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What Is a Limit Switch in a Furnace?

Your furnace’s ability to warm the interior of your home is dependent upon a functional limit switch. Also known as a fan switch, it’s an essential furnace component. Whether you have a gas or electric furnace, there’s probably a limit switch inside of it. What is a limit switch exactly, and how does it work?

The Definition of a Limit Switch

A limit switch is a device that’s designed to regulate a furnace’s blower and protect the furnace from overheating. Furnaces, of course, contain a blower. The blower lives up to its namesake by blowing air over the heating element. As air moves over the heating element, it will pick up heat. This newly produced hot air will then travel into the ductwork where it’s distributed throughout your home.

The primary purpose of a limit switch is to synchronize a furnace’s blower with its heating cycles. The blower will automatically turn on when heat is needed, and it will automatically turn off when heat is no longer needed. If the temperature inside of your home is lower than the temperature for which your thermostat is set, the furnace and blower will run. The limit switch simply regulates the blower.

While they are used primarily to regulate the blower, limit switches also protect against overheating.  Furnaces can get hot when running. If a furnace gets too hot, though, it may sustain damage. Limit switches protect furnaces from heat-related damage by automatically shutting them down. If the temperature of a furnace rises exceeds its normal and safe operating temperature, the limit switch will shut it down.

How a Limit Switch Works

Limit switches work by measuring the temperature of the air inside of the furnace. Your furnace won’t release the heated air immediately. Rather, the heated air will typically remain trapped inside of your furnace for a short period. After it has reached an appropriate temperature, the heated air will then be released. The limit switch is a device that tells your furnace when to release the heated air.

There’s a temperature sensor on limit switches. The temperature sensor will measure the temperature of the air inside of your furnace. If the air needs to be hotter, it won’t activate the blower. The limit switch will only turn on the blower once the air has reached an appropriate temperature. After kicking on, the blower will move the heated air through the ductwork and into your home’s living spaces.