Honeywell 51370B Manal page 42

Control Sequence of Operation
Start up Sequence
Upon powering up, the control checks for the presence of the vapor sensor, if the
resistance is in the expected range the control will begin normal operation after 5 to 8
seconds.
Normal Heating Sequence
1. The thermostat senses a need for heat.
2. The control checks the pressure switch condition.
3. If the pressure switch is open, the control sends power to the blower motor.
4. The blower starts moving combustion air through the combustion system.
5. The pressure switch closes.
6. The control senses the closed pressure switch and starts the ignition process by
providing a spark at the pilot electrode and allowing gas to flow to the pilot.
7. When the pilot is lit the gas control senses the pilot flame and opens the main gas
valve.
8. The main burner is lit.
9. The main burner and blower continue to operate until the thermostat is satisfied.
10. When the thermostat is satisfied the main and pilot gas valves close.
11. The blower operates for a short post purge period before shutting down.
12. The water heater remains in the stand-by mode until the next call for heat.
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