Furnace High Limit Switch Tripping? Here’s What to Do

Furnace not working? It might be the high limit switch.

The high limit switch plays a critical safety role in a furnace.

It senses the temperature in the air plenum– and shuts off the burner if the temperature gets too high.

The result is no heating in your home– and an uncomfortable environment.

In this article, I’ll tell you what to do if your furnace’s high limit switch is tripped. I’ll also discuss the reasons a high limit switch might trip, and what it’s used for.

What to do if your furnace’s high limit switch is tripping

If your furnace’s high limit switch is tripping, the first thing that you should check for is airflow restrictions.

Airflow restrictions are almost always the cause of high limit switch trips.

Here are the steps you should take if your furnace’s high limit switch is tripping.

Check the air filter

The first and easiest thing to check is your filter. Dirty filters will impede airflow, and excessively dirty filters will destroy furnace performance.

Check for airflow bloackage

Another thing to check for is blockage in your ducts and vents. Make sure that your diffusers are open and there is nothing blocking their airflow.

Also, check for blockage at your air vents and registers. Ensure that your vents and registers are open and unobstructed.

Check the blower fan

One last thing to check is your furnace’s blower fan. If your blower fan isn’t producing enough airflow, then your high limit switch will trip.

If your high limit switch is still tripping after you check everything in the list, then your limit switch might need to be cleaned or replaced.

Note: If you’re not comfortable working on your furnace, then call an HVAC professional to check it out instead.

Clean the high limit switch

In some cases, the high limit switch itself might be dirty. If enough soot covers the surface of the high limit switch, the switch can falsely trigger when it’s not supposed to.

How to clean or replace a furnace high limit switch

The high limit switch in a furnace can sometimes get dirty. When the limit switch is dirty, it will have trouble working properly, causing your furnace to trip out excessively.

To clean the high limit switch sensor, follow these directions:

1. Shut off power to the furnace and remove the access panel.
   
   
2. Locate the high limit switch. It is usually found screwed into the plenum somewhere above the burners.
   
   
3. Remove the high limit switch from the plenum by unscrewing it and taking out the wire connectors. Note: Do not touch the sensor. Hold it by the base instead. Oil from your fingers can create a hotspot that will cause the sensor to crack.
   
   
4. Clean the limit switch sensor using fine-grit sandpaper or a scouring pad. Avoid steel wool since that will scratch the sensor surface.
   
   
5. Replace the high limit switch by screwing it back into the plenum and reconnecting the wires.

If you need to replace a high limit switch, follow the steps above. But instead of cleaning the high limit switch sensor, simply replace the old high limit switch with a new one.

How to tell if the high limit switch is tripped

Here are the most common ways to tell if your furnace’s high limit switch is tripped:

• Your furnace’s heating cycles end right after they begin.
• If the furnace won’t ignite, but the blower fan stays running, then the high limit switch might be tripped.
• Some furnaces will go into hard shutdown if the furnace’s high limit switch trips multiple times. This is to protect the furnace from damage until it’s looked at by a service professional.

What causes a furnace high limit switch to trip?

Airflow issues usually cause a high limit switch to trip out.

With less airflow going through your furnace, the heat exchanger excessively heats up and causes the limit switch to trip.

Here are a few things that will cause a high limit switch to trip:

• Dirty air filter
• Blockage in air duct
• Blocked/closed air vents
• Bad blower motor

What a high limit switch is used for

The high limit switch is used to prevent excessively high temperatures in your furnace.

But isn’t the job of the furnace to get hot? Yeah, it is. But when your furnace gets TOO hot, problems can occur.

Here are two ways that a high limit switch helps protect your furnace (and your health):

Prevents heat exchanger damage

The high limit switch helps prevent damage to your heat exchanger by shutting down your furnace if it gets too hot.

Excessive temperatures in your furnace might cause the heat exchanger to crack.

A cracked heat exchanger will allow flue gases to infiltrate the supply air into your home. Nasty gases such as carbon monoxide will get into the air you breathe.

The high limit switch helps to protect the heat exchanger from getting cracked by excessive temperatures. 

If excessive heat builds up in your heat exchanger, your high limit switch will trip and shut off the burners, preventing the heat exchanger from cracking.

Tells the blower when its safe to turn off

Another important function of the high limit switch is to signal to the blower when its safe to turn off.

When your furnace’s supply air temperature gets hot, the high limit switch senses this temperature and tells the blower to turn on. Once the blower is on, it circulates heat throughout your home.

After the heating cycle is over, the blower remains on to blow away residual heat. Once the high limit switch senses a cooler temperature, the blower turns off.

Are you following my furnace component blog series?

Are you having trouble with your furnace? I’ve written a bunch of blogs answering questions that people have about troubleshooting the parts in a furnace. You can check them out by clicking the links below.

• Furnace Flame Rollout Switch – Everything You Need to Know
• Furnace High Limit Switch Tripping? Here’s What to Do
• Furnace Flame Sensor – Everything You Need to Know
• Furnace Pressure Switch Stuck Open? Here’s What to Do
• Furnace Pressure Switch Stuck Closed? Here’s What to Do
• Furnace Fuse Blown? Here’s What to Do
• Furnace Blower Not Working? Here’s What to Do
• Furnace Gas Valve Stuck? Here’s What to Do
• Furnace Inducer Troubleshooting – Here’s What to Do
• How to Check If Your Furnace Ignitor Is Bad
• Furnace’s Control Board is Bad? Here’s What to Check