How Does the Limit Switch Work in a Furnace?

So have you ever wondered, on those cold winter days, why the burners begin to roar like a lion, a few minutes before the blower motor comes on?

Calm down and relax, you are not throwing your warm air out through the venting, it just means the limit switch is working correctly.

A forced air heating system

These kind of furnaces will burn natural gas, propane, or oil, and have large blower fans to push that air throughout your duct system. Instead of heating the air that circulates around the burners like an electric furnace, it warms up a metal heat exchanger, like an old steam engine warming up boiler, and the heat exchanger will warm the air inside it. The warm air will pass through the heat exchanger, which then will circulate throughout the house duct system.

Limit switch function

If the blower motor all of sudden came on at the same time as the gas burners did, this would pump cold air into your house, at first. The limit switch has a sensor in the heat exchanger that will notify it when the air is warm enough to circle through the house.

Once the air temperature reaches the operating temperature, the limit switch turns on the blower. The limit switch will also turn off the blower motor when the air cools. For safety reasons, the limit switch shuts off the gas burners when the heating cycle achieves its maximum temperature.

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