Furnace Heat Limit Switch

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

Furnace heat limit switch.

Fan limit switches are used on all types of building heating furnaces. 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. It reads the temperature of the plenum and waits until it gets appropriately warm. 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 oil or gas burner. When the thermostat calls for heat the furnace burners ignite and begin heating the heat exchanger. 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. A limit switch is a device that s located under your furnace s supply plenum.

Watch the dial on the fan limit switch moving to confirm its operation. This happens because the flames are not being drawn into and throu. High limit switch a high limit control switch will turn off the gas to a furnace if the temperature becomes too high. 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.