What Does A Furnace Limit Switch Do? (Find Out Now!)

Ryan Womeldorf
by Ryan Womeldorf

There are various moving components within your furnace. When they are all working together and efficiently, they keep your home nice and warm even when temperatures are plummeting. It is when one of those components stops working that you run into problems.

The limit switch is one of those components. The limit switch’s job is to read the temperature and signal to the air handler that it can begin running. By getting yearly tune-ups, you can ensure that the limit switch stays in good shape for longer. When it goes bad, having it replaced is the only course of action to keep your furnace running efficiently again.

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What Does the Furnace Limit Switch Do?

When your furnace kicks on and starts its cycle, you may notice that it takes a little bit of time before you feel warm air. That’s because it takes time for the burners to heat. Because of this, the air handler has to wait because it won’t know if it is delivering lukewarm or cool air into your home.

The limit switch is located under the supply plenum in your furnace. Its sole job is to read the temperature of that plenum and wait until it hits the appropriate temperature. When the plenum hits that temperature, the limit switch lets the air handler know that it can run, delivering the warm air that you want into your home.

The limit switch is basically a safety device as well. When the plenum temperature gets too hot, the limit switch shuts off the supply of gas to the burners. By shutting off the gas supply, the limit switch prevents the heat exchanger and furnace from overheating and breaking down.

What if the Furnace Limit Switch Isn’t Working?

Like any other component in your furnace, the limit switch can start experiencing problems over time. Most of them are due to issues with airflow in your system. Without proper airflow, typically through a dirty filter, the heat exchanger will overheat repeatedly.

As a result, the limit switch will experience excessive wear and tear, causing it to malfunction sooner rather than later.

When the limit switch isn’t able to do its job, the air handler might not turn on when your furnace cycles on. Without those signals from the limit switch, it won’t know if the temperature is okay before distributing air. In extreme cases, your furnace can overheat, shutting down the entire system because the limit switch doesn’t shut the gas off to the burners.

How Do I Keep My Furnace Limit Switch in Good Condition?

In order to keep the other components of your furnace from overheating and malfunctioning, it is essential to keep the limit switch in good condition. There are a few ways to achieve this: preventative maintenance and air filter changes.

Clean/change the air filter. The air filter is responsible for a lot of things. And when it is dirty or clogged, it can be responsible for a lot of issues, too. If you notice that airflow is restricted in your home or the performance of your furnace is dipping, check the air filter.

Depending on its condition, you may be able to simply clean it. Use pressurized air and a damp cloth to get the job done. Should the filter be too dirty or worn, you will need to replace it. It is a good idea to replace air filters bi-monthly at worst anyway to prevent issues within the rest of the system.

Preventative maintenance. It is always a good idea to have an HVAC technician perform preventative maintenance. This can be in the form of a check-up or tune-up. They will inspect the limit switch, ensure that airflow is good, and even clean the system.

Preventative maintenance is essential for keeping your HVAC system operating efficiently. Without that maintenance, either by you or a technician, you are leaving the door open for potential HVAC issues.

What Does the Furnace Limit Switch Look Like?

If you plan on performing preventative maintenance or cleaning your HVAC system, it is important to know what the limit switch looks like. It is generally a rectangular-shaped device that can be found under the supply plenum in your furnace.

The limit switch is generally mounted to another part of the furnace and has a wire running from it. You can even lookup the model of your furnace to see what the limit switches for that model look like. When in doubt, call in an HVAC technician to perform the task.

What Are Some Common Issues with Furnace Limit Switches?

No matter the model, aging furnaces will begin to experience issues. Those issues can be particularly impactful for things like the sensor on the furnace’s limit switch. There are a couple of reasons why the limit switch may stop working optimally.

  • Dirt and grime. When there is an issue with your furnace, the most common problem is a buildup of dirt, dust, and grime. When that happens, various components can become bogged down by that debris, making them run inefficiently. A thorough cleaning of each component should get them working optimally once again.
  • Blower motor. When the furnace keeps running and shutting off during the day, it could be a sign that the blower motor is bad. When the blower assembly gets overheated, it will constantly trip the furnace limit switch. Not only will that overheat and potentially damage the limit switch, but it can actually shut down your entire system.
  • Clogged/dirty filter. As mentioned above, a dirty filter can cause a lot of issues. Thankfully, it is a relatively easy fix. Either clean the filter or replace it entirely (which you should do bi-monthly anyway).
  • Other issues. When the limit switch stops working, it can also be due to faulty circuits, gas valves, or even the thermostat. When troubleshooting the issue yourself, start with the air filter. Look for excessive dirt and debris on other components in your furnace. When it gets past that, identifying the issue can be tougher.

A Limit Switch Stuck Open

From time to time, the limit switch can go bad and keep the furnace from operating at all. When that happens, the switch has either failed or it is stuck open. When it is stuck open, the furnace will not turn on at all.

At the same time, the limit switch can keep tripping during the heating process. For most modern furnaces, if the system trips four times, the control computer will sense that there is a problem. When this happens, the entire furnace will go into a “hard shutdown” which means that the furnace won’t turn on again until the issue is resolved and the system reset.

Performing a Replacement

Replacing a faulty limit switch can be done on your own, but it is a bit tricky. Performing the replacement requires a solid understanding of general electrical issues. Not only that, it means understanding how to properly use a multimeter as well.

The multimeter is used to check the continuity of the limit switch. Should the multimeter show infinite resistance, the limit switch has gone bad and needs to be replaced. If you feel comfortable enough, it is simply a matter of locating the part, unscrewing the plate, and then swapping out the bad piece for a good one.

How Much Does It Cost to Replace a Limit Switch on a Furnace?

Because it is such a crucial component, replacing the limit switch is not a cheap endeavor. Remember, in addition to signaling the air handler that it can do its job. Moreover, it is a crucial safety component that keeps your furnace from overheating.

Though the part itself is small, it can be costly. Depending on whether you do the replacement yourself or bring in an HVAC technician, you can expect to pay anywhere from $150 to $400. The part itself is around the $150 mark and anything after that would be cost of labor; probably a few hours at the most.

Can I Bypass the Limit Switch on My Furnace?

This can be tricky and should only be done if you think that there are other components at fault. It is recommended that you do not simply bypass the limit switch on a regular basis. It plays too important a role in the safety aspect of the furnace to simply be ignored.

Start by turning the power to the furnace off. If you have an ohmmeter handy, you can use it to measure the resistance across the switch. Should the temperature be below the setpoint, then the resistance across that switch will be zero. If no electrical meter is available, bypass the switch temporarily to see if the furnace still works.

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Can You Reset a Limit Switch?

As mentioned previously, the limit switch can get stuck open. Most of the time, it means that the limit switch is bad and needs to be replaced. There are times, though, where it can be reset. Find the little red reset button located on the switch and use a paperclip to press it down. Resetting the limit switch should bring it back to the original closed state.

Ryan Womeldorf
Ryan Womeldorf

Ryan Womeldorf has more than a decade of experience writing. He loves to blog about construction, plumbing, and other home topics. Ryan also loves hockey and a lifelong Buffalo sports fan.

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