Furnace Heat Limit Switch

High limit switch a high limit control switch will turn off the gas to a furnace if the temperature becomes too high.

Furnace heat limit switch.

A limit switch is a device that s located under your furnace s supply plenum. 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. When the thermostat calls for heat the furnace burners ignite and begin heating the heat exchanger. 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.

It reads the temperature of the plenum and waits until it gets appropriately warm. Watch the dial on the fan limit switch moving to confirm its operation. One common symptom of a failing or blocked heat exchanger is the roll out switch tripping. Watch the oil or gas burner.

The limit switch plays an important role in every normal cycle of the furnace. 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 control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace are too high. The fan limit switch on a heating furnace controls when the furnace blower fan turns on and off.