How Does a Gas Furnace Work?

Derived from the Latin word fornax, meaning oven, a furnace is a large heating appliance designed to evenly distribute heat throughout a building. Furnaces accomplish this by burning fuel internally to warm air, water or steam, then directing this warmth into various parts of a building to maintain a comfortable indoor temperature.

Most furnaces are called forced-air furnaces because they direct heated air into different rooms with a blower. Although some furnaces generate heat through electricity, or by burning wood, coal or oil, they most commonly burn natural gas or liquid propane gas (LPG). If you own a gas-burning forced-air furnace or are thinking of buying one, understanding how they work is an important first step.

How a Gas Furnace Works: The Heating Cycle

Let’s begin by understanding the heating cycle. A pipe entering your home from the outside connects the source of the gas to the furnace. LPG burning units need an outdoor storage tank, while natural gas models feature underground connections to a natural gas pipe network.

• After the gas enters your furnace from a storage tank (LPG) or the local gas supply network (natural gas), the burner lights it.
• Cold air from your house enters the furnace, where the burning gas warms it within the heat exchanger.
• Exhaust from the combustion is piped out of the furnace through the vent and exits the home via an exhaust pipe.
• The warm air is directed into various parts of your house by the blower fan, depending on where thermostats detect the need for heat.
• The internal air temperature gradually increases as the warm air is distributed. Cold air is redirected to the furnace through return ducts.
• Once the thermostat senses the set temperature has been reached, it switches off the gas valve to prevent the flow of warm air.

Parts of a Gas Furnace

Thermostat: An electronic device that measures and controls indoor temperature, regulating the furnace’s heating cycle according to its set point. (The thermostat is located remotely and hard-wired to the furnace.)

Control board: A small circuit board designed to interpret various electrical signals from inside and outside the furnace. It sends signals in response, causing various things to happen, like opening the gas valve and turning on the burners and blower fan when the thermostat calls for heat. (Not shown in illustration.)

Burners: Small outlets where gas emits within the furnace and is ignited into even, controlled flames.

Igniter: The device responsible for lighting the gas emitted from the burners. Furnace igniters work by creating a spark to cause ignition, or by producing an extremely hot surface that ignites the gas as it passes.

Blower fan: A small electric fan and motor that directs warm air from the furnace to various parts of the home according to heat demands.

Flame detector: Designed to prevent gas from building up dangerously if the igniter fails, flame detectors shut off gas flow if no heat is detected.

Heat exchanger: A series of thin-walled metal tubes that keep the combustion process separate from the air entering the home via the blower fan. Cold air is blown over the outside of the heat exchanger, warming it before it is redirected into the home.

Plenums: The supply-air plenum distributes warmed air to the ducts. The return-air plenum carries air from rooms back to the furnace to be heated.

Gas valve: Regulates the pressure of gas entering the furnace from an outside source, either a tank of LPG or a natural gas supply line.

Air filter: A membrane designed to keep dust and debris from entering the furnace with the cool air.

Transformer: These supply electricity to the furnace and regulate it down to the correct voltage.

Furnace Maintenance and Safety

As with all combustion-based home heating appliances, gas furnaces require some regular maintenance and safety precautions.

• Keep all flammable items well away from the furnace.
• Schedule regular cleaning and maintenance calls with a certified technician.
• Change the air filter often. During regular use, swap it out at least every three months.
• Outfit your home with at least one carbon monoxide (CO) detector. Test it regularly.
• Keep furnace registers (louvered openings in walls or floors where warm air from the furnace enters the home) clean and clear of obstruction, and don’t close too many at once. Keep at least two-thirds of them open at all times to avoid excessive heat buildup in the furnace.