The Fundamentals Of Furnace High Limit Switch Issues

27 August 2021
 Categories: , Blog


With summer's heat rapidly waning and the cool temperatures of fall preparing to set in, this is the time of year when most homeowners start testing their furnace to make sure that it's ready for the cold season. Although most homeowners understand that every furnace is vulnerable to malfunction, many don't understand some of the components that can cause such problems. For example, one thing you should understand is your furnace's high limit switch. Here's a look at some of the things that you should know about your furnace's high limit switch and how it can fail.

What Is A High Limit Switch?

A high limit switch is a safety feature on your furnace that helps to prevent the unit from overheating and potentially causing a fire or other serious damages. Essentially, the high limit switch monitors the air temperature inside the furnace. When the air is heated enough to be released into the heating ducts, it triggers that release. Meanwhile, it continuously monitors the temperature and engages the burner when the temperature drops too low, or it turns the burner off, shuts off the fuel supply, and engages the blower fans if the temperature reaches the high limit level. 

What Are The Signs Of A Failing High Limit Switch?

When you know what to look for, it's fairly easy to identify a failing high limit switch. However, you'll still need to have it tested by a furnace repair technician to ensure that it actually is the source of the problem.

If you notice that the blower fan on your furnace doesn't shut off, that may be an indication that the high limit switch is faulty. The blower fan should only run when the temperature reaches a trigger point for the high limit switch sensor. Then it should disengage when the temperature is reduced. 

Another common indication that your high limit switch is failing is if the air coming out of your heat vents doesn't seem warm. If this is the case, the high limit switch is allowing air to bypass even though it hasn't reached the right temperature. The sensor may be failing, or the switch may be stuck open.

Short-cycling is another common indication of a failing high limit switch. Short-cycling refers to the process of the furnace turning on and off in short bursts. If the thermostat triggers the furnace to run, but the high limit switch is mistakenly reading high temperatures, it will turn the furnace back off again, which leads to the short-cycling problem.

If you're experiencing any of these problems, reach out to a furnace repair technician as soon as possible.