Why does the furnace switch keep tripping? This guide explains high limit switch tripping – what that really means, causes of the problem and solutions with costs. So:
About the Furnace High Limit Switch
Like many kinds of mechanical equipment, the nature of a furnace is one of potential: Potential for good (heats your house…), and potential for danger (with fire!)
To operate safely in residential homes, certain specific functions are required by law. The high limit switch (also called the ‘temperature,’ ‘high temperature,’ and simply, ‘limit’ switch) is a key component in the safety system of every furnace.
The function of a high limit switch is to turn off the burners and turn on the blower fan in a furnace that has reached an unsafe temperature. When the temperature reduces to normal, the burners will come back on.
Limit switch lockout: In some models, if the furnace overheats and trips the high limit switch several times in a row, the furnace may go into lockout mode and will not turn on until it is serviced and reset. You might try to reset the furnace by turning it off at the furnace or electrical panel and turning it back on after a few minutes.
Heat exchanger damage: An overheated furnace can crack the heat exchanger, which essentially ruins the furnace and could allow carbon monoxide gas into the home. That’s a bad deal. So, if your furnace limit switch keeps tripping, it’s not enough to keep resetting it and thinking you are good to go. The issue needs to be resolved.
Pro Tip: Furnaces operate using lethal electrical voltages and highly flammable gas. If you are uncertain about any procedures described here, do not attempt them yourself; consult a local professional.
How to Know your Limit Switch is Tripping
You will know your high limit switch is engaging or tripping when your furnace “short cycles.” The furnace will turn on normally, run for a little while without producing heat – or without bringing your home’s temperature up to the thermostat set point – and then turn off. The cycle repeats again every few minutes.
Why the Limit Switch Shuts Down the Furnace
The high limit switch is probably turning the furnace off because of one of four reasons…
- The flame sensor rod is dirty
- The heat exchanger is experiencing low air flow, and is overheating
- The limit switch is defective or worn out
- The furnace is too big (uh oh!) – But while the costliest repair, it is the most rare cause of a limit switch shutting off the furnace.
Let’s take these one at a time with solutions and the cost of making the repair or replacement.
Troubleshooting a Flame Sensor
To check the flame sensor rod, locate the high limit switch. The high limit switch is usually attached to a metal plate, which is bolted to the outside of the furnace plenum (the large metal box where air is heated.) You will have to remove the furnace cover to find it. Here is where the limit switch is found.
And here is what most limit switches look like.
Solution: The flame sensor rod is the part of the high limit switch that extends into the plenum to register temperatures. Remove the high limit switch by removing the two screws that hold it to the plenum. If the flame sensor rod is dirty, you can try cleaning it gently with steel wool or sandpaper, or replace the part with an identical new part. Refer to the furnace manual because parts vary by model and manufacturer. If you have concerns about proper high limit switch replacement, consider consulting a local professional.
Pro Tip: Cleaning is OK for the short-term, but the part is so cheap, you might as well replace it. Over time, these sensors wear out, and cleaning won’t help. However, if you want to clean it until you can buy a replacement, do not touch the flame sensor. Hold it by the base, and clean it. Why? Oil from your fingers will be left behind, and it will burn on the sensor and create a hot-spot that will cause it to crack/fail pretty quickly.
DIY Cost: The cost of a flame sensor ranges from about $14 to $35 depending on the style and model. You might as well replace the limit switch while you’re at it. Cost is $12 to $30 depending on the style.
This video shows a slightly different style and gives tips for replacing it.
Pro Cost: In addition to the cost of the parts, a service call for replacement of them will run $80 to $150 for about an hour’s labor, at most. This will bring the total cost of the job to $110 to $225.
Solving Airflow Issues that Trip the Limit Switch
Reduced airflow means heat doesn’t get pushed out of the furnace fast enough, and the furnace overheats. Pretty simple.
Solutions: There are two things a homeowner can do simply and inexpensively to increase air flow. The third solution is costlier.
1. Check and/or replace the furnace filter regularly. The furnace filter removes particles in the air that can cause furnace damage and health hazards. A dirty furnace filter restricts air flow and can cause overheating.
Cost: $3 to $25. Cost is based on the size of the filter, and more importantly, on its effectiveness to remove particles from air. The smaller the particles it traps, the higher the cost. Our Furnace Filter guide has lots of useful information including choosing the right filter for your needs.
2. The second thing a homeowner can do is to open the air supply vents in every room of the house, to help create a maximum flow of cool air into the furnace.
3. A dirty furnace blower will also cause overheating. Blowers will collect dirt over time, which slows down the blower and reduces air flow over the heat exchanger.
Solution: Have your HVAC system cleaned as part of annual maintenance.
Cost: According to the PickHVAC Furnace Tune Up Guide, the average cost is about $115 to $200. Your furnace should be cleaned and tuned each year. It will run more efficiently, break down less – and your warranty might indicate that you must have annual service to keep the warranty in effect.
The Limit Switch Needs Replacement
Over time, high limit switches can wear out and weaken. A faulty high limit switch can trip at lower temperatures, causing short cycling and triggering lockout mode even though the furnace is not actually overheating. As discussed above and shown in this video, this is a DIY job for handy homeowners.
DIY Cost: $12-$30
Pro Cost: $100 – $200
Your Furnace Might Be Too Big!!!
Short cycling may indicate a furnace that is too large for the home. A big furnace in a small home will heat the house too quickly, which causes the furnace to turn on and off too frequently.
Solutions: If short cycling has long been a problem for you, or if you just installed a new furnace, consult a professional for a “Load Calculation,” which will help determine if incorrect furnace size is causing the short cycling.
If the load calculation shows your furnace is oversized by a lot, then you’ve got decisions to make. It might be possible to replace the gas valve on your furnace with one with less capacity. In other words, if you have a 120,000 BTU furnace but an 80,000 or 100,000 BTU model would be better replacing the gas valve with a smaller one.
Load calculation cost: A manual J or similar load calculation costs $100-$150.
Gas valve replacement cost: $335 – $600 depending on the size and brand of the furnace.
Gas furnace replacement cost: According to our most recent Gas Furnace Reviews and Buying Guide, the cost of a new gas furnace installed is about $2,200 to more than $5,000 depending on size, performance, brand and other factors – all discussed in the Guide.
There are two other low-flame causes of short cycling that are less common, but possible.
Short cycling caused by low flame is less common, but possible.
If the gas pressure is too low, there may be not be enough flame to be detected by the flame sensor rod. If the burners light incompletely, there may not be enough flame for the flame sensor rod to detect.
Solutions: Have a furnace technician inspect and test the furnace. It’s possible the gas pressure can be adjusted – or the gas line is the wrong size.
If low flame is a suspected problem, remove and clean the burners. If cleaning the burners doesn’t work, thoroughly examine the heat exchanger, and consider a carbon monoxide check.
Short cycling is sometimes caused by problems with ductwork. Check to make sure inflowing and outflowing vents are not dented or blocked by furniture or other household items. Clean out accumulated dirt, dryer lint, animals/waste/nesting, and other material.
Cost of inspection and adjustment: $90 – $175
Cost of replacing a gas line with the right size: $200 – $450 depending on its length and location.
Cost of furnace cleaning: $115 – $200
Cost of vent cleaning: $150 – $500 or more depending on how far into the ducts the cleaning must go, the number of vents and how dirty they are.
When in doubt, consult a pro! Choose the Free Local Quotes option to get your questions answered, discover the cause of the issue and get it solved at a competitive price.