Operating Sequence - Flow Chart - A.O. Smith SERIES 120 Service Handbook

OPERATING SEQUENCE - FLOW CHART

Self diagnostic check is performed
(Correct state of Blower Prover Switch and ECO is verified)
(polarity , earth ground , and Igniter presence are verified )
Blower Prover Switch (N.O.) Contacts
Close at – 2.85" W.C. ± 0.13" W.C.
Sends Voltage (80-120 VAC)
to Igniter for 17-20 Seconds.
Ignition Control Board Energizes
Igniter De-energized After 3 Seconds
Ignition Control Board Waits 3
More Seconds For "Strong" Flame
Sense Signal (2.0 uA DC or more)
Tank Heated to Set -Point
De-energizes Gas Valve
Blower Runs 60 Seconds
Blower Prover Switch Contacts Re-open
Ignition Control Board Goes into Standby Mode
Servicing should only be performed by a Qualifi ed Service Agent
OPERATION AND SERVICE
Power On
Call for Heat
YES
Blower (Draft Inducer)
Motor is Energized
YES
Ignition Control Board
Gas Valve (24 VAC)
YES
Ignition Control Board
Post Purge Cycle and
Shuts Off
Control Locks Out - LED
Displays 2, 6, or 8
NO
Flash Error Code
(See Error Code Chart )
Control Locks Out
NO Displays 3 Flash
Error Code
Below 0.1 uA DC Flame Sense:
Heater Will Try to Ignite 3 Times
Control De-energizes Gas Valve
Blower Runs for Post Purge Period
Control Locks Out for 1 Hour
Displays 1 Flash Error Code
NO
Between 2.0 and 0.1 uA DC Flame Sense
Heater Will Continue to Operate
Displays 7 Flash Error Code
(Weak Flame Sense)
Figure 11
15