The heat is supplied by the burning of natural gas or propane. A mixture of gas and air flows into the burner and is ignited by the pilot. Combustion occurs, and warm air from the burner flame rises to fill a chamber known as a heat exchanger.

The heat exchanger becomes hot. Air passing around the heat exchanger absorbs that warmth, continues into the air ducts and the heat is distributed through the house.

The by-products of combustion pass upward through a venting system and escape through a vent in the roof or the wall.

There are three main types of gas furnaces. Their ratings are based on AFUE, Annual Fuel Utilization Efficiency, which is the standard measurement of efficiency for gas and oil-fired furnaces. Given in percentages, this number tells you how much of your fuel is used to heat your home and how much fuel is wasted. The higher the AFUE rating, the greater the efficiency. AFUE ratings range from the minimum industry requirement of 78% up to 95%. If you have an older furnace with an AFUE of approximately 60%, you could save up to 40% on your heating bills by replacing it with a new high efficiency furnace. The cost to replace your old, inefficient furnace is paid back through lower utility bills.

Some advantages of gas furnace include:

Could be vented through the wall

95% maximum efficiency

Some disadvantages of gas furnace include:

Produces very dry air