Testing a Limit Switch on a Furnace

Have you ever had a furnace limit switch test done in your home? It’s an essential test that has to be conducted to ensure that the furnace is working properly. If it’s not, you may experience issues like low airflow or no heat at all.

These issues could be dangerous, and in extreme cases, could result in an electrical fire. Most furnaces have some sort of mechanism in the furnace control board that will alert you when the temperature has reached a certain point. If you hear that alert sound, and your thermometer indicates that it’s extremely hot, this problem should be checked out right away.

This article explains exactly how to test a limit switch on a furnace to ensure your home’s comfort and safety are not compromised.

But before we delve any deeper, let’s discuss what a limit switch is. 

What is a Limit Switch on a Furnace?

A limit switch is an electronic device that acts as a safety circuit. When the temperature rises above the upper limit, it turns off the heating element and prevents further damage.

This safety device prevents the furnace from starting up if it detects an overload or short circuit. It will stop the furnace from turning on until the problem is fixed.

Limit switches are used in furnaces to detect when the temperature has reached a predetermined limit. When the temperature reaches the limit, the switch will trigger and activate the furnace’s heaters. This prevents the furnace from overheating and potentially causing damage.

Types of Limit Switches

There are two types of limit switches: manual and automatic. Manual limit switches require someone to open the temperature control dial to change the setting, while automatic limit switches will change the setting automatically based on the temperature detected.

How to Test a Limit Switch on a Gas Furnace?

Furnace limit switches are usually used in gas furnaces and air conditioners to turn off the furnace or air conditioner when a temperature sensor or switch has been activated. In order to perform a proper test, you need a known working sensor or switch to verify its functionality.

You can then use your furnace limit switch tester to perform an internal combustion engine (ICE) burnout test, which simulates the operation of the control device before it turns off the system.

To test a manual limit switch, turn on the furnace and wait until it reaches the desired temperature. Then, open the temperature control dial to the lowest setting and hold it there for at least three seconds. If the limit switch has been activated, you will hear a clicking sound as it changes settings.

To do a furnace limit switch test on an automatic switch, mount the sensor inside of your furnace or air conditioner and open the thermostat to set the desired temperature.

 Common Problems with Limit Switches

One of the most common problems with limit switches is that they can break down. This can lead to hazardous situations if the switch is not properly reset.

Also, as your furnace ages, the sensor on the furnace fan limit switch starts having problems. This might be caused by easy-to-fix issues like the accumulation of grime or dirt. 

Therefore, to fix the sensor, all you need is to properly clean off all the residue or materials covering the limit switch that might lead to the furnace not functioning properly. 

One way to know that the limit switch has a problem is if your furnace keeps going on and off throughout the day. Usually, when the blower is overheated, it trips the limit switch causing the furnace to shut off even before the cycle is over.

A furnace limit switch can also be affected by numerous other problems, including faulty gas valves, circuits, and even the thermostat. Also, if you’ve not checked the air filter for a while, you certainly should do. This is because a clogged air filter easily causes your furnace’s plenum to reach unsafe temperatures, making the limit switch to deactivate the furnace.

How to Test Furnace Limit Switch

If the internal temperature is either too high or too low, whether due to a flame failure or anything that disturbs the normal air flow through the unit, your furnace limit switch will do its job and trip.

For your furnace limit switches to work properly, there has to be an absolute minimum air leakage down into the plenum vacuum lines on those switches. If the plenum temperatures reach too high, the limit switch turns it off.

You might also notice that the blower keeps running constantly. If this is the case, the limit switch might have a problem and might need adjustment or replacement. However, you should note that even a wrong thermostat setting might lead to a blower running continuously.

How Can you Prevent Limit Switches from Failing?

Limit switches are used to stop the flow of a controlled substance or material. They can fail if they are left open, if the pressure is too high, or if the temperature is too high.

If you want to prevent limit switches from failing, it is important to ensure that they are properly installed and maintained. Additionally, it is important to monitor pressure and temperature levels so that they do not become too high.

What Should You Do If a Limit Switch Fails?

If your furnace limit switch fails, the most logical thing to do is to replace it. Usually, replacing a limit switch requires the work of a professional HVAC technician. However, if you have the know-how, you can also conduct a DIY limit switch replacement yourself.

The process will require you to disconnect and remove the switch first. Secondly, you’ll have to check for continuity. For this reason, you need to know how to use a multimeter.

If the multimeter indicates infinite resistance, know that the switch is bad. From here, you can simply insert the new switch using the mounting screws and reconnect the wires.

If you are doing the repair yourself, ensure that the new limit switch is an exact duplicate of the old switch. This is in terms of temperature, range, and voltage ratings.

But if you think you are not up to the task, don’t risk adding more problems to your system. Contact a professional who will ensure that everything is fixed in a professional way and that your furnace is up and running again.

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