Furnace Limit Switch Troubleshooting [In 10 Easy Steps]

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Nothing is as frustrating as finding your furnace system not working because of a limit switch malfunction. Thus, knowing how to fix it is the thing you'll often wonder about. Well, wonder no more! Because we already have the answer below!

To safely troubleshoot your furnace limit switch, you'll need to:

  1. Turn off your furnace power breaker first.
  2. Find your furnace heating duct and use a screwdriver to open its panel.
  3. Search the limit switch on the top of the heating duct panel.
  4. Open the switch by pulling its body to reveal two metal arms.
  5. See the components settings in the middle of the two arms.
  6. Identify each arm's settings.
  7. Set the components settings temperature to 105 - 115 degrees Fahrenheit.
  8. Change the setting on the left arm to 90 and 200 degrees Fahrenheit on the right.
  9. Put the furnace to an automatic mode setup by pulling out the white switch.
  10. Close the panel using your screwdriver and turn on the furnace power.

That was easy. Continue reading as we dive deeper into what causes your limit switch to trip. We will also touch on some related queries that most people have. With that said, let's jump into this discussion!

Hvac specialist running some diagnostics on the furnace. Furnace Limit Switch Troubleshooting [In 10 Easy Steps]

What Causes Limit Switch Problems?

There are various reasons why a switch "trips" or shuts down the system suddenly, other than the switch itself going defective. A temperature sensor, for example, provides input to the limit switch.

The limit switch can trigger if the sensor becomes dirty, which can happen among HVAC units.

Technician checking the furnace wiring

Airflow problems within the furnace might also cause the switch to trip. Moreover, when furnace filters become clogged, insufficient airflow issues can occur, causing the switch to trip.

This device is vital for the system to maintain and detect unusual temperature changes in the unit. So figuring out what causes this issue and fixing them immediately will help prevent significant damage to your unit.

How Can You Tell If A Limit Switch Is Bad?

You should attach a digital voltmeter and test the limit switch's two networks while the furnace is fully operational. The standard voltage running is less than or equal to 0.1 Volts of Direct Current (VDC).

However, if the voltage had a loss equal to 0.2 VDC or above, your limit switch will likely become bad. The contacts can become damaged, rusted, or interrupted over time.

It's simple to replace or change the old limit switch before it may cause severe damage to the unit.

See this digital voltmeter on Amazon.

See this visual voltmeter tutorial on YouTube:

Signs Of A Broken Limit Switch

  • Your furnace unit blowers run constantly.
  • It will start blowing cold air even though your AC unit isn't switched on.
  • The furnace system is running for a while and then abruptly turns off.

How To Replace A Furnace Switch Limit

You may not test your furnace because the limit switch failure signs are generally obvious. Instead, you may think of replacing the limit switch directly.

To replace the limit switch correctly, you'll need to:

  1. Shut off the power of the furnace unit.
  2. Find the switch location and open the access panel using a screwdriver.
  3. Conduct a Multi-meter test if there's still return current left.
  4. Take a note or have a picture on the switch for reference before disconnecting the wires.
  5. Unscrew the switch and detach it.
  6. Install your new limit switch.
  7. Securely reattached the switch to its mounting plate.
  8. Then cover the panel and turn the power on again.

Check out this limit switch on Amazon.

View this video on replacing a light switch below:

How Much Does Replacing A Limit Switch Cost?

You should only purchase a limit switch on your furnace from the same manufacturer. According to your original equipment maker, this part could cost anything between $4 to $25.

But of course, this may vary based on your brand and unit model.

Can You Reset A Limit Switch?

If you have the latest model, you can try shutting the power off for 30 seconds and then turn it on again to reset the furnace limit switch.

However, if this still doesn't work or the furnace keeps shutting down, it's never been a bad idea to hire a heating specialist to reset the limit switch for you.

Technician using tester to check for voltage on the furnace

How Many Limit Switches Does A Furnace Have? How Do They Work?

Usually, it consists of a high limit switch and the fan control limit switch. You can reach these switches by pulling the cover panel from the furnace.

Additionally, these switches have a sensor probe attached to a mounting plate.

Now that you know the basics, it's essential to understand how limit switches work. We'll discuss some of them below:

High Limit Switch

Controls the furnace unit gas supply; for instance, if a furnace's temperature rises too high, a high limit switch will control the system and shut off the gas supply.

Therefore, if no air or gas goes through the furnace unit, the heat will not be able to rise as a result of overheating the unit.

Fan Control Limit Switch

The fan switch detects the temperature inside a furnace and tells the blower motor to start on, which circulates warm air throughout the house.

A fan switch speaks for its name; when the inside temperature reaches below the setpoint, it will enable and prevent cold air from going through the house.

While gas flames are still working to warm up and turn on.

Common Types Of A Limit Switches

Hvac specialist running some diagnostics on the furnace

Electronics Switches

Electromechanical or electronics switches have a circular dial and adjustable high and low setpoints. These switches can power the fan by 120 and 24 volts for the gas valve.

Solid-State Switches

A solid-state switch limit, which is smaller and circular in layout, is another common type of switch. There are no moving parts in solid-state switches.

Furthermore, an optical detector device uses solid-state parts, including resistors, capacitors, and transistors, to detect and switch objects.

Solid-state devices are more expensive than electromechanical devices because they have longer operating lives.

Are Furnace Limit Switches Interchangeable?

Technician checking the wiring of the furnace

The furnace limit switches are entirely different, although sometimes people mistakenly refer to pressure switches as one, which is not.

In addition, pressure switches are a device mechanism triggered to shut off your furnace when fluid pressure reaches a specified setpoint.

Generally, you can access the pressure limit switch inside the motor chamber, while the limit switch is more obvious to spot as it this placed on top of the furnace panels.

Can You Clean A High Limit Sensor Rod Switch?

You can gently clean the flame sensor rod with a steel wool sponge or sandpaper if it is dirty. The flame sensor rod, which reaches into the plenum to register temperatures, is a part of the high limit switch.

To access the flame sensor rod, you'll need to loosen and remove the two screws that hold the high limit switch to the plenum box.

See this flame sensor rod on Amazon.

What Is Furnace Short Cycling?

Technician going through his checklist before starting the machine

Short-cycling occurs when your furnace switches on and off frequently for short periods, preventing the house from reaching its proper temperature.

To put it another way, the heating and cooling cycles are too short to accomplish the required temperature set point in your furnace system.

A typical house furnace cycles on and off three to eight times each hour, producing seven to twenty minutes of heating cycles.

6 Potential Causes Of Why Short Cycling Happens

  • A damaged igniter.
  • A faulty flame sensor.
  • The air filter is too dirty.
  • The furnace thermostat has gone wrong.
  • Having an Improper size of furnace unit.
  • The furnace flue limiter or vent pipe is damaged.

To Wrap Up

Hvac specialist running some diagnostics on the furnace

Throughout this article, we've learned what causes your furnace to trip the limit switch. You've also learned how to troubleshoot and replace faulty components.

Moreover, we've also discussed how to prevent your furnace unit from facing this possible threat to your HVAC system. We do hope that learning this information will take you on a positive outcome on how you maintain your HVAC unit.

Made it to the end? Well, check out this related helpful post below!

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