Below are the steps demonstrating how a typical gas furnace operates upon turning up your thermostat.
1. The booster assembly sends air to the burner for combustion.
2. Gas passes over the ignitor to light the burner.
3. The gas valve clicks causing a light to turn on which signifies the burner being lit.
4. Once the burner is running, the flame sensor senses the flame. If no flame is sensed, the entire furnace will shut off.
5. There is a time delay as the furnace begins to heat up. Within a few moments, the blower motor will come on to prevent cold air from continuing to get through the house.
6. The blower motor will run until the temperature of your house reaches what is set on the thermostat.
7. The burner shuts off at the same time the gas valve shuts off.