Service Handbook - AO Smith Water Heaters

Gas Pressure
The water heater covered in this Service Manual is rated at 100,000 Btu/hr input. It is certified 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 manual for technical assistance.
Table 2
†Manifold Offset Pressure Minimum Supply Pressure Maximum Supply Pressure
NATURAL GAS PROPANE GAS NATURAL GAS PROPANE GAS NATURAL GAS PROPANE GAS
0.24” W. C. (0.056 kPa) 0.17” W. C. (0.042 kPa) 3.5” W. C. (0.87 kPa) 8” W. C. (1.20 kPa) 14” W. C. (3.49 kPa) 14” W. C. (3.49 kPa)
† Manifold offset pressures will vary. See the Service Notes below.
Service Notes:
• The manifold "offset" gas pressure is factory set and cannot be adjusted in the field.
• The manifold offset gas pressure and the supply gas pressure can be measured at two pressure test
ports on the water heater’s 24 Volt Gas Valve, see Figure 27 below. The manifold offset pressure test
port is closest to the Combustion Blower, see Figure 28 on page 26. 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.
• Manifold offset gas pressure will run close to 0” W.C. or lower (in a vacuum) depending on the current
operating state, see Operating States on page 48. This pressure will be considerably lower, -5.00” W.C.
to -7.50” W.C. during the Pre- Purge and Post-Purge operating states when the Combustion Blower is
running at high speed and the 24 Volt Gas Valve is closed, see Venturi on page 22.
• When the 24 Volt Gas Valve opens gas entering the Venturi will cause a rise in manifold offset gas
pressure. Manifold offset gas pressures will typically be +0.24” W.C. on natural gas models and +0.17”
W.C. on propane gas models during the Heating operating state. Keep in mind these pressures are
approximate and will vary depending on the equivalent length of the vent and/or intake air pipe installed.
• There is usually a drop in supply gas pressure noticed when the water heater’s 24 Volt Gas Valve opens
during ignition. Seeing a corresponding rise in manifold offset pressure during ignition confirms the valve
is opening and gas is flowing to the Burner, see Gas Flow Test on page 27.
• 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 flame 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).
PRESSURE
TEST PORTS
COMBUSTION
BLOWER
CONNECTION
24 VOLT GAS VALVE
SUPPLY GAS
CONNECTION
VENTURI
COMBUSTION
AIR INLET
Figure 27
Servicing should only be performed by a Qualified Service Agent
25