Basics of Furnaces

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    Gas and Oil

    • Gas and oil furnaces burn their fuel inside a heat exchanger. Air is heated by conduction as it passes by the heat exchanger and enters ductwork, which sends the heated air throughout the home. The main difference between a gas and an oil furnace is the burner. The burner for a gas furnace sends natural gas into a compartment where it is mixed with air and ignited. The burner for an oil furnace uses a pump to send the oil quickly through a nozzle. The oil is converted into a spray of tiny droplets and ignited. In both cases, a flue or vent carries the byproducts, such as carbon dioxide, outside the home.

    Electric

    • Electric furnaces operate under the same principles, but do not require any fuel. They have no heat exchanger or burner. Instead, air passes over metal which is heated by electricity, similar to a hair dryer. This process eliminates the need for a vent as there are no byproducts from heating the furnace elements by electricity. In most places, electric furnaces are more expensive to use because the electricity is more costly than gas or oil.

    Efficiency

    • Furnace efficiency is the percentage of heat generated by the furnace that reaches your home. In other words, a furnace with 90 percent efficiency loses 10 percent of the heat it produces by expelling it along with the harmful byproducts via the flue. Electric furnaces have 100 percent efficiency because the do not need to produce any byproducts. For gas and oil furnaces, the greater the efficiency, the less fuel required to heat your home, resulting in lower utility bills.

    Safety

    • Some gas and oil furnaces are equipped with a sensor to detect ignition. It would be dangerous to continue sending gas or oil into the burner within the heat exchanger if the fuel was not being burnt. The ignition sensor might be a part of the burner or it might be a sensor in the ductwork that stops the burner if the air being sent into the house is not warm (a sign that the fuel is not being burned).

    Fan/Limit Switch

    • A furnace that uses a fan to send air past the heat exchanger, through the ductwork and into the house is called a forced-air furnace. Some forced-air furnaces have a fan/limit switch that delays turning the fan on when the burner begins and delays turning the fan off when the burner turns off. If the fan were to start immediately with the burner, it would push cold air into the house during the time the heat exchanger is heating up. If the fan were to stop when the burner turned off, the furnace would be wasting the heat as the heat exchanger cooled.

    Air Quality

    • Two components are used to improve the quality of air entering the house. Air filters avoid the build-up of particulates inside the furnace. Humidifiers, which do not come with every furnace, help moisten the air to combat the crispness of cold weather.

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