Gas Furnace Sequence of Operation

While most furnaces will follow the below sequences, 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. The inducer motor begins to spin and creates a vacuum to ensure the leftover gasses go out the exhaust pipe.



3. 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.



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



5. 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.



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



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



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

Summary
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.