What Is a Furnace Limit Switch?

There are many parts of a furnace whose roles are pretty well known. Most homeowners already know what a pilot light does, or a heat exchanger, or an air filter. The furnace limit switch is rarely regarded as a major part of how the furnace operates. However, the limit switch serves a vital role in proper furnace operation. Let’s examine what the furnace limit switch is, what it does, and what can happen when it malfunctions.

What is the Limit Switch?

The furnace limit switch is a control circuit that determines whether to turn the furnace fan on or off. It is also responsible for turning off the burners. When the heat is turned on, the limit switch is the object that actually starts the fan in the furnace to that air can begin circulating. When the thermostat determines that the target temperature has been reached, it tells the limit switch to shut off the fan and the burners.

The limit switch is also attached to a separate thermostat, which measures the internal temperature of the furnace. The switch usually doesn’t turn the furnace fan on right away. Instead, it waits until the internal furnace temperature gets warm enough to begin heating the home. If the furnace’s internal temperature rises too high, the limit switch will shut off the furnace to prevent overheating. This brings us to the major issue that can occur with furnace limit switches.


Short-cycling is the name for when your furnace turns itself on and off rapidly and repeatedly throughout the day. This is caused by the limit switch registering an internal furnace temperature that is too high. When that happens, the switch shuts the furnace down to prevent overheating damage. After the furnace cools off, however, it starts up again and the cycle goes on indefinitely. This causes quite a bit of damage to the furnace, and should not be allowed to continue for any length of time. You’ll need a professional to determine whether the limit switch or the furnace is responsible for the short-cycling.

If you need to schedule service for your furnace in the Portland area, call Clean Air Act. We offer heating services throughout Portland, OR.

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