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How to Locate & Reset Your Furnace Limit Switch

In this article, I will guide you through how to safely reset a furnace limit switch, troubleshoot common problems, and determine when it's time to call in a professional.

How to Reset a Furnace Limit Switch

Here's a quick overview of the steps we'll cover for resetting your furnace limit switch:

  1. Locate the Limit Switch: We'll show you where to find it.
  2. Inspect for Issues: Look for any obvious problems before resetting.
  3. Press the Reset Button: A simple push (sometimes a hold) can do the trick.
  4. Test and Monitor: Make sure everything's working correctly.

Let's dive into the details and get your furnace running smoothly again!

Table of Contents

What is a Furnace Limit Switch and Why Does It Trip?

The limit switch is a small, usually manually resettable switch that monitors the temperature inside your furnace. If the temperature inside the furnace gets too high, the limit switch trips, cutting off the power to the burners and preventing further heating.

Limit Switch Types

There are several types of limit switches, including the high limit switch, rollout switch, and air pressure switch.

  • The high-limit switch is the most common type and responds directly to excessive heat.
  • The rollout switch is located near the burners and is designed to detect flames that roll out of the burner compartment.
  • The air pressure switch, also known as a sail switch, ensures proper airflow within the furnace. If airflow is restricted, this switch can also shut down the system.

Why Your Furnace Limit Switch Has Tripped

A tripped limit switch isn't necessarily a sign of a major problem, but it's definitely a signal that something isn't right. Here are some common reasons why a limit switch might trip:

  • Overheating: A dirty air filter, blocked vents, or a malfunctioning blower motor can all restrict airflow and cause the furnace to overheat.
  • Faulty Blower Motor: If the blower motor isn't working correctly, it can't circulate air properly, leading to heat buildup and a tripped limit switch.
  • Blocked Vents or Registers: Closed vents or registers can restrict airflow and cause the furnace to overheat.
  • Flame Rollout (for rollout switches): This dangerous situation can be caused by delayed ignition, a dirty burner, or other issues within the burner compartment.
  • Restricted Air Intake or Exhaust: Problems with the furnace's air intake or exhaust can also disrupt airflow and cause the limit switch to trip.
  • Faulty Limit Switch: In some cases, the limit switch itself may be faulty and need to be replaced.

Understanding what causes a limit switch to trip is essential for troubleshooting the problem and preventing it from happening again.

Finding the Limit Switch on Your Furnace

Before you can reset the limit switch, you'll need to locate it. Its location can vary depending on the make and model of your furnace. However, there are a few common places to check.

Here's a general guide to help you find the limit switch:

Near the Blower Motor

Often, the limit switch is located near the blower motor compartment.

  • Look for a small switch with a red or sometimes white reset button.
  • It may be attached directly to the blower housing or nearby.

On the Burner Assembly

Another common location is on the burner assembly itself, usually close to the burners.

  • Again, look for a small switch with a reset button.
  • The rollout switch, a specific type of limit switch, will always be located near the burners.

Inside the Furnace Plenum

The plenum is the large metal box that houses the heat exchanger. In some furnaces, the limit switch might be located inside the plenum, near the heat exchanger.

Be extra cautious when accessing this area, as it can still be hot even after the furnace has been turned off for a while.

Near the Vent Pipe

Less common, but still possible, is a location near the vent pipe. Check the area where the vent pipe connects to the furnace.

Once you've located the limit switch, take note of its position and the type of reset button it has. This will prepare you for the next step: resetting the switch.

Step-by-Step Guide to Resetting the Limit Switch

Now that you've located the limit switch, you're ready to reset it.

Here's a step-by-step guide to resetting your furnace's limit switch:

First Inspect the Area

Before resetting the switch, visually inspect the area around the limit switch.

Look for any obvious signs of damage, such as:

  • loose wiring
  • burnt components
  • or blockages

If you notice anything unusual, do not attempt to reset the switch yourself. Call a qualified HVAC technician.

Resetting the Limit Switch

  1. Let the Furnace Cool: If the furnace was recently running, allow it to cool down for at least 30 minutes before touching any components.
  2. Press the Reset Button: The reset button is usually red and may be a small button or a small switch that you need to flip. Gently press the button. You may hear a faint click.
    • In some cases, you might need to hold the button for a few seconds.
  3. Test the Furnace: Turn up your thermostat and observe the furnace. It should start up normally.

If the limit switch trips again shortly after resetting it, this indicates an underlying problem that needs to be addressed. Do not repeatedly reset the switch. Refer to the next section on troubleshooting or call a professional HVAC technician.

What if the Limit Switch Keeps Tripping?

If your furnace limit switch keeps tripping, it's a clear sign of an underlying issue that needs to be addressed. Do not simply keep resetting the switch. Repeated tripping can damage the switch and, more importantly, indicates a potential safety hazard.

Here are some common reasons why a limit switch might keep tripping and how to troubleshoot them:

Dirty Air Filter

A dirty air filter restricts airflow, causing the furnace to overheat. Check your air filter and replace it if it's dirty. This is the easiest fix and often solves the problem. Aim to change your air filter every 1-3 months, depending on usage and air quality.

Blocked Air Vents or Registers

Closed vents or registers can also restrict airflow.

  • Make sure all your vents and registers are open and unobstructed.
  • Furniture, rugs, or other objects blocking vents can cause overheating.

Overheating

Overheating can be caused by several factors, including:

  • a faulty blower motor
  • restricted airflow
  • or a malfunctioning thermostat

Solution:

  • Check for obstructions around the furnace itself.
  • Ensure the area around the air intake is clear.

If you suspect a problem with the blower motor or thermostat, contact a professional HVAC technician.

Faulty Blower Motor

If the blower motor isn't working correctly, it can't circulate air properly, leading to overheating.

Flame Rollout (for rollout switches)

A dirty burner or other burner-related issues can cause flames to roll out of the burner compartment, tripping the rollout switch.

This is a serious safety concern. Do not attempt to fix this yourself. Contact a qualified HVAC technician immediately.

Restricted Air Intake or Exhaust

Problems with the furnace's air intake or exhaust pipes can also disrupt airflow.

  • Check the intake and exhaust pipes for any obstructions, such as leaves, debris, or bird nests.
  • Ensure the pipes are clear and properly connected.

If you've tried these troubleshooting tips and the limit switch continues to trip, it's time to call in a professional. A qualified HVAC technician can diagnose the problem accurately and perform the necessary repairs.

Don't risk further damage to your furnace or compromise your safety.

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