Understanding how your furnace ignites can help you know if it is working correctly or if its time to call an expert for service. Most manufacturers who use a hot surface or direct spark ignition. The sequence of operation can on occasion vary depending on type or brand. But the generally-accepted 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. 30 seconds – 1 minute blower motor comes on.
10. Once thermostat is satisfied, the burner shuts off
If the inducer comes on but your hot surface ignitor never glows, there is a good chance that you have a bad pressure switch or your vent motor is not moving enough air (vent pipe stopped up, blower wheel stopped up).
Knowing how the furnace works, helps know when is the right time to call for service from a professional.