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What Does a Furnace Limit Switch Do?
The furnace limit switch either shuts the gas furnace down if the heat exchanger is getting too hot or it cycles the blower fan on and off based on the heat exchanger temperature. Most fan-limit switches do both.
The two types of furnace limit switches that we’ll discuss in this article are the furnace high-limit switch and the furnace fan-limit switch.
What Does a Furnace Fan Limit Switch Look Like?
The furnace high-limit switch resembles a flat, silver rectangle and has two wires connected to it.
The fan-limit switch looks like a black rectangular box with a clock-like dial on the top portion and wires going into the bottom portion.
Where is the Limit Switch on a Furnace?
Both limit switches would be installed in the burner section on the front wall, typically above the burners. Both limit switches have an arm that extends into the heat exchanger section with a temperature sensor on the end of it.
Why Do You Need a Furnace High-Limit Switch?
You need a furnace limit switch to protect your home from the heat exchanger overheating and causing a fire. The furnace blower fan is meant to remove the heat from that section, but the furnace will overheat if there is an issue with the airflow.
Why Do You Need a Furnace Fan-Limit Switch?
A furnace fan-limit switch is needed to do two things: turn the blower fan on once the heat exchanger gets warm enough and protect your home from the furnace overheating and catching fire.
Adjust the Furnace Fan Limit Switch to Control the Blower
The furnace fan limit switch can be adjusted to turn the blower on at different temperatures and trip the furnace off at different temperatures.
Please remember that the manufacturer sets the temperature at which the furnace limit switch should trip the furnace off, and that temperature should not be adjusted.
Common Problems With a Furnace Limit Switch
As the furnace gets older, it is typical for the furnace limit switch to get dirt, grime, and other materials built up on it. If your gas furnace is cycling on and off, it could be due to various issues, ranging from dirt and grime build-up to blower motor issues or a dirty furnace filter.
Any of these issues would cause the furnace limit switch to cycle the furnace on and off before the end of the heating cycle. If this is the case, you’ll need to troubleshoot each of the problems I mentioned.
Other issues that could cause the furnace limit switch to trip would be a faulty gas valve, a hole in the heat exchanger, or a blocked supply or cold air return registers. The first place to check is always your furnace filter, as people often forget to change it regularly.
Furnace Limit Switch vs. Furnace Pressure Switch
They have very similar names with very different use cases. The furnace limit switch protects the furnace from overheating, while the furnace pressure switch protects the home from the release of products of combustion into the home.
Furnace Limit Switch Repair and Replacement Cost
You cannot repair a furnace limit switch unless it’s simply a broken wire leading to it. To replace a defective furnace limit switch, you’ll need the model and the serial number of your furnace.
Then call a local HVAC supplier to get a price on it. The cost is usually around $35-$135 dollars depending on the furnace brand and whether it’s a high-limit switch or a fan switch. The above price does not include labor.
The labor cost to hire a professional HVAC contractor to install your new furnace limit switch will be based on your geographic area. I suspect the hourly rate could range from $75.00 to $300 per hour.
How Do I Know If My Limit Switch is Bad?
To determine if your furnace limit switch is functioning properly or if your furnace has other issues causing it to trip, you’ll need a temperature probe.
First, remove the defective limit switch from the furnace housing, leave the wires connected, and put the temperature probe into the furnace. You will not want to leave the furnace in this condition, as it creates a very hazardous operating condition.
Now, turn the heating system up on the thermostat and wait for it to begin heating. With the furnace running, you’ll be monitoring the temperature probe to see how hot the air temperature gets.
If the furnace temperature exceeds the furnace limit switch setting, you may have an issue with airflow, the air filter, or other internal components.
Can I Replace a Furnace Limit Switch on My Own?
Yes, you can replace the furnace limit switch yourself. However, performing furnace repair on your own can be dangerous as you’re working with combustible gas and electricity, so be careful,l as the entire furnace has gas and electricity throughout.
When to Hire a Furnace Repair Specialist
If your furnace isn’t delivering enough warm air, or if you’re unsure about working safely around gas or electrical components, it’s best to call in the professionals. A malfunctioning furnace limit switch can affect both performance and safety.
For expert diagnosis and repair in the North Okanagan, contact Vernon Air Conditioning, Plumbing & Electrical Services. Our licensed HVAC technicians are here to keep your home warm, safe, and efficient throughout the entire winter season.
