How Do Forced Air Furnaces Work?

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A forced air furnace pulls cold air into ductwork that runs throughout your home to the furnace to be heated. When the air is heated, it travels back through other ductwork and forced out through heat registers, warming rooms.



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Several types of fuel can be used to heat the air, including oil, coal, wood, electricity or gas. Most forced air systems use natural gas, because it is considered to be the least expensive option and is widely available. A natural gas system moves fuel through a pipe from the supplier into a combustion chamber, which has a pilot light for igniting gas being fed to a burner. A thermostat that senses temperature and usually is mounted on a wall in a living space controls the supply of gas to the burner. The thermostat is set to maintain a desired temperature, and when the air temperature falls below the setting, it supplies more gas to the burner, which then heats the air that has been drawn into the combustion chamber. The thermostat also activates a limit-control switch that activates a fan which forces the hot air through the ductwork.


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Fumes from the burning of the fuel are vented from the combustion chamber through a flue pipe that vents either through the roof of your home or through an exterior wall.

Ductwork and Registers

After the air is heated in the combustion chamber, it is forced through the ductwork to each room through the heat register, which can be adjusted to control air flow. Heat registers are in each room that is to be heated. The number of registers and cold-air returns will depend upon the size of your home, the size of the rooms, and the number of floors in your home.



Furnaces run more efficiently if cold-air returns or heat registers are not impeded, ensuring maximum airflow.

If rooms in your home are warmer than others, especially on upper floors because hot air rises, partially close registers in the warmer rooms to divert air to other rooms.


Air circulating through the furnace goes through a furnace filter, which collects dirt and dust. A dirty air filter restricts air circulation and reduces the furnace's efficiency. Change furnace filters as they become dirty.



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