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
- The thermostat reaches a set temperature and sends a signal for the furnace to come on.
- The inducer motor begins to spin and create a vacuum to insure the left over gasses go out the exhaust pipe.
- 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.
- 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.)
- 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.
- The blower motor starts blowing air through the vents. Some furnaces have different start times for the blower motor.
Lenox furnace sequence of operation