On furnaces, a fan limit switch uses a heat sensor to detect the temperature of the air passing through the valve that the switch is affixed to. Its purpose is to detect when the air in the furnace is warm enough to circulate into a home. Without the fan limit switch, the furnace would blow cold air.
The fan limit switch in a furnace is also able to detect when the air temperature has fallen below a certain limit. When this occurs, the fan limit switch instructs the furnace to turn the blower off. The switch doubles as a safety measure that turns off the burner in the event that the furnace's heat temperature reaches a dangerous level.
Furnaces make a familiar noise for several minutes before they begin to release air. This is the time during which the furnace is heating the air to an acceptable temperature. The fan limit switch is responsible for withholding the air from circulation until the furnace has heated it sufficiently. If a furnace blower begins to run continuously without turning off, a homeowner needs to check to make sure the fan limit switch is in proper working condition. If not, the homeowner must call a professional to make the necessary repairs.