What is Furnace Limit Switch (Common Problems & Replacement Cost)

The humble furnace limit switch. It is present on every furnace, yet most consumers give it little thought, and less attention. Year after year, season after season, it does its job keeping you safe and comfortable. But one day it might stop doing its job. Perhaps the limit switch on your furnace has failed, and that’s why you’ve found this PickHVAC furnace troubleshooting FAQ guide.

Following is everything you ever wanted to know about your furnace limit switch, and more. We will help you locate it and discuss how to replace it if necessary – It’s a fairly easy DIY repair for a careful, moderately experienced homeowner.

A Glossary of terms is below intended to assist the Do It Yourselfer in finding your way around the inside of your furnace, understanding its operation and carrying out maintenance and repair items like how to replace a furnace limit switch – discussed below.

The furnace limit switch is also known as a high limit switch, fan limit switch, high temperature limit switch, or simply, limit switch. This article will refer to the part as a “limit switch.”

What the Limit Switch Does – Safety and Comfort in One Switch

The furnace high limit switch helps to ensure your safety and the comfort of your home. Pretty handy for a part that costs as little as $4.

It senses the furnace temperature in the plenum, and tells the blower fan to turn on and off as needed, according to pre-set temperature settings. It can also shut down the furnace is it overheats due, for example, to a very dirty air filter restricting airflow, so the created heat can’t get out of the furnace. That’s a very useful safety feature.

When the thermostat calls for heat, the limit switch causes the furnace burners to ignite and begin to warm the furnace as super-hot combustion gases travel through the heat exchanger on their way out of the house.

Comfort: At this point, if the furnace blowers and furnace burners were to come on at the same time, the system would pump cold air through the house. Air in the plenum isn’t hot enough yet and must be heated, so the limit switch keeps the blower fan off. This optimizes indoor comfort.

When air in the plenum reaches the high limit setting on the limit switch, the switch signals the control board to start the blower fan, which circulates air across the heat exchanger for warming, then on to the rest of the house through ductwork. Cool air simultaneously pulled from the house returns through air intakes, and circulates into the furnace to be heated. We talk about this as if it is a two-part process – warm air is pushed out, and cool air is pulled in. However, this is really continuous circulation in a forced air heating system.

When the thermostat reports the indoors temperature has reached its designated setting, the furnace burners turn off. The fan blower stays on briefly to distribute the heat still stored in the heat exchanger. When the limit switch senses that the plenum air supply has reached its lower temperature setting, the limit switch shuts off the blower fan so cool air won’t circulate through the ducts.

Safety: As a safety device, the role of a limit switch is to shut off the gas supply to the furnace burners, to prevent furnace and heat exchanger overheating.

The limit switch shuts down the furnace burners if the heat exchanger gets too hot, which can crack heat exchanger pipes and ruin the furnace. And when a heat exchanger cracks, deadly carbon monoxide can and will leak from it into the air in your home.

Overheating commonly happens for these reasons:

1. The blower fan is not working right, so heated air isn’t getting forced out – and cool air isn’t being pulled in
2. The furnace filter is dirty – really dirty – so that it is restricting air flow through both the furnace and the heat exchanger, causing heat exchanger cooling failure. Improper airflow causes the heat exchanger to overheat repeatedly, and excessively wears on the limit switch, eventually causing malfunction.

What does a limit switch look like? Where is the Furnace Limit Switch?

The furnace limit switch is attached to the furnace plenum inside the cabinet.

You will have to take off the furnace cabinet cover or door to locate it. This diagram shows what most of them look like and gives you an idea where to find it. Note that brands differ a little.

It features a long temperature sensor probe attached to a mounting plate. The mounting plate is fastened outside of the furnace plenum, but inside the furnace cover, and the probe goes through the furnace housing into the plenum. The mounting plate receives two control wires for the blower fan and gas valve. Location of the limit switch varies by make and model, but is usually attached to the plenum above the combustion chamber and/or heat exchanger.

Furnace Limit Switch Troubleshooting

Troubleshooting the most common limit switch problems takes knowing what to look for.

1. Furnace Blower Fan Won’t Turn Off

A common issue with the limit switch is when the furnace blower fan continues running, even after the burners have turned off and the air in the plenum has cooled. This problem often occurs because the limit switch is stuck in the “on” position, which can prevent the blower from turning off as it should. The likely causes are:

• Stuck or malfunctioning limit switch
• Dirty furnace filter causing improper airflow
• Damaged fan limit switch that no longer properly regulates the fan’s operation

2. Furnace Won’t Turn On

If your furnace isn’t turning on at all, it could indicate a failure of the limit switch, particularly when the furnace is in the “off” position. If the system detects excessive heat or other problems, the limit switch may prevent the furnace from igniting as a safety precaution. This failure typically requires professional servicing or replacement.

3. Furnace Shuts Off Due to Overheating

Sometimes, the limit switch may correctly detect an overheating issue, shutting the furnace down to prevent further damage. However, if this happens repeatedly, it could signal an underlying problem:

• Clogged or dirty air filter reducing airflow and causing the heat exchanger to overheat
• Faulty blower fan that isn’t circulating air properly
• Improperly sized furnace that struggles to meet the heating demands of your home

4. Furnace Limit Switch Failure

As your furnace ages, the limit switch can become worn out and fail to function properly. If you notice a persistent failure to regulate temperature or a constant shutdown due to overheating, it may be time to replace the limit switch.

Furnace Limit Switch Maintenance

Regular maintenance is essential to ensure the furnace limit switch operates effectively. Here’s how to maintain your limit switch:

1. Annual Furnace Inspection: Schedule a professional furnace tune-up every year to inspect the limit switch, ensure the air flow is clear, and replace any parts that are showing wear.
2. Check the Air Filter: A clogged or dirty furnace filter is one of the primary causes of overheating. Regularly replace or clean your filter to ensure proper airflow and prevent strain on the limit switch.
3. Clear the Ductwork: Blocked ducts or vents can also cause poor airflow, leading to overheating. Make sure your ducts are clean and free of debris to avoid putting additional stress on your furnace.

Do It Yourself Limit Switch Replacement

If you’re comfortable with basic DIY repairs, replacing the furnace limit switch is relatively simple and affordable. Follow these steps:

Test the Furnace: Once the new limit switch is in place, restore power to the furnace and turn it on to check if it operates correctly.

Turn Off the Power: Before doing any work, turn off the power to the furnace at the breaker box to ensure your safety.

Locate the Limit Switch: Open the furnace door and locate the limit switch. You’ll likely find it attached to the plenum near the heat exchanger.

Test the Switch: Use a multimeter to check for continuity. A limit switch with no continuity (infinite resistance) is bad and needs replacing.

Remove the Old Limit Switch: Unscrew the limit switch mounting plate and disconnect the control wires.

Install the New Limit Switch: Install the new limit switch by connecting the control wires to the appropriate terminals. Make sure it matches the part you removed in both model and specifications.

Here’s a handy find/remove/replace limit switch video that is less than 4 minutes long.

Furnace Limit Switch Replacement Cost

The cost to replace a furnace limit switch is relatively low, especially if you choose to handle the replacement yourself. Here’s a breakdown:

Professional installation: If you hire a technician to replace the limit switch, labor costs typically range from $100 to $300, depending on your location and the complexity of the job.

Cost of a new limit switch: Between $4 and $25, depending on the manufacturer and model.

Common Causes of Furnace Limit Switch Failure

Understanding what causes a furnace limit switch to fail can help you prevent issues before they occur. Here are some common culprits behind limit switch malfunctions:

1. Dirty Air Filters: A clogged filter restricts airflow, leading to overheating in the heat exchanger. This can force the limit switch to activate prematurely, or even cause it to fail after prolonged exposure to high temperatures.
2. Malfunctioning Blower Fan: If the blower fan isn’t circulating air properly, heat buildup can occur in the furnace, which could overwork the limit switch. A faulty motor or damaged fan blades can result in inefficient heating and strain on the switch.
3. Improper Furnace Size: If your furnace is too large or too small for your home, it may be working harder than necessary, which can lead to overheating and stress on the limit switch. This can lead to frequent shutdowns or burner failure.
4. Overheating Due to Blocked Vents: Blocked vents can prevent the hot air from circulating effectively throughout the home, causing the furnace to overheat and triggering the limit switch. Always ensure vents are clear to maintain optimal airflow.

Can a Furnace Limit Switch be bypassed?

Can you bypass furnace limit switch? Yes – but we do not recommend it.

Here is a straightforward response from an HVAC technician: “Why do you need to know how to bypass anything in a furnace? Planning to burn a house down? Bypassing most things in a home heating system would create a very real and very dangerous fire and safety hazard.”

There is no good reason for a consumer to bypass a furnace limit switch. Furnace switches are reliable, inexpensive to replace, and are designed for protection of the consumer. Bypassing a furnace limit switch or any home heating component can create dangerous safety and fire hazards. In limited instances, HVAC professionals may use the technique to isolate or diagnose specific system problems.

Glossary of Furnace and Furnace Limit Switch

These terms will assist you in finding, diagnosing and replacing a furnace limit switch.

Blower Fan: When engaged by the limit switch, the furnace blower fan circulates cool air across the heat exchanger and through the ductwork distribution system; the fan simultaneously circulates cool air from the house, which is received through air intakes.

Combustion Chamber: The part of the furnace where the gas is burned to generate heat.

Continuity (electrical): Testing of electrical current to determine resistance, and detect (limit switch) malfunction.

Forced Air: A blower system to “force” heated or cooled air through a house using ductwork. Alternative systems include hot water radiators, electric baseboards, and wood stoves and pellet stoves.

Furnace Filter: A replaceable/recyclable/disposable fiberglass filter that cleans the air going into the furnace for heating circulation.

Heat Exchanger: Also called the air exchanger, cool air is blown across the heat exchanger, where the air is warmed. Warm air is then blown through the plenum and ductwork.

High Limit (and Low Limit) Setting: The limit switch may shut down the furnace for repeatedly exceeding the dangerous “high limit” or “low limit” settings, which may indicate any of several possible part or system failures.

Multimeter (also, multi-meter): Measures electrical current, voltage, and resistance. It can be used to determine whether the limit switch is faulty.

Plenum: The sheet metal ductwork encasement above the heat exchanger, the plenum holds air for warming and circulation through the heating system, and serves as a central manifold for other ductwork.

Temperature Sensor Probe: The part of the limit switch that extends through the furnace housing and into the plenum to measure the air temperature, which determines subsequent actions of the limit switch.

Thermostat: Senses the temperature of a heating or cooling system, and maintains the system temperature near a desired level.