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A high-limit switch is an essential part of a heating furnace. In conjunction with the thermostat, the switch regulates the temperature of the air being circulated in a home. Most importantly, this switch will shut off the furnace blowers and burners if the unit overheats thus protecting your family and property from the risk of fire.
The high limit switch can send your furnace into a lockout mode if unusual behavior is detected in the system. In most cases, this happens when the limit switch keeps tripping or after you’ve made several attempts to turn on a furnace that is not responding.
Your furnace will remain in the ignition lockout state until servicing and manual reset of the unit are done. However, this may not help if the high limit switch is faulty. A damaged or worn-out limit switch is likely to trigger a false alarm that will lead to a furnace lockout.
CAUTION: When working on your furnace, always disconnect power to avoid electrocution or damage to the property. Further, go through your owner’s manual for specified recommendations regarding your type of furnace.
What to do before you Reset A Furnace Limit Switch
You’ll first need to understand why your furnace has gone into lockout mode. Most modern models have a visual display that will generate an error code. For example, a blinking LED light may help in revealing the exact problem in the component as explained in the user manual.
The next thing is to diagnose and resolve the problem affecting your furnace. This can be done with the help of a professional HVAC technician or by referring to the owner’s manual. But some of the important fixes that can be done include the following:
Solve any airflow issues
Furnaces rely on a steady flow of air for the circulating heat being generated by the burners. If the amount of air is reduced, heat will not be blown out of the furnace fast enough. Eventually, there will be heat buildup that will make your furnace overheat. In response, the high limit switch will stick open.
To improve airflow into the furnace, the following will need to be done:
- Inspect and/or replace the furnace filter regularly. Air filters help in trapping dust, debris, and other forms of dirt from entering the furnace to cause damage. This makes air filters get clogged over time.
- Ensure the high-efficiency furnace’s vents are not obstructed. Any blockage in the air vents caused by leaves, animals, and insects can prevent the flow of air into the unit. Manually inspect and remove any form of blockage in the air vents. Additionally, install a vent screen to prevent future occurrences.
Clean The Flame Sensor Rod
The flame sensor rod is part of the high limit switch which extends into the plenum to register temperatures. It occasionally comes into contact with flame and it’s common for soot to build up on it. This will make your furnace cycle on and off quickly a scenario that will likely send it into lockout mode.
The solution to this problem is to locate the flame sensor rod and clean it. Access the blower compartment and remove the screws holding the rod on the plenum. Use steel wool, or a scouring pad to clean the rod without involving any liquid. Once done, screw it back into its position.
Check For A Bad Limit Switch
Over time a limit switch will wear out just like any other parts in the furnace. This will make it keep tripping at lower fun temperatures or fail to signal the blower fan to turn off when the unit overheats. This can put your family and property at risk of fire and exposure to carbon monoxide gas.
You can tell that your furnace is overheating from the burning smell all over the vents or the loud humming noise from the furnace. If your furnace will not shut off automatically, you should consider turning off the power at the circuit breaker and contact a professional HVAC expert.
How to Reset Furnace Limit Switch
Once the problem affecting your furnace has been identified and corrected, you should reset your furnace to continue enjoying the warmth of your home. In most gas furnaces, this is done by simply shutting off the power supply and turning it back on after 20 or so seconds.
For some models, the controller will unlock the furnace after one or two hours. If this does not work for your furnace, you will have to manually reset the furnace limit switch. Here is how it’s done:
- Begin by shutting off the power to your furnace and/or close the gas valve
- Locate your furnace’s access panel usually on the side of the furnace housing or on the bottom near the return air vent or duct. Use a screwdriver to open it. Some furnaces use push or twist locks.
- In the panel, find a limit switch usually in a white or silver cover. Remove the cover to access the main body of the high limit switch.
- The limit switch has two arms, one controls the furnace’s low temperatures, and the other controls the high temperatures. Between the arms, there is a component controlling the furnace’s fan.
- Now set the temperature on the middle component to approximately 105 to 115 degrees Fahrenheit. This will signal the fun to switch on and off at the right time.
- Next set the left arm to approximately 90 degrees Fahrenheit. This will signal the furnace to stop blowing cold air once it reaches that set temperature.
- Set the right arm of the switch to approximately 200 degrees Fahrenheit. This will signal the furnace to shut off completely once the burner reaches this temperature. (refer to user manual)
- Finally, pull out the white/silver switch to set the furnace into automatic mode and close back the panel. If you happen to push this switch in, then all your settings will be cleared.
How to Replace Furnace Limit Switch
To replace a limit switch, you’ll need a new one, a screwdriver, and a multi-meter. Once you have accessed the limit switch as directed above, disconnect the wires. Next is to test the limit switch using a multi-meter. Set your multi-meter to the lowers ohms of resistance and touch the probes on each terminal.
The multi-meter will show infinite resistance if the limit switch is stuck open or faulty and therefore there is a need to replace it. If the limit switch has continuity (is closed), the multimeter will read zero ohms. You may still need to replace it if it prematurely or falsely shut down the furnace.
Install a new limit switch and secure it in place with screws. Connect back the wires and replace the upper cover. Open the gas valve and/or turn on the power to your furnace. By now it should be running normally.
The heating furnace comes with numerous sensors for enhancing safety in the unit. The high limit switch is one little-known sensor that will shut off the furnace when it overheats. This switch, however, can send your furnace into a lockout mode.
To resolve this problem, you’ll need to service the furnace thoroughly and then try to reset the limit switch manually. If this doesn’t help, you will have to replace the limit switch. We recommend this to be done with the help of an HVAC expert or after you’ve read and understood the owner’s manual.
HVAC tech with over 30 years of experience. Retired and doing repair work on the side around Madison County, AL.