An Overview Of How The Gas Furnace Heats The House

A gas furnace requires regular maintenance for efficient and safe operation. Understanding how your gas furnace works can help you maintain it properly. Below is an overview of the major steps involved in a gas furnace's operations.

Thermostat Switching

The heating process begins when you turn on the heat at the thermostat. Turning on the heat signals the furnace that it is time to begin heating the house. The thermostat sends the relevant signal to the furnace's mainboard, which controls the furnace's operations. The mainboard performs a fast safety check to ensure everything is alright and then signals the furnace to begin heating.

Call for Heating

When the furnace receives a call for heating, the draft inducer starts running even before the burners come on. The draft inducer fan clears air and combustion air remnants from the combustion chamber. The draft inducer's action ensures that the gas remnants don't ignite and explode before the furnace is properly running.

The draft inducer also draws in the fresh air the furnace needs for combustion. As such, the draft inducer continues running even after the remnant gases are all out of the furnace, and the burners are up and running.


A few seconds after the draft inducer starts, the pressure switch closes, and the ignition starts to heat up or produce sparks. The exact operation depends on the type of ignition system. Once the ignition is hot enough, the gas valve opens to supply the burners with fuel, which ignites them and starts burning the air-fuel mixture. The whole sequence of events takes a few seconds.

Blower Start

Once the burners light up, the flame sensor senses the burner's flame and signals the furnace mainboard to start the blower motor. The blower motor starts running the blower fan that blows air over the heat exchanger and into the ductwork. The ductwork circulates the heated air into different parts of the house through supply registers.

Air Circulation

As heated air enters the duct system, the byproducts of combustion enter the flue pipe and exit the house. At the same time, the warm air that the registers supply to different rooms cools and enters the return duct through the return registers. The cool air returns to the furnace for further heating before it returns to heat your house.

The process continues until the thermostat registers that your house's temperature has matched the set temperature (on the thermostat). At that point, the thermostat signals the furnace to stop further heating. The burners stay off until the temperature falls, and then the furnace starts again. The on-and-off cycles continue until you turn off the furnace.

If you suspect a problem with your furnace, contact a furnace repair service in your area to schedule an inspection.