Service Handbook - AO Smith Water Heaters

Gas Flow Test
The following illustrations show the approximate pressures for the manifold offset and supply gas for the
Standby, Pre-Purge and Heating operating states during a normal heating sequence. Notice how the manifold
offset pressure starts at 0" W.C. in the Standby operating state, falls to a deep vacuum during the Pre-Purge
state and then rises to a slight positive pressure when the 24 Volt Gas Valve is energized during the Heating
state. Also notice how the supply gas pressure remains constant until the 24 Volt Gas valve is energized at
which point it drops slightly.
Follow the procedure to connect two digital manometers to the 24 Volt Gas Valve in the Gas Pressure Test
on page 26. Observe the pressure changes during the three operating states listed in Figure 29 below.
The drop in supply pressure and corresponding rise in manifold offset pressure, between Steps 2 and 3,
proves the 24 Volt Gas valve is opening and that fuel gas is flowing through the valve.
1
OPERATING STATE:
• STANDBY
CONDITIONS:
• COMBUSTION BLOWER OFF
• 24 VOLT GAS VALVE OFF
PRESSURE READINGS:
• MANIFOLD OFFSET:
• SUPPLY GAS:
0.00” W.C.
+8.41” W.C.
2
OPERATING STATE:
• PRE-PURGE
CONDITIONS:
• COMBUSTION BLOWER ON
• 24 VOLT GAS VALVE OFF
PRESSURE READINGS:
• MANIFOLD OFFSET:
• SUPPLY GAS:
-5.33” W.C.
+8.41” W.C.
3
OPERATING STATE:
• HEATING
CONDITIONS:
• COMBUSTION BLOWER ON
• 24 VOLT GAS VALVE ON
PRESSURE READINGS:
• MANIFOLD OFFSET:
• SUPPLY GAS:
0.07” W.C.
+7.75” W.C.
Figure 29
Servicing should only be performed by a Qualified Service Agent
27