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Sequence of Operation – Gas Furnaces

Author: Brian Baker, HVAC Tech Representative

Knowing the sequence of operation of a gas furnace will save time and money on troubleshooting calls.  We see time and time again, that technicians order multiple parts to repair a gas furnace.  Did the gas valve, control board, ignitor, and limit switch really all fail at the same time?  Probably not.  You may laugh but we get orders for these parts on a regular basis.  The sequence of operation may vary depending on type or brand.  But for the most part, manufacturers have all gone to 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. 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, in this case, gives me a good idea of what is wrong in the first 5 minutes of the service call.