Fan limit switch troubleshooting diagnosis & repair FAQs:

Frequently asked questions about how to test and diagnose a fan limit control.

This article series describes in detail the testing and diagnosis of problems with warm air heating furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems. As a working example we examine the Honeywell combination fan and limit control type L4064B.

The photo at the top of this page shows all of the controls and wiring terminals in a Honeywell combination fan and limit control type L4064B.

Q&A on Testing the Warm Air Furnace Fan Limit Switch

These questions & answers about testing the fan limit switch were posted originally at FAN LIMIT SWITCH TROUBLESHOOTING - please be sure to see the fan limit control installation, diagnosis, and repair procedures given there.

[Click to enlarge any image]

On 2020-10-28 - by (mod) -

how to slow the "rotation" of the fan limit switch?

Pam:

You can't slow the rate of rotation of the sensor in the fan limit switch directly, as that's determined by the rate of air flow through the plenum, the return air temperature, and the input BTU capacity of the burner that's trying to re-heat the air.

Indirectly, if the blower fan permits adjusting its speed you can set the speed lower (that may be a switch or a jumper on a control board), but of course you're also reducing the rate of delivery of warm air to the occupied space.

You might start by asking for help from your heating service tech to get to the underlying issue here.

If this has always been a problem then it may be that the house loses heat too fast, drafty or poorly insulated, or it may be that the ductwork or heater are not properly sized. An over-sized furnace will cause the fan to shut off quickly.

For a "Fan Won't Stop" problem you should be sure to read these diagnostic articles


FAN WONT STOP - topic home

FAN WONT STOP - THERMOSTAT SWITCH - switches right on the thermostat can force the fan to run

HEAT WON'T TURN OFF for other reasons that a heating system won't stop running

Watch out: take great care when fooling with the settings on the fan limit control; if you bend the spring in the control you force it out of calibration, making the system dangerous and risking heater damage or even carbon monoxide poisoning should the heat exchanger crack due to excessive temperatures.

On 2020-10-28 by Pam

I reset the limit switch 100 low, 150 high. It fixed the constant blower problem, but it seems like it rotates very quickly from 150 down to 100. Is that normal? Can I slow down the rotation.

Also I have a programmable Honeywell thermostat. It is set at 63, turns on at 62, off at 64, but it feels more like 75+ in the house. How do I know if i have the correct size thermostat.

On 2018-04-30 - by (mod) -

I would look for thermostat wires shorted together or a bad fan control module

On 2018-04-28 by Kirk

My hvac squirrel cage fan comes on intermittently even if the thermostat is completely disconnected.

On 2018-04-04 - by (mod) -

Ed

The manufacturers of fan limit controls know their equipment better than anybody else, and they also have a lot at stake in assuring that their controls are installed and operated safely. Take their advice.

The videos you cite are examples of dangerous advice that might be offered by someone with a neat idea but who is not fully informed. It's a classic problem: smart people don't know that they don't know something important.

Rotating the dial on the fan limit switch by hand risks bending and thus de-calibrating the bimetallic spring that operates the switch in response to furnace supply plenum temperature. It is patently obvious that if we de-calibrate a temperature-sensing safety control then the control cannot be relied on to do what it's supposed-to.

For example bending the spring in the fan limit switch might result in an over-temperature in the furnace that in turn cracks the heat exchanger.

A cracked heat exchanger can send fatal carbon monoxide gas into the occupied space.

While not every uninformed trick we try has fatal consequences, that's the risk: bent switch, dead people. Don't do it.

On 2018-03-26 by ed

my fan/limit switch (an old delco unit) has the normal 'do not rotate' imprinted on the temperature dial, yet i see many videos of people 'testing' their fan/limit switch with an ohm meter by rotating the dial to verify the fan and limit switches are turning on and off as they should. is this a good way to check the switch? most have the wires disconnected when they are testing the switch..

On 2018-01-07 by Jeanette

We have gas heat/electric air. Heat is running fine. But like every other day it does not come on. I flip it off to on and it continues to work fine until the next time it decides not to run. Replaced the thermostat but still having the same problem. Do you know what we would check next? Thanks

On 2017-10-17 by (mod) - fan switch making loud clicks?

Marie,

That's not an issue that I've seen discussed. But you might ask your service technician to check the system start-up - something else might be making an abnormal noise, such as a spark or relay.

On 2017-10-17 by Marie

I just started my furnace for the year and I feel that the electrical « click » sound that the fan limit switch(Honeywell) makes when it tells the blower to go off/ on is much louder than it used to... anyone else?

On 2017-08-23 by Tom b

I have a Carrier model 38YZA heat pump outside unit with a Carrier model FV4 inside unit in the attic for the upstairs level of my home in Huntsville Al.. Yesterday I noticed the upstairs temperature was well above the set temperature of 72 degrees

. I did not hear the inside blower motor running and went outside to check on the a/c unit. It too was not running but I saw ice on the refrigerant line leading to the house. I turned off the system at he thermostat immediately and left it to access later in the evening when it became cooler.

Later in the evening I checked to verify the circuit breakers were in the on position which they were. I switched the system on again at the thermostat but nothing started, no inside blower (even when switched from auto to blower), nor the outside fan or compressor - all silent.

My question is can a failed "outside" compressor capacitor cause silence at the outside unit fan and compressor as well as the inside fan motor to not start? When I turn the system on at the thermostat, I hear a slight click within a few seconds as if the thermostat is calling for the rest of the system to turn on.

On 2017-05-01 by Lynn

I have a Johnstone Model U-30 Gas Furnace that the blower will not engage. I can hear the pilot kick in but it will not blow hot air. This just occurred last night. I have made sure everything is cleaned out and that everything is connected. What else could it be? Help!

On 2017-03-18 by (mod) - what tells the flame to turn off

The furnace burner will turn off when

- the room thermostat is satisified and stops calling for heat, AFTER an additional brief cool-down run time in the air handler.
it is
the fan limit control that's keeping the fan on for that additional cooling-off period.

The burner will also turn off for other reasons, of course including unsafe conditions sensed by any of several controls (*varying by type of fuel, e.g. gas vs oil)

On 2017-03-17 by stan

what tells the flame to turn off the heating setting up stairs on the ther.

On 2017-03-09 by (mod) - New Honeywell Limit switch with the brass jumper broke off by mistake

Dave:

The company, wanting to avoid liability for a fire or injury will say no, buy a new unit, but I suspect if you can make an electrically-secure connection the switch will work fine.

On 2017-03-09 by Dave

I have a New Honeywell Limit switch with the brass jumper broke off by mistake, is there anyway I can use it by maybe putting a jumper wire between the red and black wire??

On 2017-02-09 by (mod) -

See the moderator's reply just below. Start by checking for a loose or corroded electrical connection

On 2017-02-09 by Rachel

@WAYNE, My wood furnace doing the same. did you find the problem?

On 2017-01-27 by (mod) -

Sounds like a bad fan limit switch but first check for a loose or corroded electrical connection anywhere in the controls.

On 2017-01-27 by Michael

@Roy,
I have the exact same issue. My 1985 York furnace. The thermostat calls for heat, the heat comes on but when the thermostat is satisfied the blower continues to run. Tapping (lightly) on the cover of the switch turns is off. Or, turning off the breaker will do the same thing. So, is it the limit switch or a board? I think the switch is sticking.

On 2017-01-11 by Anonymous

Wayne

If the blower fan is cycling on and off when the thermostat is not calling for heat I suspect a limit switch or fan switch or control board wiring error or control defect

On 2017-01-10 by WAYNE

I INSTALLED A NEW FAN LIMIT CONTROL SWITCH FOR BY WOOD FURNACE ADD ON.
NOW THE FAN CYCLES ON AND OFF EVERY COUPLE OF MINUTES.
THE ON OFF SETTING ARE SET TO FACTORY SETTINGS 100 AND 140 F.
ANY IDEAS ABOUT THE PROBLEM?

On 2016-12-18 by Patty Taylor

I replaced the control board in my Goodman furnace and now my fan runs continuously. Did I hook it up wrong?

On 2016-12-02 by Bill

Thanks Dan! I cleaned the filter and the fan and still have the problem. Can I clean the thermal sensor, Or do I need to buy a new limit switch? Thanks

On 2016-12-01 by (mod) -

Probably not, Bill. Usually when I see this problem, residual heat in the heat exchanger is re-heating the air in the supply plenum after shut-down. That causes the fan to cycle on to cool down the heat exchanger - reducing chances of its damage from thermal stress.

I'd look for a dirty air filter or dirty blower fan that's slowing air movement through the heat exchanger in the first place. If there's no other air obstruction through the system then a dirty thermal sensor on the limit switch could be at fault.

On 2016-12-01 by Bill

I have an old Carrier up flow furnace. When I turn off the heat at the thermostat the unit shuts off. Then immediately the fan starts for a couple seconds and then shuts off. It does this a couple times. Is the problem my limit switch?

On 2016-09-30 by (mod) -

Sounds like there may be a loose electrical connection or a failing switch. I would NOT try to service a limit switch. Bending or fouling up the spring can make it unsafe.

On 2016-09-30 by Roy

The blower in mine runs on after the heat cycle. If I give the furnace a gentle tap, it will shut off. I'm just wondering if the switch can be cleaned, or should I replace?
Thank you

On 2016-09-24 by Linda

It is Saturday--do you answer on the weekends? Can't wait for Monday for answer . . . Thank you

Sept. 24/16 My forced-air heater in mobile home is making clicking noises--what is it? A timer? Any danger of explosion? Thank you . . .

On 2016-05-08 by (mod) - fan rotates only slowly

Fred

If you mean the fan rotates only slowly, if it's a belt driven model check for a loose pulley or belt.

Also ask your electrician to check for low voltage or a bad motor start/run capacitor.

If it's a direct drive motor-fan unit then the motor or capacitor to it may need replacement.

On 2016-05-06 by Fred Bonk - Miller gas furnace has slow fan

I have a Miiler gas furnace in a mobile it is 29 yrs, old the furnace turns on but fan turns slow, the motor was replaced about 10 yrs. Ago.

On 2016-04-08 by (mod) - blower turning on and off

Anon: I can't tell from your comment if the problem is that the furnace is short cycling - that is the burner -, the thermostat is shorting, or if there is a problem with the blower control switch or control board. Indeed it sounds as if an onsite repair tech is needed.

On 2016-04-08 by Anonymous

My blower comes on at any given time and then turns off then back on again, it will keep doing that unless we unplug the power and turn off fan switch

On 2016-03-30 by Anonymous

Thanks I will look into that. It is definitely a goofy setup. I had thought maybe there was a blower designed to only rotate in one direction. The damper idea sounds promising. Thank you

On 2016-03-28 by (mod) -

Mike:

This sounds like a questionable duct design: if multiple suites in a building are sharing return air that means that any contaminant, odor, etc. in any tenant's space will be blown into everyone else's space. It also means that any tenant who blocks return air can interfere with heat or cool air delivery to everyone else.

That gripe stated let me get to your question.

The backwards fan rotation is most likely due to the shared return air, as you suggest and as I already condemned. (Other causes of an electric motor running backwards on its own are discussed at https://inspectapedia.com/electric/Electric_Motor_Direction.php )

But backwards fan spinning also happens in larger HVAC systems that use multiple blower fans in one duct system.

And depending on the motor type, a fan motor that is OFF but has been spinning "backwards" may keep going backwards even when your thermostat calls for the fan to run. Not all motors will be damaged by this SNAFU but it means that the blower won't push heated or cooled air where it's needed.

It *might* be possible to change the squirrel cage fan cage itself to one that uses a different blade design to reduce the susceptability of the fan to be driven by duct airflow when its motor is off.

But I'd try a different idea that might totally stop the problem: ask your HVAC installer if it is possible to locate an electrically-opearated "automatic duct damper" near the inlet to your fan, or more normally, on the outlet side of your blower system.

Your thermostat is wired to open the duct damper and then turn on the air handler for heating or cooling. With the damper closed air should be almost completely (except for small leaks) blocked from passing through common air return system and into your individual supply duct system.

That ought top stop the fan from blowing (mostly if the damper is on the supply or outlet side and entirely if the damper is installed on the inlet side of your blower)

Keep us posted.

On 2016-03-28 by Mike Krueger

My suites furnace has a common return with several others. Each suite has its own thermostat.

My problems is I can't turn the fan off ever as the blower will begin to rotate backwards which throws a saftey switch and can't be restarted without going to the basement ,removing the door , resetting the blower, replacing the door, each time

.The blower is too strong and loud to want on all the time. Is there a way to prevent this without reducting the entire system to a more normal setup?

On 2016-03-27 by (mod) -

Good going Cory. As what you found will help other readers I'll post this conversation at

https://inspectapedia.com/heat/Fan_Limit_Switch_FAQs.php

- good debugging. Loose wiring connections are perhaps the most common fault on many systems.

On 2016-03-25 by Cory

Ok so I went over everything again and I ended up finding a faulty connection with my wire going to my gas valve,it is a push in and lock stlye connection so to the naked eye it looked like it was in correctly ,you live you learn I guess thanks for all the help it is appreciated.

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Reply:

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Question: burner won't re-start

(Jan 6, 2013) Tiffany said:

Something is wrong with my furnace. The burner will ignite and the fan will come on and go off when the set temperature is reached.

However when the temperature drops below the set temperature the burner will not re ignite and the fan will not come on again until I turn the temperature setting all the way down and the switch to off and let it remain off for a few minutes then set the switch to the desired temperature back to where I wanted it and the switch back to heat.

Anybody have any thoughts as to what could be the problem? Until about a week ago it was working normally then it started doing this.

Reply:

Sounds as if the system is overheating and may be unsafe. It's time to call a professional.

Question: furnace fan won't start

(Jan 22, 2013) Erica Hartley said:

My furnace fan does not come on at all. My husband reached in and made sure it was not locked up. What could this be

(Jan 29, 2013) Steve said:

I instaled a new L4064 because my fan would not come on. Now the burner ignightes and after it warms up the fan comes on, problem is, the fan continuously.I am not 100% sure that I didn't cross up the RED and the YELLOW wires. Could that mistake cause this problem?

There are three adjustments, I left them as they were right out of the box because they were the same as the old settings

. I called an expert, he said to move the one on the left closter the the one in the center ( center set at about 140-150) so I mover the one on the left from about 100 to about 120 degree and the fan keeps running

.This is an area of my home that I rent to someone. I told them to let me know if the heat was working as they are used to. I have heard nothing from them for 4 days, so I am assuming that they are comfortable and that they are not getting cold air from the furnace, but I am not sure about that.

Reply:

Erica and Steve we diagnos this condition

at FURNACE FAN WONT START

Question: no manual fan - on button

(Feb 19, 2013) Carmen said:

I pushed the button in (manual) on my fan limit switch but I am unable to pull it out (auto) because there is not a white button on it. Now when the thermostat is at the set temperature the burners do not go out. What can I use/do to switch it back to auto.

Reply:

Carmen some fan limit switches do not include the FAN ON manual control.

Question: burner won't re-start

(Feb 2, 2014) Anonymous said:

(Jan 6, 2013)Elliott said:
Something is wrong with my furnace. The burner will ignite and the fan will come on and go off when the set temperature is reached.

However when the temperature drops below the set temperature the burner will not re ignite and the fan will not come on again until I turn the temperature setting all the way down and the switch to off and let it remain off for a few minutes then set the switch to the desired temperature back to where I wanted it and the switch back to heat.

Anybody have any thoughts as to what could be the problem? Until about a week ago it was working normally then it started doing this.

Reply:

Anon I don't have a clear picture of what's going on from just your note. It sounds as if the heater is going off on a safety reset. If this is the case, you might get it going again TEMPORARILY by a power-off, wait, and power-on cycle. But such as system is unsafe and needs professional inspection and service.

Question: blower runs but there is no heat.

(Feb 21, 2014) Anonymous said:

blower will come on but heat will not

Reply:

Anon this sounds like the problem is not the blower but the heater - see our NO HEAT diagnostic articles.

But what's odd is that the blower would come on at all if there is no heat: typically the limit switch won't turn on the blower until the plenum has reached a minimum temperature - to avoid blowing cold air on occupants.

Question: hot blower housing

(Mar 31, 2014) Darren said:

My Intertherm E3 electric furnace and blower function normally.

However when both the blower and elements are off the blower housing becomes to hot to touch.

When I cycle the blower the housing cools down but immediately heats back up once the fan is off. I can visually see that the elements are off when this occurs. Any suggestions? Thanks.

Reply:

Darren I'm not expert on this equipment but what you describe doesn't sound safe. I'd think that when the blower is OFF the heating elements should be OFF for safety and to avoid damaging the equipment. There may be a problem with the limit switch or with wiring.

I'd contact your heating service company or the company - and I'd turn the thing off until this is resolved.

Intertherm Heating and Cooling
Customer Service
8000 Phoenix Parkway
O'Fallon, MO 63368
www.intertherm.net

Question: how to turn on the gas A/C unit

6/10/14 xenaon said:

i rent from a bad guy. He shut off the wall switch - which shut off the fan. Before, if I turned on the wall switch - the AC went on.

Now, it doesn't. Inside my apt the AC and Furnace breakers are fine. 3 hours later, the bully shut off the gas water tank downstairs - turning the switch horizontal, then setting it to off and vacation, so the pilot light is out.

I cannot relight the pilot - turn on the gas line, hold down the pilot valve - won't light.

I cannot find a way to turn on the Rheem Criterian II gas/AC unit. What else needs to be turned on? Thermostat (Honeywell) does not ignite the AC or fan or heat - but it could not possibly be the thermostat as the person who shut off the fan switch - also turned off the water heater.

Reply:

I'm a bit nervous about getting between you and the landlord on how to control your heating and cooling, but watch out for unsfae DIY wiring of the controls.

Question: carrier heat pump with electric heat. The blower fan will not run with the heat pump

(June 16, 2014) Lee Ann said:

I have a carrier heat pump with electric heat. The blower fan will not run with the heat pump, but will run with the electric heat. Any suggestions?

(June 26, 2014) jay said:

I have an ac unit and the blower upstairs in unit itself starts for a second or two then shuts off. Has anyone experienced this problem.the fan and compressor works its just the blower motor cycling on and off.

Reply:

Lee Ann that sounds like a control or control board problem, as you know that the blower itself will operate.

A possibility to check first is that the heat pump is actually working. If we are trying to heat a space and if the heat pump is unable to raise temperatures inside the air handler to a sufficient level, a fan limit switch might keep the blower from ever turning on - as it doesn't want to blow chilly air on people who are clamouring for heat.

Jay

this sounds like a control relay problem. Check too for loose wiring connections.

Question: we need to replace a limit switch

(Oct 25, 2014) Anonymous said:

we need to replace a limit switch on our oil furnace blower. One of the prongs broke off. We have since lost the part and eager to get a new one. But without he number for the switch. It is hard. I have a pic but numbers are faded. I can make out a few numbers but that is all.

Reply:

You can use the email found at our CONTACT link at page bottom to send us a photo if you like - but limit switches on residential furnaces are pretty standard.

An easy solution would be to give the manufacturer of your furnace a call, giving them the model and serial number of your unit - they can tell you which limit switch they prefer.

Question: heat pump works but back-up gas furnace does not work properly

11/25/2014 Jane Metzger said:

My furnace works fine when set on the heat pump. When it switches to the gas auxilary the gas ignites, but the fan doesn't come on until the high heat turns off the gas. Then the blower comes on and runs until the air starts cooling.

The AC guy was here and thinks it is the control board. but I want to make sure. We had a problem with a clogged condensation pipe last year, but there doesn't seem to be any water issues this time.

Reply:

Jane

In heat pump mode heat is coming from your heat pump and the coil in the air handler - on the gas furnace the fan limit switch is probably not being called on to do a thing - since its job is solely to protect the heat exchanger in the gas fired furnace.

But when the system switches to aux heat then the gas furnace wants to operate (or you want it to) and the fan limit switch is on-duty.

I'm not smarter than your on-site AC guy but you'd think that if the control board in the heat pump is turning on the gas furnace - that is the gas furnace tries to run and its burner ignites, then at that point it's the fan limit switch that actually turns on and off the blower (in response to furnace heat exchanger temperature).

I'm not quite clear on what's happening but can offer a diagnostic question or two.

What is turning off the gas: the fan limit switch?

On most gas furnaces the blower will run continually during a call for heat and will run for a short time after the thermostat is satisfied - to cool down the furnace heat exchanger (avoiding damage to it).

The limit switch will shut down the burner if its sensor thinks that the heat exchanger temperature is dangerously high.

I'd start by checking that the wall thermostat switches are all set properly.

I'd continue by watching the fan limit switch during a heating cycle to see if its dial rotates and noting its position when the burner operates, burner stops, fan comes on or off. (the limit switch dial should start moving until the burner has begun heating the plenum)

Question: broken fan limit switch AUTO button

(Nov 26, 2014) Sabrina said:
Hello,
I accidentally broke off the fan limit switch (the white switch) and it's set on manual. the fan has been on for a few days. Is there anyway I can set it up to auto without having to replace it?

Reply:

Sabrina

Pending replacement of the broken switch (which I recommend, using a trained heating service tech), if the switch was pulled "out" to force the fan on (as is often the case) you may be able to push the remaining stub "in" to turn the "FAN-ON" state back to AUTO

. If your switch works backwards - that is it's pull for auto and push for "Manual On" and if you can grasp and pull the switch out with insulated needle nose pliers that might work. Else you're stuck and need to move on to my first suggestion.

Question:

(Feb 27, 2015) Rich said:
gas forced air -fan turns on and off every few seconds before it gets up to heat thermastate calls for

Reply:

Check for a loose wire, temperature sensor, or faulty limit control.

Check for an obstruction in the warm air supply ducting or a dirty filter.

Question:

(Mar 4, 2015) BOB said:
L4064b, 8-inch. Installed about 1 month ago, worked ok.

Last week it began stopping and restarting fan at both heat up and cool down. I watched the dial and at heat up the dial just slightly moves toward lower temp, but not nearly enough to reach the low temp shutoff setting. It is just the opposite when cool down shutoff occurs.

To me it acts like the switch is not latching/holding after activation in either case. Should it latch or am I confused as to how these work? I have installed many of these over the years as they seem to last between 3 and 5 years; I have never had this problem. Could the unit be defective? Thanks

Reply:

yes

Question: which wires go where on the fan switch?

3 Oct 2015 Harold said:
I have a very old fan limit switch with only 2 wires a blue and white wire, it went out, bought a new one but it requires 4 wires to hook up how do I wire this?

Reply:

Harold,
see the fan limit switch wiring article found

at FAN LIMIT SWITCH INSTALLATION & WIRING

...

Continue reading at FAN LIMIT SWITCH TROUBLESHOOTING or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see FAN LIMIT SWITCH TROUBLESHOOTING FAQs-2 - more-recent questions & answers about testing a fan limit controller.

Or see these

Recommended Articles

• FAN LIMIT SWITCH
• FAN ON AUTO MAN THERMOSTAT SWITCH
• FAN ON SWITCH ADDITION
• FAN WONT STOP - THERMOSTAT SWITCH
• UNDERSIZED RETURN DUCTS - not enough return air

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