December 24, 2022

How Do You Know If A Furnace’s Limit Switch Is Bad

If you have recently purchased a furnace, it must have come with a limit switch. It is an important safety feature that gets the majority of homeowners confused. On top of that, it can also go bad and need replacement that’s done by furnace repair services. That said, we are going to look at the limit switch in a bit more detail and see how it’s important for your safety.

What Is The Purpose Of The Limit Switch?

To determine whether your furnace needs a new limit switch, you first need to understand its purpose and how it works. The first thing you need to know about the limit switch is that it has two primary objectives – comfort and safety.

When it comes to comfort, the limit switch senses the temperature in the plenum and signals the blower fan to turn on or off. Furthermore, it can also shut off the furnace, if it overheats due to a particular reason such as a dirty or clogged air filter.

So, the limit switch prevents both the furnace blowers and burners from turning on at the same time. The reason is that it would pump cold air through the house. As long as the air inside the plenum isn’t hot, the limit switch will keep the blowers off to keep the indoor environment comfortable.

On the safety side, the limit switch shuts the supply of gas to the burners to prevent the heat exchanger and furnace from overheating. If the heat exchanger gets extremely hot, the limit switch turns off the burners which can otherwise crack the heat exchanger pipes and damage the furnace.

Upon cracking of the heat exchanger, carbon monoxide will enter your house and can be deadly if inhaled for a long period of time.

Where Is The Limit Switch Located?

Since the limit switch needs to be cleaned and maintained as well, you should be aware of its location. Typically, the limit switch is attached to the furnace plenum. If you wish to access it, you will need to remove the furnace cabinet. Depending on the manufacturer, the shape and size may vary but are more or less similar. Follow the user manual if you can’t find it.

How To Identify A Bad Limit Switch?

As mentioned in the beginning, a limit switch does not last for life and will need a replacement. Therefore, you should be aware of the signs that indicate that you need to replace the limit switch. If you notice that your furnace blowers are turning on and off without any sequence, it means the limit switch is malfunctioning.

In addition to that, if you observe that the furnace is turning extremely hot but not shutting down, it most probably points to a limit switch that needs replacement.

While identifying the signs that point to a bad limit switch are important, it is equally essential to know the reasons as well. A limit switch can go bad if there are airflow issues in the furnace or a dirty air filter or temperature sensor. You should know that furnaces require proper airflow to distribute and produce cool and hot air.

If there is a problem with the airflow, the furnace will constantly overheat which will add stress to the limit switch. As a result, the wear and tear on the limit switch will increase. This is why it is important that you schedule expert maintenance for your furnace.

Components like a limit switch, air filters, and temperature sensors are sensitive. You cannot be sure about their lifespan. So, to prevent any mishaps, thorough maintenance ensures that components as such are identified and fixed before they can create a threatening situation.

Can A Limit Switch Be Bypassed?

Yes, a furnace limit switch can be bypassed but is not recommended. Fooling the system just to save some money on a replacement won’t bring you any good down the road. Furnace limit switches aren’t expensive to replace but offer a huge value in terms of safety and protection. Therefore, if you were thinking of bypassing the limit switch, think twice.


That’s it! Make sure you keep a lookout for unusual furnace activity and consider professional maintenance from furnace services Long Island to keep your family members and the house safe.

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