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, then you’ll need to check for a few things. Restrictions to airflow, in particular.
- The first and easiest thing to check is your filter. Dirty filters will impede airflow, and excessively dirty filters will destroy furnace performance.
- 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 they are open and blowing air.
- 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.
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:
- Shut off power to the furnace and remove the access panel.
- Locate the high limit switch. It is usually found screwed into the plenum somewhere above the burners.
- 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.
- Clean the limit switch sensor using fine-grit sandpaper or a scouring pad. Avoid steel wool since that will scratch the sensor surface.
- 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
The most common way to tell if a limit switch is tripped is to observe the behavior of your furnace:
- 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.
Airflow issues can be caused by many different things such as:
- Dirty air filter
- Blockage in air duct
- Blocked/closed air vents
- Bad blower motor
What a high limit switch is used for
Why is a high limit switch so important? It protects both your furnace and your health. Here are a couple of reasons why the high limit switch is so important:
Prevents heat exchanger damage: 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 from excessive temperatures.
If an excessive amount of heat builds up in your heat exchanger, your high limit switch will trip and shut off the burners. This is done to prevent the heat exchanger from cracking.
Tells blower when to turn on and off: Another important function of the high limit switch is to signal the blower to turn on or off.
When the supply air temperature gets hot enough, 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 circulate residual heat. Once the high limit switch senses a cooler temperature, the blower turns off.