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A furnace high-limit switch that opens and closes frequently clearly indicates something is wrong with the unit or its safety mechanism. It may fail to heat your home consistently and can even mean your house is at risk of catching fire. So, why does a furnace high-limit switch keep tripping?
A high-limit switch will keep tripping due to a dirty flame sensor, short cycling, obstructed airflow, or a defective switch. These lead to overheating within the gas furnace that triggers tripping as a safety measure to prevent fires. Clean and replace dirty and worn-out parts to fix the problem.
How the Furnace High Limit Switch Works
A furnace high-limit switch is an in-built safety mechanism that controls the opening and closing of the heating system. The switch will automatically shut off (close) when the furnace reaches an unsafe operating temperature.
The limit switch in most residential homes has a temperature sensor that’s usually connected to the blower motor. It, therefore, performs two crucial functions as follows:
The high-limit switch controls the on-and-off rhythm of the blower fan, which helps maintain the temperature in your house. The temperature sensor detects if the air supply is too cold and signals the blower fans to turn when the air has been heated, ready to be supplied into your home.
The temperature sensor on the high-limit switch also detects if the air supply is too hot, which means the furnace is overheating. The response is to shut down the system as a safety measure.
It may be normal for the switch to close once in a while, but a high-limit switch that trips frequently is a sign something isn’t working correctly. Apart from shutting off the blower fans, the high-limit switch may also lock the whole system and stop it from operating until the whole unit cools down to safe temperatures.
What causes the furnace high limit switch to keep tripping?
A tripping switch in a gas furnace is a safety mechanism, but when it is erratic, it could be a sign some components of the furnace are not working correctly. Typically, tripping is because of one of three reasons: an inefficient furnace filter, faulty wiring or installation, or not enough airflow coming into the furnace.
Here’s what causes the furnace high limit switch to keep tripping:
1. Short cycling
The high-limit switch is designed to shut down the furnace if it senses something has gone wrong. Short cycling, which means an excessively rapid fluctuation between heating and cooling cycles, can cause tripping of the HLS sooner than expected and disrupt your home’s living temperature.
Short cycling occurs when either too much heat or not enough heat builds up in the air ducts during each cycle. This typically happens with furnaces that are bigger than required for a house. The result is overheating and overcooling within a short period, forcing the high-limit switch to close and open within very short intervals, leading to tripping.
You might have installed a 100,000 BTU furnace in a house that needs an 80,000 BTU unit. To help you determine why your high-limit switch trips too often, you need to have a load calculation done.
Most contractors use the Manuel J as a basis for residential load calculations for HVAC systems.
2. Dirty flame sensor rod
The flame sensor rod is a metal rod that regulates the draft for gas furnaces. When the sensor is dirty, it may fail to detect the temperature of the air flowing in the furnace. The result is erratic responses and frequent tripping.
If you haven’t serviced your HVAC system for more than 2 years, the HLS tripping problems are likely caused by a dirty flame sensor rod from the buildup of dirt, soot, and even dust.
You might be able to clean the flame sensor if you can open the access panel and identify this part. However, if you’re unsure about what you’re doing, I recommend getting a technician to help with the process.
3. Airflow issues
A dirty or incorrectly installed air filter sometimes causes an overheating furnace. In turn, restricted airflow leads to erratic tripping and opening of the high-limit switch in your furnace.
Dirty or clogged air passages in the furnace can restrict its ability to sense when it should operate, and so can cause problems with your furnace’s high limit switch.
If you are experiencing frequent tripping of your HLS, make sure to clean out any dust from around the system. It may be necessary to have an experienced technician do this if you cannot access certain areas on your own where there is likely more buildup than just what’s visible at eye level.
4. Overheating
Overheating is common in old HVAC systems and those that are rarely serviced. Dirty filters, a caked sensor rod, dirty blowers, and a faulty pilot light are all reasons that can lead to overheating that can lead to the high limit switch tripping so often.
They are also significant signs that your home heating system has not been serviced for a long time. If overheating is the issue, your furnace needs to be serviced as soon as possible to improve efficiency and cut costs.
If you have an older unit in your home, it may take even less time for overheating issues to arise. Regular maintenance of your heating system will help prevent overheating and tripping from becoming too big of a problem and potentially costly by ensuring enough fresh air gets into the heater through its ducts while minimizing potential fires.
5. Faulty limit switch (worn out)
The limit switch is a safety device that monitors the temperature in your furnace. Once it reaches a certain point, it will activate and turn off power to the heating system before an overheating scenario can develop into a hazardous residential fire.
Limit switches wear out over time and can even become defective, especially when other parts of the gas furnace are faulty too. Defects usually come from overuse – where the HLS trips too often – and lack of maintenance.
A faulty limit switch can trip prematurely without any clear warning signs of heat build-up within its ducts. Sometimes false alarms within the HVAC system can make the limit switch close erratically.
Pro tip: One way of knowing you have a worn-out limit switch is when it trips even when there’s a low temperature in the house yet you’ve set your thermostat to “AUTO” and not just “ON.”
Note that when the high-limit switch trips several times in quick succession, it will lock the whole heating system in lockout mode. It means the furnace is overheating to a severe level that can easily start a fire in your home.
Allow the furnace to cool off completely before turning it back on. If it won’t turn on, get a professional to service your gas furnace and reset it. That’s when it will start heating your home again.
How to Fix a Limit Switch that Keeps Tripping
The proper fix for a tripping high-limit switch depends on the actual cause. It is important to diagnose the system before attempting any repairs or fixes I’ve discussed below.
Here’s how to fix a high-limit switch that keeps tripping:
1. Clean the flame sensor rod
90% of the time, a gas furnace cycling on and off is a sign the flame sensor rod is dirty and needs cleaning. If you can open the access panel, you can clean the sensor yourself, but if you don’t know how to access it, I’d suggest you get a technician to help you with the process.
Once you remove the flame sensor rod, use an abrasive material, such as very fine sandpaper, to clean off any debris and caking of the flame sensor’s head. Please do not touch the sensor with your hands, as they can leave the oil on it and make it fail when re-install.
When cleaning the flame sensor, please do not be too aggressive, as this can shorten its lifespan. I recommend cleaning this rod 1-2 times yearly to keep it in good condition and working as expected.
2. Replace the high-limit switch
If cleaning the flame sensor didn’t solve the problem, it means the problem lies elsewhere – probably an old and worn-out high-limit switch. The proper fix, therefore, is to replace the limit switch with a new one.
The high-limit switch on a gas furnace is located at or near the burner, with two wires coming out of it and leading back into the panel box. It will be mounted either vertically or horizontally, depending on how you look at it. The new one should come with instructions from its manufacturer to help you with the replacement.
Follow them carefully, as these units need special installation procedures that could void your warranty if done wrong. I recommend replacing this part once every five years for safety reasons and peace of mind when running your system next winter season.
Here’s a video on how to replace your gas furnace high-limit switch:
3. Clean the dirty filters and fans
The dirty filters and fans are the following parts of your gas furnace you want to clean. We’ll start with the filter at the bottom of your furnace cabinet or in a compartment near it.
If this filter is clean, chances are it is not the cause of the high-limit switch tripping and quickly turning your gas furnace on and off. If it has some visible dust, remove it and vacuum the dust out of it.
You can clean the air filter regularly to prevent the accumulation of dust and other toxins in the filter. A clogged filter’s danger isn’t limited to causing overheating in your gas furnace; it can obstruct airflow or let dirty air into your home.
The next part of cleaning is the blower fan. It also plays a vital role in keeping the air supply healthy within your HVAC system constant. Blower fans can stop working due to the accumulation of dirt.
Simply clean them and check if they’re rotating as expected. If faulty, you might want to have them serviced or even replaced.
4. Match the HVAC BTU to the house size
The final fix for a furnace high-limit switch that keeps tripping is to match the HVAC BTU size with your heating and cooling needs.
If you have an older home, chances are you need more BTUs than newer homes because they don’t insulate as well. To find out what size of the system should be installed to properly heat or cool your house, have a professional do a load calculation for you.
The total square footage of your house should be well-heated during winter to keep your house comfortable. The calculation or estimate will tell you how large of a gas furnace is needed based on the sq-ft calculation. A larger wattage will help keep it running longer without cycling on and off – which means more comfort, but it can also mean inefficient energy usage.
5. Service the entire HVAC system
It is crucial to service your entire HVAC system yearly. This service should consist of cleaning, inspecting, and testing the components to ensure they are working correctly.
A professional will inspect the gas furnace for any abnormalities or signs that it may be on its way out soon to prevent issues such as tripping high limit switches and other problems.
If you’ve made all these fixes, but your high-limit switch keeps tripping, replace it with a new one, which is most likely worn out from years of use and needs replacement. After installing this part again, check each component in case another problem lurks elsewhere. A thorough inspection can save your home from an energy crisis when winter hits, so do not skimp on this key maintenance task.
Pro tip: If the high-limit switch keeps opening and closing, you don’t want to switch it back on every time it trips. Some parts, such as the heat exchanger, when damaged, can cause the release of poisonous carbon monoxide into your home. I highly recommend getting a professional to check, service, and repair the gas furnace before using it again.
References
- Christopher J. Brown et al, CPSC: Furnace CO Emissions Under Normal And Compromised Vent Conditions
- L. Brand and W. Rose, US Department of Energy: Measure Guideline: High Efficiency Natural Gas Furnaces
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.