Operation And Service; Sequence Of Operation - A.O. Smith SERIES 120 Service Handbook

OPERATION AND SERVICE

SEQUENCE OF OPERATION

1. Call for heat is activated when temperature sensed from Thermistor Probes falls below
the Thermostat setting.
2. Blower (Draft Inducer) is energized.
3. Ignition Control Board verifies that the Blower Prover Switch is closed and that the
Blower (Draft Inducer) is operating.
4. Igniter is energized.
5. Gas Valve is energized and burners ignite
6. Ignition Control Board verifies flame at burners.
7. Water is heated to Thermostat set point.
8. Gas Valve is de-energized.
9. Blower runs for purge period and is de-energized.
10. Ignition Control Board goes into standby mode and Blower Prover Switch Opens.
OPERATION AND SERVICE
Blower (Draft Inducer)
Thermistors
(probes)
Main Gas
Valve
Servicing should only be performed by a Qualifi ed Service Agent
Main
Hot Surface Igniter
Burner
(Not Shown)
Figure 9
12