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.

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 inside the combustion chamber, creating heat that is transferred to 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.

One of the benefits of a forced-air system is that it can include an air-conditioning unit, a humidifier, and an air filter—and all of these can take advantage of the system's ducts for delivery of conditioned air to rooms. Ductwork is generally metal, wrapped with insulation to help keep heat in. In some cases, flexible insulation-style ductwork is preferred.

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.

An electric-resistance furnace works more like a hair dryer: Electric heating elements heat the air, and a blower pushes the warmed air through the system. Most problems are caused by an interruption in the delivery of power, faulty heating elements, or blower breakdowns.

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. In terms of repairs, gravity furnaces have the same combustion problems as forced-air systems, but they don't have blower breakdowns because they don't have blowers.

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