A high-temperature limit switch also called a fan limit switch or simply a limit switch is an important device used in gas and heating oil forced-air furnaces. Simply put, it senses the temperature of the furnace and tells the blower fan to turn on and off at the appropriate times. It also can shut down the furnace burners if things are getting too hot inside the furnace.
How to Identify a Limit Switch
A typical limit switch consists of a long temperature sensor probe attached to a mounting plate. The probe extends through the wall of the furnace housing, and the plate fastens to the outside of the furnace. The mounting plate usually contains two or more wire terminals that receive control wires for the blower fan and the furnace's gas valve. The location of the switch varies by the furnace model, but it is typically located in the hot-air supply plenum, above the combustion chamber or heat exchanger of the furnace.
What a Limit Switch Does
The limit switch plays an important role in every normal cycle of the furnace. When the thermostat calls for heat, the furnace burners light 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. When the air in the plenum reaches the high limit setting on the limit switch, the switch turns on and activates the blower fan, circulating air through the heat exchanger on its way to the house while simultaneously pulling cool air from the house and into the furnace. When the house temperature reaches the setting on the thermostat the burners turn off, but the limit switch keeps the blower running for a little while to extract as much heat as possible from the heat exchanger. When the air in the supply plenum drops to the lower setting on the limit switch, the switch shuts off the fan until the next cycle begins.
The other important function of the limit switch is to shut off the burners if the heat exchanger gets too hot, which can crack the piping in the exchanger, effectively ruining the furnace. Overheating can occur if there is a problem with the blower fan or if the furnace filter is so dirty that it restricts airflow through the furnace and exchanger so that the exchanger doesn't cool as it should.
Signs of a Bad Limit Switch
One common symptom of a malfunctioning limit switch is a blower fan that doesn't shut off. This happens when the switch simply fails to shut off the fan after the burners have stopped and the air exchanger is sufficiently cool. A bad switch also can prevent a furnace from operating at all. If the switch has completely failed and is stuck in the open circuit, or off, position, the furnace won't run. The same effect can result from repeated overheating. If a limit switch (doing its job) hits its top limit and has to shut off the burner four or so times, the control computer of the furnace may go into a "hard shutdown" mode so that the furnace won't turn on until the unit is serviced.