furnaceUncategorizedFurnace Repair Guide: The Limit Switch

Though it doesn’t get a whole lot of attention, the furnace limit switch is one of the most important parts of the entire system. Without the limit switch, your furnace would be running without a major safety measure, one that could prevent a major breakdown or even a fire. Let’s take a look at what the limit switch is, what it does, and one of the major ways in which it can warn you that something is wrong with your furnace.

What the Limit Switch Does

The furnace limit switch is a small device that is connected to the main chamber of the furnace, called the plenum. Its job is to monitor the furnace’s internal temperature. If the temperature inside the furnace plenum gets too high, the limit switch activates and shuts down the entire system. It does this in order to prevent the furnace from overheating, which could create a fire risk and cause damage to the system. Though this seems like a pretty straightforward function, there’s a reason that you need to know about it as a furnace owner. Though the limit switch can prevent the system from overheating, it does not treat the cause of the problem. That leads to a furnace behavior known as “short-cycling.”


When the limit switch shuts down the furnace, it’s really only a stopgap measure. The switch cannot diagnose or solve whatever issue is causing the temperature in the furnace plenum to rise beyond the safety limit. That means that when the furnace cools off enough to start up again, it will more than likely exceed the temperature limit again. The limit switch will then shut the system down again, and this cycle will keep going indefinitely. This places a huge amount of stress on the furnace, and if it goes on for too long it can drastically shorten its lifespan. For this reason, it is very important to call a professional immediately if you notice that your furnace is turning itself on and off over and over again throughout the day.

If you think your furnace may be overheating, call G&S Heating, Cooling & Electrical, Inc. We provide heating repair throughout North Seattle.


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