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How a Forced-Air Furnace Works

by Don Vandervort, © HomeTips

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, at the request of a thermostat, by a pilot light, a spark, or a similar device.

forced air-heating furnace diagramA 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 gases 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.

forced air heating diagramFurnaces 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 gases outside, so its installation is 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 gases are vented through a flue in the roof or, in some newer homes, through a wall.

upflow furnace diagramUpflow Furnaces

An upflow furnace draws cold air in through the bottom and sends heated air out the top. Upflow furnaces are often used in houses that have basements or that deliver heat through overhead ductwork. Downflow or counterflow furnaces draw cool, return air through the top and deliver heated air out the bottom. This type is favored where there is no basement or where the air ducts are located in the floor.

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