to full speed, record the pressure reading.
6. Repeat this test for all three pressure switches as necessary.
7. Compare the pressure readings taken to the activation pressures in the Table above.
8. Ensure all sensing tubes are properly routed and reconnected when testing is complete. The pressure
switches will not operate properly if there are leaks in the tubing. Reset the Operating Temperature Set
Point if it was adjusted. Run the water heater through a complete heating cycle before leaving to ensure
it is operating properly. Draw water from the nearest faucet to make sure that the water temperature does
not pose a scalding hazard due to the increased temperature set points that were used in this procedure.
Results/Actions
Blower Prover: If the pressure reading taken at the Blower Prover switch sensing tube is at or above the
activation pressure shown in the above Table AND the switch contacts did not close during the "Continuity
Test During Operation" shown on Page 13 - the switch is defective and must be replaced. If the pressure
reading taken does not reach or rise above the activation pressure in the above Table the pressure switch IS
NOT defective and should not be replaced - call the toll free phone number on the back cover of this Service
Handbook for further technical assistance. Have the complete Model, Series and Serial number (located on

15
Servicing should only be performed by a Qualied Service Technician
the water heater's rating label) for the water heater being serviced on hand before calling.
Blocked Air Intake: Note these are negative pressures; in a vacuum. If the pressure reading taken at the
Blocked Air Intake Switch sensing tube does not reach or drop lower than the pressure shown in the above
Table AND the switch contacts were opening during the "Continuity Test During Operation" shown on Page
13 - the switch is defective and must be replaced. If the pressure reading taken reaches or drops lower
than the pressure shown in the "Pressure Switch Settings" Table on Page 14 and the switch contacts were
opening during the "Continuity Test During Operation" shown on Page 13 - the switch IS NOT defective and
should not be replaced. Check for restrictions at the intake air connection on the water heater, check for too
small of pipe installed, too many equivalent feet and/or too many elbows in the intake air piping. Check for
low points in the intake air piping where water may be collecting and blocking the intake air piping. Check for
any restrictions or debris at the intake air termination outdoors. Check for condensate/water in the sensing
tube/switch, if the sensing tube has water in it, replace the Blocked Air Intake Switch and the tube. Ensure
the sensing port is oriented properly, see "Intake Air Connection" on Page 34.
Blocked Exhaust: If the pressure reading taken at the Blocked Exhaust switch sensing tube does not
reach or rise above the activation pressure given in the above Table AND the switch contacts were opening
during the "Continuity Test During Operation" on Page 13 - the switch is defective and must be replaced. If
the pressure reading taken reaches or exceeds the pressure shown in the "Pressure Switch Settings" Table
on Page 14 and the switch contacts were opening during the "Continuity Test During Operation" on Page
13 - the switch IS NOT defective and should not be replaced. Ensure the condensate drain connected to the
exhaust elbow on the water heater is not clogged and is draining freely. Check for restrictions or blockage in
the vent pipe, check for too small of pipe installed, too many equivalent feet and/or too many elbows in the
vent piping. Check for low points in the vent pipe where water may be collecting and blocking the vent piping.
Check for any restrictions or debris at the vent termination outdoors.

16 Servicing should only be performed by a Qualied Service Technician
3. BLOWER PROVER OPEN
CHANGE BACK ADVANCED
Blower Prover Open
Fault occurred 2 mins ago
The blower prover switch remains
open after the blower has been
energized.
Call a service professional:
Your Company Name Here
(press [DOWN] for more....)
CAUSES:
• The control has detected an open Blower Prover
Pressure Switch during operation.
• Blower Prover Pressure Switch sensing tube
disconnected or kinked.
• Loose or open wiring connections to Blower
Prover Pressure Switch.
• Blower not running when expected.
• Blower Prover Pressure Switch not operating
(Replace Blower Prover Switch).
CHECK/ REPAIR:
• The blower prover pressure switch will close its contacts on a pressure rise. See "Pressure Switch Tests"
section on Page 12 thru Page 15 under "Blower Operation Error" on Page 11.

17
Servicing should only be performed by a Qualied Service Technician
4. IGNITION FAILURE
CHECK/ REPAIR
• Determine whether or not the burner is igniting by looking through sight glass. Because the control system
will shut off the Gas Control Valve if 2-3 seconds if ignition is not detected, you must look in the sight glass
at the exact moment the Gas Control Valve icon appears on the display screen.
TESTING FOR IGNITION (FLAME):
• Set the Set Point Temperature high enough so there is a call for heat.
• Recycle the unit off and back on by turning the Enable/Disable switch to Disable for one second and then
back to Enable.
• The unit will begin an ignition cycle.
• When the Gas Control Valve icon appears on display screen, immediately look through the sight glass for
a brief blue ame (if you are getting an Ignition Failure error, the ame, if the burner ignites, will only be
present for 2-3 seconds).
BURNER IGNITES (BRIEFLY)
• If the burner ignites briey then shuts off with an “Ignition Failure” error message, follow these steps:
• Remove the connector from ame sense rod [picture of connector showing location on unit]. Inspect
the connector for corrosion. Replace if corroded or damaged. Remove Flame Sensor and clean metal
end with steel wool.
• Check insulator for soot, cracks or other damage and replace if necessary.
CHANGE BACK ADVANCED
Ignition Failure
Fault occurred 2 mins ago
Flame not detected. Clean flame rod.
Check gas supply.
Call a service professional:
Your Company Name Here
(press [DOWN] for more....)
CAUSES:
• On an “Ignition Failure” error, either the burner is
not igniting or the burner is igniting, but the Flame
Sensor is not detecting ignition (ame).
SPARK
IGNITER
BURNER
FLAME
BURNER
(MUST BE GROUNDED)
CERAMIC INSULATOR
FLAME SENSOR
METAL CONDUCTOR
CONTROL SYSTEM
APPLIES AC CURRENT
TO FLAME SENSOR
A
C CURRENT FLOWS FROM THE FLAME
SENSOR TO THE BURNER. THE BURNER
MUST BE GROUNDED FOR CURRENT TO
FLOW. DURING THIS PROCESS THE AC
CURRENT IS “RECTIFIED” AND BECOMES
A
DC CURRENT. DC MICRO AMP CURRENT
THEN FLOWS BETWEEN THE FLAME SENSOR
A
ND THE BURNER.

18 Servicing should only be performed by a Qualied Service Technician
• Reinsert connector on Flame Sensor and make sure the electrical connection is clean and tight.
• Recycle unit again to start another ignition cycle. If the burner ignites but the ame sense rod still fails
to detect ame, contact Commercial Technical Support.
BURNER DOES NOT IGNITE
Failure to ignite (no ame) usually means a gas supply or Gas Control Valve problem. If you do not see a
brief blue ame through the sight glass during the ignition cycle (at the moment the Gas Control Valve icon
appears on the display), follow these steps:
• Remove and reinsert the Gas Control Valve electrical connector. Make sure the electrical connections
(pins) are clean and that the connector ts tightly.
VENTURI
LOW GAS
PRESSURE SWITCH
SUPPLY GAS
CONNECTION
GAS CONTROL
VALVE
COMBUSTION
AIR INLET
24 VAC SOLENOID
& WIRE HARNESS
GAS CONTROL VALVE ASSEMBLY: BTH 120 - 250 MODELS
LOW GAS
PRESSURE
SWITCH
SUPPLY GAS
CONNECTION
24 VAC SOLENOID
& WIRE HARNESS
GAS CONTROL VALVE ASSEMBLY: BTH 300 - 500 MODELS

19
Servicing should only be performed by a Qualied Service Technician
• Remove and reinsert the circuit board end of the Gas Control Valve connector. Make sure it is connected
tightly and that the pins are clean.
• Recycle unit again to start another ignition cycle. If you still do not see a brief blue ame through the sight
glass at the moment the Gas Control Valve icon appears on the display, verify that 24 VDC is present on
the connector pins 1 and 5. Use care to not bend or damage the pins while testing.
• Connect test leads to the Gas Control Valve connector’s pins. Gently connect probes to Gas Control Valve
connector’s two outer pins. Set the meter to DC volts.
• Recycle unit again to start an ignition cycle.
• At the moment the Gas Control Valve icon appears on the display, you should see 24 VDC on your meter.
This electrical signal will only last for 2-3 seconds (if the burner does not ignite or the ame sensor does
not detect ignition).
24 VDC PRESENT
• If 24 VDC is present on the Gas Control Valve connector during the beginning of an ignition cycle, reinstall
the connector and recycle the unit again.
• Check the gas supply pressure during ignition using test procedure from "Gas Pressure Test" on Page 9.
• During the ignition cycle, the gas supply pressure should drop about 0.5” to 1.5” W.C. when the Gas Control
Valve icon appears if the Gas Control Valve is working. If the gas supply pressure does not drop at the
beginning of the ignition cycle, the Gas Control Valve may be defective—contact Commercial Technical
Support. A pressure drop of more than 1.5” W.C indicates an issue with the gas supply (pipe too long or
too small, bad regulator, etc.).
24 VDC NOT DETECTED
• If 24 VDC is not detected on the connector terminals when the Gas Control Valve icon appears on the
display, contact Commercial Technical Support.