Test and Replace the Fan Limit Switch on a Furnace

By | April 27, 2019

Furnace fan fan limit switch
What is a Fan Limit Switch?
A furnace has two switches called 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 in to 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 deferent points in the furnace.
 
Example Limit Switch On Amazon
Honeywell L4064B2236 Combination Fan and Limit Furnace Control

 
Things To Check First

  • Furnace Filter
  • A clogged up furnace filter will block air flow into a furnace and raise the temperature inside it. If the temperature inside the unit becomes to high the High limit switch will turn the gas off to the unit. Be sure the filter is good.
     

  • Blower Motor
  • 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 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 than that doesn’t mean the motor is bad, other components can cause a blower motor to stop working.
    thermostat blower motor test
     
    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 a 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.
 
Testing The Limit Switch
Once you know the blower motor is working, and filter is clean then the limit switch will need to be tested.

Types of limit switches

  • Electromechanical
  • Old furnace fan electromechanical limit switch
    There are different types of limit switches the first is the electromechanical switches that have a circular dial with a mechanical low set point and high set point that can be adjusted. With these type 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 multimeter will have to be used to see if there is continuity when it reaches the preset temperatures on the low and high side.

     

  • Solid State
  • Furnace high limit switch
    Another common type is the solid-state high limit switch which is smaller an 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 recommend some service techs in the field will use a jumper wire to jumper the connections on these type 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 good than other possible problems may be the contactors, circuit board, or gas valve and possibly the thermostat.
 
Where can you buy a Fan Limit Switch?
Fan limit switches can be bought at a HVAC supply stores 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 buy from Amazon or eBay as they have a big selections.
 



 




 

11 thoughts on “Test and Replace the Fan Limit Switch on a Furnace

  1. Ken

    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

    Reply
  2. Orly

    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 🙁

    Reply
  3. James R. Gamelli

    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

    Reply
  4. Kurt

    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?

    Reply
  5. Peter

    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?

    Reply
  6. Barney

    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?

    Reply
    1. Keith

      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

      Reply
  7. E Coot

    too many lawyers? everyone says don’t adjust hire a profesional. BULL! The “pros” I find are dumber than bricks.

    Reply

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