How Does a Gas Furnace Work?

By Robert Korpella

Gas -- either natural or propane -- provides the source necessary to warm a home's cold air during the heating season. The furnace system makes the conversion and distributes the warm air into the home's living spaces. Furnaces powered by natural gas and propane operate in much the same way. With either system, the home's thermostat begins the heating cycle as temperatures in the room fall below the setting you chose. That sends a signal to fire up the burners.

HVAC technician
credit: Salvador Burciaga/Hemera/Getty Images
Gas-powered furnaces operate in cycles in order to heat the home.

Ignition Source

Older gas furnaces used a pilot light as an ignition source when it was time for the furnace to produce heat. A regulator supplied a small flow of gas to keep a short flame burning so it would be ready to ignite the gas when it was time to heat the home. More efficient and modern gas furnaces use a "glow stick" made of silicone nitride as the ignition source. Electricity passes a current through the glow stick only when the furnace calls for heat.

A Combustion Chamber

Air must be present for proper combustion. Gas mixes with air inside the combustion chamber, which is also called a burner. The heating cycle begins when a small amount of the air and gas mixture enters the combustion chamber. The glow stick or pilot light ignites the mixture, which burns in a controlled fire inside the combustion chamber as more of the gas and air mix moves into the chamber.

The Heat Exchanger

A heat exchanger mounts above the combustion chamber. This allows the exchanger to absorb as much heat as possible from the combustion process. The cold air inside the exchanger warms almost immediately as the heat from the combustion process rises into the exchanger. Once that air reaches a level preset by the furnace manufacturer, an electric motor kicks in and powers a blower fan that forces the warm air into the home's heating ducts and out through the registers. The combustion sequence ends before the blower motor stops. This allows the blower to distribute every bit of warm air into the home before it, too, stops running and awaits the next heating cycle.

Dealing with Gases

The byproducts of combustion include carbon monoxide and other harmful gases as well as a small amount of unburned natural gas or propane. High-efficiency gas furnaces capture the exhaust gases and compress them before igniting them in a second combustion chamber. This squeezes every bit of energy from the unburned gases. Any remaining byproducts must be vented away from the home, typically through a flue pipe.

Furnace Plenum

As the furnace distributes warm air throughout the home, the cold air it displaces flows through the cold air return ducting and back to the furnace. That cold air collects inside a box called a plenum, which sits next to the heat exchanger. The plenum is the last stop for cold air moving through the furnace system. Since cold air from the home also contains dust particles, the air must be filtered before it enters the plenum. Furnace filters are located either at the cold air return grate in the living space or at the entrance to the plenum. Pressure differences between the plenum and the heat exchanger allow the air inside the plenum to warm. As it does, that air moves into the duct and out through the heat registers.