Knowing the sequence of operation on a gas furnace will help you diagnose it right the first time - saving time and money on troubleshooting calls.
The sequence of operation may vary depending on type or brand. But for the most part, manufacturers mostly use a hot surface or direct spark ignition.

The sequence of operation is as follows:
1. Heat call from the thermostat
2. Control board does a safety check (limit switches, roll out switches)
3. Inducer motor comes on
4. Pressure switch must close to go to the next step
5. Hot surface ignitor begins getting hot / direct spark ignitor begins to spark
6. Gas valve opens
7. Furnace lights
8. Flame sensor senses flame
9. Blower motor comes on between 30 seconds to 1 minute
10. Once thermostat is satisfied, the burner shuts off


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