How Gas Furnaces Work
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A Proven Heating Technology
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Natural gas furnaces are a dominant type of home heating in the United States. They operate on the straightforward principle of forced air heating, in which cool air is sucked in, heated by direct application of flame, and blown through ducts to all areas of a house or building. For decades, gas furnaces competed for sales primarily with electric or oil furnaces, as well as traditional boiler/radiator steam heat. However, the market has expanded to include new technologies such as heat pumps and floor radiant heat systems, as well as alternative sources such as wood stoves and pellet or corn burners. Nevertheless, a great many homes still use natural gas or propane furnaces, which for many years had clear cost advantages over electric and oil units due to the higher cost of those fuels.
Heat Exchange and Air Intake
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The gas furnace heating cycle begins when the thermostat senses the temperature has slipped below the current setting. The thermostat electrically triggers an inducer fan, which pulls air through the heating chamber as a valve opens to allow gas to flow into the burners. A pilot flame or spark (depending on the model) ignites the gas, which quickly heats up the surrounding air. A short time (determined by temperature or a timer) after the gas flame begins to heat the air, the unit's blower motor goes into operation. As the blower pushes warm air out, cold air is sucked in through return ducts in each room. In some installations, an adjustable humidifier is added to the intake duct near the furnace so that one of the drawbacks of gas heat--the drying out of the air--can be mitigated. Such humidifiers are usually connected directly to the water line using a small copper pipe, and they use another pipe to drain away the condensation created from their operation.
Filtration and Warm Air Distribution
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Before the cold air reaches the furnace, it must first be filtered clean of dust and particulate matter which, if allowed to enter the furnace, could quickly have an adverse effect on its operation. Air filters, made in many different sizes to match the various dimensions of intake ducts, are available in many price ranges. The most inexpensive filters use flimsily encased woolly material, while more costly and sophisticated filters have rigid construction, thicker filtration pads, and in some cases, anti-allergenic characteristics. As the incoming cold air is heated by the gas, the blower pushes it back out through a separate series of ducts into the rooms. Some installations have hand-adjustable valves in the ductwork near the furnace so that the output of hot air may be balanced between rooms at various distances, or so the heat can be virtually shut off to a room that has been closed for the winter.
The duct cover plates or grilles in each room typically also have shutoff valves to regulate the amount of heat that comes through. After a cycle that usually lasts several minutes, the thermostat senses that the temperature has returned to the preset level and it turns off the gas valves. The blower, however, continues to operate for a few moments more so that all of the warmed air can be forced through the ducts.
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