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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