Gas Furnace Sequence of Operation

By | May 11, 2024

Furnace parts
There are different types of furnaces and manufacturers, but below are the most common steps for a furnace to go through from start to blowing out hot air.

While most furnaces will follow the below sequences of steps, there may be variations in models and manufacturers.

Gas Furnace Sequence of Operation

  1. The thermostat reaches a set temperature and sends a signal for the furnace to turn on.
  2. Gas Furnace Sequence of Operation Thermostat Turns On

  3. The inducer motor begins to spin and creates a vacuum to ensure the leftover gasses go out the exhaust pipe.
  4. Furnce Inducer motor

  5. A pressure sensor detects if there is a vacuum created by the inducer motor; if there is, it closes a switch; if there is no vacuum, the switch is left open.
  6. Vacuum sensor

  7. An igniter comes on depending on the furnace; this can be a Hot Surface Igniter, Spark Igniter, or Pilot Light.
  8. hot surfce igniter

  9. The flame sensor will attempt to detect the igniter/flame source; if there is a working flame source, it will allow gas to flow and light it; if not, it will keep the gas off.
  10. flame sensor

  11. If all tests are good, gas will start to flow, and the furnace flames will light.
  12. furnace flames will light

  13. The blower motor starts blowing air through the vents. Some furnaces have different start times for the blower motor.
  14. motor

  15. Once the furnace is working, it will continue to work until the thermostat cuts off a signal to it.

A gas furnace sequence of operation can help to troubleshoot a problem that may arise.

While furnaces can vary in build, there is a common sequence of steps that happen before the flame is lit.

It starts with the thermostat, which has a preset temperature.

Next, the inducer motor will start spinning, creating a negative pressure in the burning chamber, so gases are vented.

A pressure switch will detect if there is a vacuum; if not, it will not allow the flame to be lit.

An igniter will then come on, which can be a pilot light or spark igniter.

The flame sensor will verify a flame is present or that the igniter is working and allow gas to flow into the burner.

Once the flames are lit, the blower motor will come on and blow hot air into a home.

Category: HVAC how to

About Aaron H. Benetti

Aaron H. Benetti an HVAC technician who has worked in the field since 1991. He began his career as an HVAC installer and later began doing troubleshooting and repairs. Around 2010, he began to write books on HVAC topics, mainly tutorials and how-to information. Currently, Aaron works as an HVAC tech and also writes many articles and posts for websites as well as books and videos on HVAC topics.

3 thoughts on “Gas Furnace Sequence of Operation

  1. Craig

    Is there a specific sequence of operation fir a Comfortmaker gas furnace or is it about the same ? I think it’s an 80+ or 90+ gas furnace. Could u help me. I don’t have the owner’s manual any more. It got damaged to a flooded basement where I had all my tech info boxed up.


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