Furnace Heat Limit Switch

Watch the oil or gas burner.

Furnace heat limit switch.

The furnace oil or gas burner should turn on and subsequently the fan limit control should turn the burner back off when the temperature in the supply air plenum reaches the fan limit set point. One common symptom of a failing or blocked heat exchanger is the roll out switch tripping. The limit switch plays an important role in every normal cycle of the furnace. Fan limit switches are used on all types of building heating furnaces.

Initially the air above the heat exchanger in the supply plenum is not warm enough to blow into the house so the limit switch keeps the blower off. A furnace has two switches called a fan limit switch and a high limit switch these switches can be separate or in one package called a combination switch. High limit switch a high limit control switch will turn off the gas to a furnace if the temperature becomes too high. Once the plenum reaches a specific temperature the limit switch sends a signal to the air handler to begin running and delivering warm air to your home.

Watch the dial on the fan limit switch moving to confirm its operation. The fan limit switch on a heating furnace controls when the furnace blower fan turns on and off. It reads the temperature of the plenum and waits until it gets appropriately warm. A limit switch is a device that s located under your furnace s supply plenum.