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Kilowatt Heating, Air Conditioning and Electrical Blog

What Does the Furnace Limit Switch Do?


A gas furnace contains many important components: the gas line, gas burners, control board, blower fan, combustion chamber, heat exchanger, inducer fan, flue, electronic ignition system, flame sensor, etc. They all work together to provide a home with warmth, convenience, and safety.

Today we’re going to look at one of the key components that allows a furnace to synchronize its parts and deliver even and energy-efficient heating to the house. It’s also a part that helps the furnace to run safely with little risk of it overheating. This part is called the furnace limit switch.

Furnace Limit Switch Basics

The furnace limit switch (a.k.a. the furnace high limit switch or furnace fan limit switch) is designed to monitor the temperature inside the furnace so it turns on and off parts of the furnace to regulate the heat in a home. It’s located in the plenum of the furnace, which is the metal box that connects to the HVAC system where air moves out of the furnace and into the ductwork. The switch is small: an exterior plate over a temperature sensor that’s set within the plenum.

How the Furnace Limit Switch Operates

We’ll go through a basic heating cycle for a gas furnace to show the important job of the limit switch. When the thermostat signals to the HVAC system that heat is required, gas flows to the burners and the ignition system lights them. The heat exchanger of the furnace (where heat will be transferred to the air) starts to heat up. When the limit switch registers the furnace has heated up enough, it closes and connects a circuit that turns on the blower fan. When the thermostat detects that the house has warmed up to the temperature setting, it will turn off the gas. However, the blower will continue to run to remove heat from the heat exchanger. When the furnace cools down enough, the furnace limit switch will open and deactivate the blower. This helps to create an even flow of heat into the house and improve energy efficiency by getting as much heat out of the furnace after the gas turns off.

The Limit Switch and Furnace Safety

We’ve explained the job of the furnace limit switch during standard heating cycles. But the limit switch helps keep the furnace operating safely. The heat exchanger or the blower can overheat, which can create a safety hazard as well as damage the furnace interior. If the furnace goes over a certain temperature, the limit switch will automatically shut it down. This overheating most often occurs when the blower isn’t working properly and cannot remove heat from around the heat exchanger. 

If your furnace begins to turn off because of the limit switch, please don’t try to continue to operate the heating system. Instead, call us for furnace repair in Valley Village, CA. We’ll find out what needs to be fixed. The problem may even be with a fault in the limit switch. 

Kilowatt Heating, Air Conditioning and Electrical serves Los Angeles and the San Fernando Valley. Rely on us for all your heating repair needs.

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