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What is a Furnace High-Limit Switch? Benefits & Troubleshooting

Furnace Limit SwitchWhat is a High-Temperature Limit Switch?

A high-temperature limit switch—also known as a fan limit switch or a limit switch—is an important piece in a furnace. Essentially, it senses the temperature of the furnace and tells the blower fan to turn on or off at the appropriate times, so that it can properly circulate air to your home. If it senses the furnace begins to overheat, it will shut the furnace off.

The high-temperature limit switch is a long temperature sensor probe connected to a mounting plate. This probe extends through the wall of the furnace; the plate is fastened to the outside of the furnace.

Where is a Limit Switch Located in a Furnace?

The high-temperature limit switch is typically located in the hot-air supply plenum which is above the furnace’s combustion chamber or heat exchanger. However, it varies with the furnace model. If you can’t find it, knowing your furnace’s model will help you do some online research to determine where it is.

What is the Purpose of a High-Temperature Limit Switch?

Its role is to keep the furnace operating safely and at the desired settings. It is what enables air from the furnace to warm up your home. The limit switch reads the temperature of the plenum, and once it hits a certain temperature, it will send a signal to the blower to begin delivering air throughout your home.

Without the limit switch, a furnace could overheat and possibly get damaged—or it may force itself into a “hard shutdown” mode and won’t turn on until it is serviced. If the limit switch is damaged, your house may not even heat up because the blower never received the signal to begin circulating warm air.

Furnace Limit SwitchHow Can Limited Airflow Overheat the Heat Exchanger?

Limited airflow in a furnace can be damaging to a heat exchanger. Airflow issues arise from dirty filters or a blocked vent; these things prevent hot air from escaping the furnace and lead to overheating. Not only will the heat exchanger then be overheated, but it could also begin developing stress cracks due to the overheating.

Read More: What is a Furnace Exhaust Pipe? Tips & How to Replace 

How Can Poor Airflow Cause Damage to the Limit Switch?

Since limited airflow causes an overheated heat exchanger, limited airflow also damages a limit switch. An overheated heat exchanger puts the high-temperature limit switch through premature wear and tear and can eventually cause malfunction.

How Can a Malfunctioning Heat Exchanger Negatively Impact the Heating Cycle?

Regardless of the cause, a malfunctioning heat exchanger will negatively impact your furnace’s heating cycle.

Because an overheated heat exchanger and a malfunctioning limit switch typically go hand in hand, it will result in issues in the furnace and prevent your house from being toasty and warm. What happens is that the furnace does not get the proper signals to either start or stop blowing hot air. This causes erratic heating, or houses that are too cold or too hot.

How to Replace a High-Temperature Limit Switch

Before anything, make sure that you know your furnace’s model, serial number, and voltage ratings. This can be found on the rating plate of your furnace. It is important to know this information so you know what kind of limit switch and parts you may need. Here are the steps to replacing a high-temperature limit switch:

1.) Shut the Power Off

For safety reasons, the first thing you should do when replacing a limit switch is to turn off the power to the furnace. You can either turn the furnace service switch off or the circuit breaker.

2.) Remove and Replace

Using a flat-head screwdriver, release the wires from the terminal clamps. Remove the old limit switch by removing the screws that secure the limit switch to the furnace plenum. Pull the limit switch straight out. Next, put the new limit switch into the spot the old one was. Rescrew the limit switch in and rewire it, making sure the proper wires go to the proper terminals.

Read More: Signs It’s Time to Buy a Water Heater Replacement

How Does a Furnace Work? 4 Main Components & AFUE RatingsHow to Install Modern Limit Switches

Installing or replacing more modern limit switches tends to be easier because there is less wiring to worry about. Usually, there will only be two wires coming off of the limit switch and only 2 screws holding it in place. They also have spade terminals, which are easier to work with because it isn’t necessary to remove the whole terminal just to rewire it.

How to Install Old Limit Switches

Old limit switches tend to be trickier. They probably have more than two screws holding it in, and three wires to take care of. Additionally, there may be a tab on the switch that might have to be snapped off, depending on the installation instructions of the type of furnace. If your limit switch is older, it is probably best to call a reliable professional to take care of it.

Different Types of High-Temperature Limit Switches

The type of limit switch you have depends on the type of furnace in your home. It is important to know which one is in your furnace to know how it operates.

Electro-Mechanical Limit Switch

These limit switches have a circular dial with a mechanical low and high set point that is able to be adjusted. On one side they have 120 volts for the furnace fan and the other side will have 24 volts for the gas valve.

Solid State Limit Switch

This type of limit switch also features a circular design but is smaller. A cool feature in these is that they have a reset button. Instead of replacing it with a new one, sometimes all you have to do is check the reset button.

Furnace Heating CyclesFurnace Repair & Installation Services

No matter the issue with your limit switch, the highly experienced technicians at ServiceMark will solve the problem. We specialize in all things furnace-related, so we’ll get the job done properly. From furnace repair to heater installation, we’ll handle it so you can enjoy a warm home in no time. Give our team of licensed technicians a call by phone at (302) 367-7915 to schedule an appointment.

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