The limit switch on a furnace serves as the primary sensor in the heat exchanger and determines if air within the furnace is warm enough to circulate. The limit switch turns on the blower of the furnace and automatically shuts off when the air temperature exceeds the maximum.
The limit switch regulates the operations of the furnace by sending a sensor to turn on the blower and sending another sensor when the air is too warm or the temperature falls below a limit that is programmed into the furnace.
Many furnaces operate with a forced heating system fueled by natural gas or oil. These systems are equipped with large blower fans that are designed to send air through a system of supply ducts to circulate into the residence or business. The air within the furnace is heated by a metal heat exchange warmed by a burner. Once the air is warm enough, the limit switch signals that the air passing through the heat exchanger is ready to travel through the supply ducts.
Furnace owners can determine if the limit switch is working if they hear a soft roar from the furnace burner minutes before the blower fan within the furnace turns on. If a roar is audible, the sound indicates that the limit switch is effectively operating.