The Inner Workings of a Furnace
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.
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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.
Source:http://www.hometips.com/cs-protected/guides/furnaces/furnace_works.html