What is a Fan Limit Switch?
A furnace has two switches, a fan limit switch and a high limit switch; these switches can be separate or in one package called a combination switch.
High limit switch
A high limit control switch will turn off the gas to a furnace if the temperature becomes too high.
For example, if the blower motor stopped running, then no air would move through the unit. The flames would raise the temperature inside the unit and have no place to go getting hotter and hotter. As a safety, the high limit switch turns the gas off when the temperature becomes too high.
Fan control limit switch
The fan switch reads the temperature inside a furnace and, once the temperature becomes warm enough, turns the blower motor on, sending warm air into a home. This is used so that cold air is not blown into a house, waiting for the gas flames to heat up enough before the blower turns on.
Depending on the furnace, these two switches can be separate or together in what is called a combination switch.
Some furnaces will have two or even three high limit switches wired in series and checking the temperature at different points in the furnace.
Things To Check First Before Replacing a Fan Limit Switch
- Furnace Filter
- Blower Motor
A clogged up furnace filter will block airflow into a furnace and raise the temperature inside it. If the temperature inside the unit becomes too high the High limit switch will turn the gas off to the unit. Be sure the filter is good.
The next thing to check is if the blower motor is working or not. If the blower motor is not working and no air is being blown through the unit it will overheat and the high limit switch will turn the gas off.
One way to check this is to go to the thermostat and turn the FAN setting from AUTO to ON. If the blower motor blows air out the vents when manually set to ON then you know that the blower motor itself is good and something else such as the limit switch is causing the problem. If the blower motor doesn’t turn on then that doesn’t mean the motor is bad, other components can cause a blower motor to stop working.
Before testing the limit switch, power off the furnace, wait a few seconds, and power it back on. Some furnaces will try a set number of times to start (Usually three); if it fails, then it will have to be reset with power off before it works again.
Keep in mind other things can also cause this problem; we are simply trying to eliminate the blower motor and filter as possible problems, as these are the most common.
To quickly rule out the thermostat read this post.
Types of Fan Limit Switches
- Electro-Mechanical
- Solid State
There are different types of limit switches; the first is the electro-mechanical switches that have a circular dial with a low mechanical set-point and a high set-point that can be adjusted. With these types of switches, one side will have 120 volts for the fan, and the other side will have 24 volts for the gas valve. To test it, a multi-meter will have to be used to see if there is continuity when it reaches the preset temperatures on the low and high sides.
Another common type is the solid-state high limit switch which is smaller a circular in design. The solid-state switches sometimes have a reset button that should be checked for before replacing. These also should be tested with a multi-meter for continuity. Although not recommended, some service techs in the field will use a jumper wire to jump the connections on these types of limit switches to see if the switch has gone bad. Don’t use a jumper switch unless you know what you are doing.
If both the blower motor and limit switch test is good, then other possible problems may be the contactors, circuit board, gas valve, and possibly the thermostat.
Testing the Fan Limit Switch
Once you know that the blower motor is working and the filter is clean, then the limit switch will need to be tested.
Both switches, Electro-Mechanical or solid-state, can be tested with a multi-meter to see if they are working.
In a multi meters continuity setting, either switch can be tested if they are good or have failed.
Where Can You Buy a Fan Limit Switch?
Fan limit switches can be bought at an HVAC supply store or online.
Some HVAC supply stores do not sell to the public because of the high returns, while others will but at elevated prices.
If you can wait a few days, I would suggest buying from Amazon or eBay as they have a big selection to choose from.
Have you had an issue with a furnace fan limit switch? Let us know your thoughts below.
my furnace keeps running when temperature is met.
Too many lawyers? everyone says don’t adjust hire a professional. BULL! The “pros” I find are dumber than bricks.
Right on the money . Expensive brick too !
E Coot, so true!
My blower is running constant. When I open the cover to the furnace and the safety switch resets as it should when I close it. The burners will then light.
The thermostat seems to work normally
The filter was real dirty and I replaced it
I think the dirty filter has caused the limit switch to fail
My control limit switch sticks as well. I have old Honeywell L4064E1785. I can’t seem to find this old model anywhere. Can anyone recommend a place or alternative part number and brand?
I have a very old Gilbert and Barker furnace the limit switch does not have a tube Probe on it it has like a capillary tube that goes into the plenum can’t find this part anywhere don’t know if there’s a way to use a different type of switch to come out with the same action that the old one did
I know that I need a new fan and limit switch but I’m not sure what to get. Mine has an 11″ insert. Will any of the electromechanical switches on Amazon work as long as they have the 11″ insert? Or are there specific models for a specific furnace?
i have a eb15b limit switch 1burnt out should I change all 3
Hi the auto switch for the furnace wont work the fun keeping noise what i need to do? The furnace working properly the heater working but the fun keep moving and noise 🙁
My furnace fan stays on, even after the furnace has cooled off, until I bang the furnace cover, then it goes off… is that the fan limit switch sticking on?
I just went though the same thing here and after a week of banging on it i called “contractor” who charged me 90 dollars and told me it was a “coincidence” yeah he needs to get a job. I found out that the spring in behind the limit switch can get fouled up and if your lucky will move when you bang on furnace. I changed switch and works better than i remember. Dangerous with chance of explosion so unless mechanically inclined hire professional
Sounds like the fan sequencer. Or a relay on the control board.
My 1978 Moncrief has a honeywell fan limit switch that is the original and still works great.
I love that i can custom tune my blower on and off settings for maximum comfort and efficiency.
I have my high limit at 160 degrees for maximum heat exchanger life
My blower on at 110 degrees
And my blower off at 90 degrees and instead of wasting thousands of BTUs up the flue before the blower kicks on, i get heat from the registers under 30 seconds after burner start.
And my furthest register on the coldest day never delivers less than 85 degree air at blower off.
And as a result, my gas bills are nearly the same as my neighbors that are running the latest “high efficiency”
furnaces.