Your furnace starts when it receives a signal from the thermostat that tells it to turn on. Depending on the temperature you set, when the thermostat detects the air temperature dropping below that number, it activates the furnace.
You may also have an issue with your thermocouple or thermopile, which helps detect whether your pilot light is on or not. Your thermocouple is an essential safety device that makes sure your gas valve is not sending gas into a furnace without a working pilot light.
What Is a Furnace?
An efficient and well-maintained heat exchanger is essential for a working furnace. During the process of heating the heat exchanger, combustion gases should be venting safely out of the house.