Forced-Air Furnace Heating Systems
More about: Typical Furnace Problems / Easy Furnace Repairs
Most homes are heated by central forced-air heating systems. Ductwork carries air to the furnace (or air-handling unit in the case of a heat pump), where the air is filtered, warmed, and blown back through ducts to rooms throughout the house.
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.
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.
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.
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