To understand how a high-efficiency furnace works, consider these basics. In principle, a forced-air furnace is a relatively simple device, somewhat like a gas oven that's hooked up to a fan. Natural gas is piped to a burner inside a combustion chamber where the gas is mixed with air and ignited by a pilot light, a spark or a related device at the request of a thermostat. A blower in the furnace pulls cool air in from rooms through air ducts, passes it through a metal "heat exchanger" where it's heated by the burner, and blows the warm air back into rooms through ductwork. Exhaust gasses from the burners are vented outside through a flue.

Additional devices can be added to nearly all forced-air systems to condition the air that passes through. An air-conditioning unit can cool and dehumidify the air, an electronic air cleaner will remove dust and particulates from the air, and a humidifier can add moisture to uncomfortably-dry winter air.

Older-style gravity furnaces, usually located in basements, offer central heating but don’t force the air--instead, they allow heated air to rise naturally into rooms through large ducts.

Furnaces can be fueled by natural gas, oil, propane, coal, wood, or electricity. Today, most use gas because it is clean-burning, commonly available, and relatively inexpensive. In a few regions, electricity is unusually affordable--in these areas, electric furnaces or electric radiant-heating may be sensible. One advantage of electric heating over gas and other combustion fuels is that electric heating doesn't require a flue to carry combustion gasses outside, so its installation can be more affordable.

A gas forced-air heating system goes into action when a room's air temperature drops below a preset level on the thermostat. The pilot light ignites a burner in the furnace's heat exchanger, a metal chamber around which air flows and is then heated. The warmed air moves into the hot-air plenum and into the rooms through ducts. The combustion gasses are vented through a flue in the roof or, in some newer homes, through a wall.