A.O. Smith BTH 120 Series How to use

MODELS COVERED: BTH 120 THRU 500
SERIES 100, 101 AND
BTH 400/500 SERIES 104
500 Tennessee Waltz Parkway
Ashland City, TN 37015
PRINTED IN THE U.S.A. 1012 316586-002
SERVICING SHOULD ONLY BE PERFORMED BY A QUALIFIED SERVICE TECHNICIAN
Service Handbook
COMMERCIAL GAS HIGH EFFICIENCY WATER HEATERS
http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

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Servicing should only be performed by a Qualied Service Technician
TABLE OF CONTENTS
INTRODUCTION ......................................................................2
QUALIFICATIONS .....................................................................2
TOOLS REQUIRED ....................................................................3
GENERAL INFORMATION...............................................................4
GAS PRESSURE SPECIFICATIONS ......................................................4
INSTALLATION QUICK TIPS – BTH 120AND 150: GAS PRESSURE.............................6
INSTALLATION QUICK TIPS – BTH 199AND 250 GAS PRESSURE .............................7
INSTALLATION QUICK TIPS – BTH 300, 400, 500 GAS PRESSURE . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
INSTALLATION QUICK TIPS – BTH 300, 400, 500............................................9
INSTALLATION – VENTING CATEGORYAND MATERIALS ...................................10
VENTING TABLES – BTH 120 - 250 ......................................................11
VENTING TABLES – BTH 300, 400, 500...................................................11
CONCENTRIC VENTING...............................................................12
VENT CONDENSATION ...............................................................13
VENTING – ALL MODELS – SINGLE PIPE DIRECT VENT – USING OUTSIDEAIR TERMINATION
CLEARANCES SIDEWALL POWER VENT.................................................14
VENTING – ALL MODELS – TWO PIPE DIRECT VENT – USING OUTSIDE AIR TERMINATION
CLEARANCES SIDEWALL DIRECT VENT .................................................15
VENT TERMINATION – DIRECT VENT – ALL MODELS ......................................16
DIRECT VENTING – ALL MODELS.......................................................17
CONTROL OVERVIEW ................................................................18
ADJUSTING TANK TEMPERATURE – OPERATING SET POINT – DIFFERENTIAL.................20
CHANGING THE DISPLAY UNITS .......................................................22
FAULT AND WARNING CONDITIONS – ADVANCED DIAGNOSTIC INFORMATION ................24
ACCESS TO THE CURRENT FAULT OR WARNING .........................................25
FAULT CODES.......................................................................26
VIEWING THE FAULT HISTORY – VIEWING INFORMATION ABOUT THE HEATER................27
CONTROL SEQUENCE (TYPICALALL MODELS) ...........................................28
CONTROL SEQUENCE FLOW CHART ...................................................29
CONTROLS – CENTRAL CONTROL BOARD – CCB.........................................30
CONTROLS – GAS CONTROL VALVE BTH 120 ............................................32
CONTROLS – GAS CONTROL VALVE BTH 150 ............................................32
CONTROLS – GAS CONTROL VALVE BTH 199 AND 250.....................................33
CONTROLS – GAS CONTROL VALVE, ORIFICE CHART - BTH 300/400 - 100/101.................34
CONTROLS – GAS CONTROL VALVE, ORIFICE CHART – BTH 400/500 ........................35
CONTROLS – PRESSURE SWITCHES – ALL MODELS ......................................36
CONTROLS – PRESSURE SWITCHES – BTH 120 THROUGH 250 .............................37
CONTROLS – PRESSURE SWITCHES – BTH 300, 400, 500 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .38
CONTROLS – CONNECTIONS, IGNITER, FLAME SENSOR, SIGHT GLASS, POWEREDANODES ...39
HOT SURFACE IGNITER / FLAME SENSOR / CONTROL TIMING..............................40
BLOWER SPEED CONTROL BTH 199 AND 250 ............................................41
VARIABLE FREQUENCY DRIVE – BTH 400 AND 500........................................42
VARIABLE FREQUENCY DRIVE – BLOWER SPEED AND PRESSURE READINGS................43
WIRING DIAGRAM – BTH 120 - 300......................................................44
WIRING DIAGRAM – BTH 400, 500 ......................................................45

2Servicing should only be performed by a Qualied Service Technician
INTRODUCTION
The service handbook is designed to aid in servicing and troubleshooting A. O. Smith Cyclone Xi BTH
commercial water heaters in the field. No duplication or reproduction of this book may be made without the
express written authorization of the A. O. Smith Corporation.
The following text and illustrations will provide you with a step by step procedure to verify proper
installation, operation, and troubleshooting procedures. Additional quick reference data is included to assist
you in servicing these products.
The information contained in this handbook is designed to answer commonly faced situations encountered
in the operation of this product line and is not meant to be all inclusive. If you are experiencing a problem
not covered in this handbook, please contact A. O. Smith Technical Information at 1-800-527-1953, by
email at [e-mail], or your local A. O. Smith Corporation representative for further assistance.
Our website at: http://www.hotwater.com is also a resource for installation and service information. This
handbook is intended for use by licensed plumbing professionals and reference should be made to the
instruction manual accompanying the product. This handbook contains supplemental information to the
product’s instruction and operation manual.
QUALIFICATIONS
ANSI Z223.1 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 of this water heater requires ability equivalent to that of a Qualified Service Technician in the field
involved. Installation skills such as plumbing, air supply, venting, gas supply, electrical supply are required
in addition to electrical testing skills. Some products may require combustion testing equipment and
certification. If you do not possess these skills or do not have the proper tools you should not attempt to
service this water heater.

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Servicing should only be performed by a Qualied Service Technician
TOOLS REQUIRED
• ELECTRICAL MULTIMETER CAPABLE OF MEASURING CONTINUITY/ OHMS, AC & DC VOLTS,
AMPERES, MICROAMPERES, MILLIVOLTS, and FREQUENCY (Hz)
• UEi Model DL289 or equivalent
• DIGITAL MANOMETER + 60" W. C. in .01" increments
Note: A digital manometer is required for testing pressure switches and can replace a gas pressure
gauge, draft gauge or slack tube manometer for checking gas pressure.
• UEi model EM200 or equivalent
• WATER PRESSURE GAUGE w/LAZY HANDAND HOSE BIBB CONNECTION
• THERMOMETER
• 1-1/16 INCH SOCKET WITH EXTENSION FOR POWEREDANODE REPLACEMENT
• SET OF NUMBERED DRILL BITS
DIGITAL MANOMETER DIGITAL MULTIMETER
WATER PRESSURE
TEST GAUGE W/LAZY
HAND AND HOSE BIBB
CONNECTION

4Servicing should only be performed by a Qualied Service Technician
GENERAL INFORMATION
INSTALLATION REQUIREMENTS FOR THE COMMONWEALTH OF MASSACHUSETTS
For all side wall terminated, horizontally vented power vent, direct vent, and power direct vent gas fueled
water heaters installed in every dwelling, building or structure used in whole or in part for residential
purposes, including those owned or operated by the Commonwealth and where the side wall exhaust vent
termination is less than seven (7) feet above finished grade in the area of the venting, including but not
limited to decks and porches, the following requirements shall be satisfied:
INSTALLATION OF CARBON MONOXIDE DETECTORS
At the time of installation of the side wall horizontal vented gas fueled equipment, the installing plumber
or gas fitter shall observe that a hard wired carbon monoxide detector with an alarm and battery backup
is installed on the floor level where the gas equipment is to be installed. In addition, the installing plumber
or gas fitter shall observe that a battery operated or hard wired carbon monoxide detector with an alarm
is installed on each additional level of the dwelling, building or structure served by the sidewall horizontal
vented gas fueled equipment. It shall be the responsibility of the property owner to secure the services of
qualified service technicians for the installation of hard wired carbon monoxide detectors. In the event that
the side wall horizontally vented gas fueled equipment is installed in a crawl space or an attic, the hard
wired carbon monoxide detector with alarm and battery back-up may be installed on the next adjacent
floor level. In the event that the requirements of this subdivision can not be met at the time of completion
of installation, the owner shall have a period of thirty (30) days to comply with the above requirements
provided that during said thirty (30) day period, a battery operated carbon monoxide detector with an alarm
shall be installed.
APPROVED CARBON MONOXIDE DETECTORS
Each carbon monoxide detector as required in accordance with the above provisions shall comply with
NFPA 720 and be ANSI/UL 2034 listed and CSAcertified.
SIGNAGE
A metal or plastic identification plate shall be permanently mounted to the exterior of the building at
a minimum height of eight (8) feet above grade directly in line with the exhaust vent terminal for the
horizontally vented gas fueled heating appliance or equipment. The sign shall read, in print size no less
than one-half (1/2") inch in size,
“GAS VENT DIRECTLY BELOW. KEEP CLEAR OF ALL OBSTRUCTIONS.”
GAS PRESSURE SPECIFICATIONS
MODELS Natural
120‑150 Propane 120‑150 Natural 199‑250 Propane 199‑250 Natural 300/400/500 Propane 300/400/500
Maximum Gas
Supply Pressure 10.5" WC
(2.62 kPa) 14.0" WC (3.49 kPa) 10.5" WC (2.62 kPa) 14.0" WC (3.49 kPa) 11.0" WC (2.74 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) 7.0" WC (1.74 kPa) 11.0" WC (2.74 kPa)
Minimum Gas
Supply Pressure
(Low Gas Press.
Switch Setting)
4.8" WC
(1.20 kPa) 11.0" WC (2.74 kPa) 4.8" WC (1.20 kPa) 8.5" WC (2.12 kPa) 5.2" WC (1.30 kPa) 11.0" WC (2.74 kPa)
Manifold Pressure 4.0" WC
(1.0 kPa) 10.0" WC (2.49 kPa) 0" WC (0 kPa) 0" WC (0 kPa) 4.0" WC (1.0 kPa) 10.0" WC (2.49 kPa)

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Servicing should only be performed by a Qualied Service Technician
GAS SUPPLY SYSTEMS
Lowpressurebuildinggassupplysystemsaredefinedasthosesystemsthatcannotunderanycircumstances
exceed 14” W.C. (1/2 PSI Gauge). These systems do not require 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 supply pressure must be stable
within 1.5” W.C. (0.05 PSI Gauge) from static to dynamic pressure to provide good performance. Pressure
drops that exceed 1.5” W.C. (0.05 PSI Gauge) 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 valves. If your low pressure system does NOT meet these requirements, the
installer is responsible for the corrections.
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).
Appliances require gas regulators that are properly sized for the water heater input and deliver 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) at any time must be equipped with a gas supply regulator.
The minimum supply pressure is measured while gas is flowing (dynamic pressure). 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) as gas begins to flow to the water heater
then the supply gas system including the gas line and/or the gas regulator may be restricted or undersized.
See Supply Gas Regulator section of this manual. The gas 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)
SUPPLY GAS REGULATOR
Install a positive lock-up gas pressure regulator in the gas supply line if inlet gas pressure exceeds the
maximum supply gas pressure at any time. see "Gas Pressure Specifications" Table on Page 4. Regulators
must be sized/used according to manufacturer’s specifications.
If a positive lock-up regulator is required follow these instructions:
1. Positive lock-up gas pressure regulators must be rated at or above the input Btu/hr rating of the water
heater they supply.
2. Positive lock-up gas pressure regulator(s) must be installed no closer than 3 equivalent feet (1 meter)
and no farther than 8 equivalent feet (2.4 meters) from the water heater’s inlet gas connection.
3. 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. Some
additional adjustment maybe required later to maintain a steady gas supply pressure.
4. When installing multiple water heaters in the same gas supply system it is required that individual
positive lock-up gas pressure regulators be installed at each unit.

6Servicing should only be performed by a Qualied Service Technician
INSTALLATION QUICK TIPS – BTH 120 AND 150: GAS PRESSURE
ADJUSTMENT PROCEDURE: MANIFOLD GAS PRESSURE BTH 120 AND 150 MODELS
Main line gas pressure to the water heater for natural gas should be between a maximum of 10.5" w.c. (2.62
kPa) for natural gas, 14.0" w.c. (3.49 kPa) for propane and a minimum of 4.8 w.c. (1.20 kPa) for Natural
Gas, and 8.5" w.c. (2.12 kPa) for Propane Gas. Also see "Gas Pressure Specifications" Table on Page 4.
Asupply gas pressure regulator (service regulator) must be installed on the gas supply line no closer
than 3 equivalent feet (1 meter) and no farther than 8 equivalent feet (2.4 meters) from the water
heater's inlet gas connection if the gas pressure is above maximum pressure or not stable.
1. Check gas line pressure with a manometer.
2. Check manifold pressure gauge (manometer) connected to the manifold pressure tap on the gas
control valve. If manifold pressure adjustment is required, remove cover screw from top of the gas
control valve. Using a small screwdriver, turn adjusting screw clockwise to increase gas pressure or
counterclockwise to decrease gas pressure to obtain 4.0" w.c. (1.0 kPa) for natural gas and 10.0" w.c.
(2.49 kPa) for Propane (LP) Gas.
3. Cycle the burner on and off several times to check for proper operation.
4. Check the high limit and operating controls for proper operation.
5. Check the vent system seams and joints and ensure that there is no leakage of flue products into the
room.
6. Verify correct input rate (see below).
HIGH ALTITUDE INSTALLATIONS BTH 120 ‑ 150
For water heater installation locations with elevations above 6,500 feet (1982 meters) consult the “High
Altitude Installation” section of the owners manual.
1. Attach a pressure gauge (manometer) to the manifold pressure tap and refer to Page 4 for correct
pressure.
2. Use this formula to “clock” the meter. Be sure other gas consuming appliances are not operating
during this interval.
Btuh = 3600 X H/ T
T = Time in seconds to burn 1 cubic foot of gas.
(With a stopwatch read the gas meter and measure the amount of time required for the heater to
consume 1 cubic foot of gas.)
H = Heating value of gas (in Btu’s per cubic foot of gas).
Btuh = Actual heater input rate, in Btuh.
EXAMPLE: (Using BTH-150 heater)
T = 25.25 seconds
H = 1050 Btu/ft.3
BTUH = ?
Compare result to the de‑rated input required for the elevation at the installation location.
If it is necessary to adjust the gas pressure to the burner in order to obtain the full input rate, the steps
below should be followed:
1. Remove the pressure regulator cover screw and adjust the pressure by turning the adjusting screw
with a small screwdriver. Turn adjusting screw clockwise to increase gas pressure and input rate
and counterclockwise to decrease gas pressure and input rate. Do not exceed 4.0" (1.0 kPa) for
natural gas models and 10.0" w.c. (2.49 kPa) for propane models.

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Servicing should only be performed by a Qualied Service Technician
2. “Clock” the meter as in step (2) above.
3. Repeat steps (1) and (2) until the specified input rate is achieved.
4. Turn the manual gas control valve to “OFF”. Replace the pressure regulator cover screw. Remove
the pressure gauge or manometer from the manifold pressure tap and replace the set screw in
the manifold pressure tap. If the gas pressure regulator cannot be adjusted to give the full input
rating with sufficient gas pressure at the valve, check to ensure the unit is equipped with the correct
orifice. Refer to "ORIFICE CHART – BTH 120 thru 500" on Page 35.
INSTALLATION QUICK TIPS – BTH 199 AND 250 GAS PRESSURE
ADJUSTMENT PROCEDURE: MANIFOLD GAS PRESSURE BTH 199 AND 250 MODELS
IMPORTANT NOTE:
THE BTH 199 AND 250 MODELS INCORPORATE A NEW GAS CONTROL, WHICH OPERATES AT A
MANIFOLD PRESSURE OF 0" w.c. (0 kPa) FOR BOTH NATURALAND PROPANE GAS. SEE THE "GAS
PRESSURE SPECIFICATIONS" TABLE ON PAGE 4. THESE MODELS ARE CONFIGURED PRIOR
TO BEING SHIPPED FROM THE FACTORY AND NO ADJUSTMENTS ARE NECESSARY PRIOR TO
STARTUP. THE CONTROLLER MONITORS THE AIR FLOW AND MAKES ADJUSTMENTS TO THE FAN
SPEED WHICH, IN EFFECT, CONTROLS THE AMOUNT OF GAS FLOW. THEREFORE, THE UNIT WILL
SELF ADJUST TO ACQUIRE THE CORRECT AMOUNT OF INPUT.
Supply gas pressure to the water heater must not exceed a maximum of 10.5" w.c. (2.62 kPa) for natural
gas, or 14" w.c. (3.49 kPa) for propane. The minimum supply gas pressure is 4.8" w.c. (1.20 kPa) for
natural gas and 8.5" w.c. (2.12 kPa) for propane gas.
Once the unit is installed and filled with water and the inlet pressures confirmed, simply turn the switch
on and observe operation. Cycle the unit off and on several times to ensure proper operation. For more
information please refer to the Instruction Manual.
HIGH ALTITUDE INSTALLATION
The BTH 199 and 250 models are suitable for installation up to 10,100 feet above sea level with no
adjustments.
GAS ORIFICE
The BTH 199 and 250 models do not have a natural gas orifice. A .230" orifice is used on Propane (LP) gas
models.
Venturi
Gasket
w/o Orifice
Gas Control Without Orifice Natural Gas
Venturi
Gasket
w/LP Gas
Orifice
(.230")
Brass
Gas Control with .230" Propane (LP) Orifice
Asupply gas pressure regulator (service regulator) must be installed on the gas supply line no closer
than 3 equivalent feet (1 meter) and no farther than 8 equivalent feet (2.4 meters) from the water
heater's inlet gas connection if the gas pressure is above maximum pressure or not stable.

8Servicing should only be performed by a Qualied Service Technician
INSTALLATION QUICK TIPS – BTH 300, 400, 500 GAS PRESSURE
ADJUSTMENT PROCEDURE: MANIFOLD GAS PRESSURE BTH 300, 400, 500
A minimum dynamic gas supply pressure of 5.2" w.c. (1.30 kPa) for Natural Gas and 11" w.c.
(2.74 kPa) for Propane (LP) Gas is required before making any adjustment to the gas control
pressure regulator. Attempts to adjust the regulator during periods of low gas supply pressure could result
in over firing of the water heater when the gas supply pressure returns to normal.
Check gas line pressure with a manometer, adjust the gas supply line pressure "Gas Pressure
Specifications" table on Page 4.
1. Check manifold pressure using a pressure gauge (manometer) connected to the manifold pressure
tap on the gas control valve. If manifold pressure adjustment is required, remove cover screw from
top of the gas control valve. Using a small screwdriver, turn adjusting screw clockwise to increase
gas pressure or counter clockwise to decrease gas pressure to obtain 4.0" w.c. (1.0 kPa) for Natural
Gas and 10" w.c. (2.49 kPa) for Propane (LP) gas.
2. Cycle the burner on and off several times to check for proper operation.
3. Check the high limit and operating controls for proper operation.
4. Check the vent system seams and joints and ensure that there is no leakage of flue products into
the room.
5. Verify correct input rate as shown on Page 6.
HIGH ALTITUDE ADJUSTMENT BTH 300, 400, 500
For high altitude adjustments, contact the help line number on the front of the water heater or contact
[e-mail].