Furnace Heat Limit Switch

One common symptom of a failing or blocked heat exchanger is the roll out switch tripping.

Furnace heat limit switch.

A limit switch is a device that s located under your furnace s supply plenum. High limit switch a high limit control switch will turn off the gas to a furnace if the temperature becomes too high. The fan limit switch on a heating furnace controls when the furnace blower fan turns on and off. Fan limit switches are used on all types of building heating furnaces.

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. 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. Watch the oil or gas burner.

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. This happens because the flames are not being drawn into and throu. 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. This control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace are too high.