Gas Furnace Sequence of Operation

By | September 1, 2019

Furnace parts
There are different types of furnaces and furnace manufacture but below is the most common steps for a furnace to go through to start.
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 come on.

  3. The inducer motor begins to spin and create a vacuum to insure the left over gasses go out the exhaust pipe.
  4. Furnce Inducer motor

  5. A 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. (A pilot light will stay on all the time.)
  8. hot surfce igniter

  9. The flame sensor will attempt to detect the 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. The blower motor starts blowing air through the vents. Some furnaces have different start times for the blower motor.
  12. motor
    Once the furnace is working it will continue to work until the thermostat cuts off a signal to it.

Lenox furnace sequence of operation




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