Furnace Heat Limit Switch

This control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace are too high.

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. The fan limit switch on a heating furnace controls when the furnace blower fan turns on and off. Watch the oil or gas burner. 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.

When the thermostat calls for heat the furnace burners ignite and begin heating the heat exchanger. 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. This happens because the flames are not being drawn into and throu.

Fan limit switches are used on all types of building heating furnaces. 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. It reads the temperature of the plenum and waits until it gets appropriately warm.