The furnace limit switch, also famous as just the limit switch or fan limit switch, is one of several safety controls built into your furnace to protect you and your family as well as the furnace itself. Without it, you’re exposed to various dangers, including health risks and house fire.
Unfortunately, like any other mechanical or electrical component, the switch can fail, often without notice. Basic knowledge on how to reset the limit switch can be invaluable in such circumstances.
Read on to learn what can cause limit switch malfunction and the step-by-step process to reset your limit switch. We also provide a few tips on when and how to bypass or jump the limit switch.
What’s a Furnace Limit Switch and What Does it Do?
The furnace limit switch is a simple logic circuit that controls your home’s maximum level of heating. Its primary purpose is to shut down the furnace if overheating is detected in your interiors.
Additionally, the limit switch also turns the blower fan on and off during regular heating cycles to prevent overheating in the first place.
Hot to Tell if Your Furnace Limit Switch is Bad
Limit switches rarely go bad. But when they do, you’ll experience severe challenges running the furnace. The following are some common symptoms of a bad furnace.
Warning Lights on the Furnace’s Diagnostic Dashboard
The first sign that you have a bad limit switch is usually an error code on your furnace. The codes vary depending on the furnace.
However, nearly all furnaces have LED lights on the control panel that blinks severally to signal specific issues.
Your job is to watch out for the lighting LEDs and count the number of times it blinks. Then refer to your owner’s manual to see if the blinking signals a broken limit switch.
Temperature Instability
You may also have a defective limit switch if your furnace has become somewhat unstable lately. Ordinarily, the furnace will keep temperatures within a safe range – above the thermostat setting but within a safe range.
However, the range may begin to swing wildly if the limit switch is failing. For instance, if you set the thermostat to 70°F, as recommended by the DOE, you might find the temperatures swinging past 75°F. This is both wasteful and unhealthy.
Interruptions on Normal Furnace Sequencing
Furnaces have a predefined operational process. When you turn on the switch, the thermostat requests heating, depending on current settings, and then the furnace ignites, lighting the burners that provide heat to warm your rooms.
However, the limit switch must be in perfect shape for the process to go on without interruptions. If the limit switch is open or faulty, the heater may go off even when it’s cold. Or it may cycle off when the thermostat is requesting heat.
Can You Reset a Limit Switch?
Yes, you can reset a bad limit switch – as long as it’s not defective. It’s a straightforward process, as we’ll see shortly.
However, keep in mind that resetting might not fix all limit switches. If the switch is broken beyond redemption, the only solution would be to replace it. Replacing the switch involves disconnecting and removing it and then attaching the new switch.
Meanwhile, resetting the limit switch involves changing the setting on the switch such that the fan picks up as soon as the furnace warms up.
Where is the Reset Button on a Limit Switch?
The reset button on the limit switch is located on a panel underneath the furnace’s heating duct. Heating ducts are typically located in the home’s basement or utility closet. In most cases, you need to open the panel (with a screwdriver) to access the duct.
How to Reset a Furnace Limit Switch – Step-by-Step
If you feel that all your limit switch needs is a reset, not a replacement, the following is a step-by-step guide to reset it.
Step 1: Turn Off the Furnace
Never troubleshoot the furnace without first turning it off. It’s too dangerous. So, begin by shutting off the electric power. Ideally, you want to shut it at the furnace breaker.
Additionally, turn off the gas supply at the gas valve. The supply is off when the lever is perpendicular to the gas line.
Step 2: Locate the Heating Duct
To locate the limit switch, you must first locate the heating ducts on your furnace. The ducts are typically located in the home’s basement or utility closet. The panel holding the limit switch is located underneath the duct.
Step 3: Open the Panel
Typically, you need to open the panel to access the limit switch. Use a screwdriver to remove the screws holding down the panel.
Step 4: Locate the Limit Switch
The limit switch will be at the top of the panel. It’s often a sizeable plate surrounded by a silver or white cover. Lift the cover upwards or pull it off to reveal the switch’s body.
You’ll know you’ve found the limit switch when you see a plate with two metal arms sticking out.
Step 5: Read the Settings Between the Two Arms
The two arms have different functions. The one on the left controls the furnace’s low temperature while the other controls high temperatures. However, you’re not concerned with the arms.
Instead, you’re concerned with the reading between the two arms – the value that controls the furnace’s fan. What’s the reading?
Step 6: Adjust the Setting as Necessary
Best practice requires that the furnace fan kicks on as soon as the furnace warms up. So, ideally, you want the fan to begin to run as soon as the temperatures within the furnace cabinet reach 105°F. To this end, we recommend setting the central component to between 105°F and 115°F.
Step 7: Set the Lower Temperature Limit
As we saw earlier, the arm that controls the lower temperature setting is to the left. We recommend setting it to around 90°F.
This way, when the furnace reaches 90°F, the left arm will signal it to start generating more heat to warm your home.
Keep in mind that the warm air from the furnace is about 20°F cooler when it exits the ducts in your home. So, effectively, you’re asking the furnace to keep your home above 70°F.
Step 8: Set the Higher Temperature Limit
The ideal high-temperature limit varies from one application to the next. However, we recommend a value of around 200°F.
When the furnace reaches this temperature, the right arm will request it stop producing heat further. So, the unit will cycle off and let the blower fan circulate the warm air already within your home.
Step 9: Set the Furnace to Automatic Mode
Many people tend to forget this part. Although you can also switch the temperatures manually to keep your home within the selected range, it’s much more convenient to automatically let the furnace handle it. If you push the white socket in, your settings will be cleared.
Step 10: Close the Panel and Finish Up
You’ve successfully reset your furnace limit switch. Now, return the panel and screw it back into place. Then turn the gas supply back on and switch on the furnace power.
You can check the video below to help you through the process
Can I Bypass a Limit Switch?
Yes, you can bypass the furnace limit switch, though we don’t recommend it. Bypassing the limit switch creates a massive fire risk as you’d essentially be running the furnace with no regard for potential overheating. What if the home becomes too hot?
Nevertheless, you can bypass the limit switch when testing the switch. You can also bypass it temporarily to troubleshoot other parts of the furnace.
First, turn off the furnace (both the electricity and gas supply). Then take out the nuts holding the service cover and remove the cover. Now, unclip the two wires from the metal contacts on the side of the pressure switch and connect them directly.
When you do this, power will flow directly from the first wire to the next without going through the limit switch. So, your furnace should continue to work even if the limit switch is broken.
How to Jump a Limit Switch
Jumping is just another word for “bypassing.” To jump your limit switch, disconnect the two wires attaching to the switch and connect them directly.
Alternatively, use a small piece of wire to connect the two wires. This eliminates the limit switch from the equation. A wire with two alligator clips is ideal.
Bypassing or jumping the limit switch can be an excellent way to test the switch if it refuses to reset. If the furnace doesn’t run in standard setup but functionals perfectly when you bypass/jump the switch, the switch is defective. If you’ve tried to reset it in vain, the only solution is to replace it.
How Much Does it Cost to Replace a Furnace Limit Switch?
Furnace limit switches cost between $5 and $25. Meanwhile, professional servicing costs $60 to $200, depending on the type of furnace and your geographic location. So, you should be prepared with at least $65 if you prefer a professional replacement.
Summary
Furnace limit switches are a critical part of the modern furnace. They protect you and your family from overheating and are also responsible for keeping the furnace ON when temperatures dip dangerously low.
Therefore, it helps to arm yourself with basic knowledge on limit switches, especially how to reset the switch. This way, you can troubleshoot and even fix your furnace personally if you encounter an issue in the middle of the night.