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]

Videos of people manually rotating the temperature dial on fan limit control - is this OK?

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 2018-03-26 by ed -

Reply by (mod) - NO that is a very dangerous procedure: don't rotate the fan limit switch dial - you could cause dangerous furnace overheating - fatal CO poisoning hazard

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.

Watch out: 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.

 

Reader Question: Fraser Johnson Furnace blower starts late then cycles at 30 second intervals; Blower doesn't start until about 10 - 20 degrees above "Fan On"

29 January 2015 Matteo said:

Hi I have a slightly different problem. Furnace is about 30 years all. My blower starts late and then cycles for about 30 second intervals.

Burner turns on normally, begins to heat. Fan limit switch moves past " Fan Off", then past "Fan On", Blower doesn't start until about 10 - 20 degrees above "Fan On"

Then blower runs on for 30 seconds and shuts off for a few minutes, turns back on for 30 seconds, and shuts off for a few minutes.

Meanwhile, Fan limit Switch reaches "Fan Limit Off", burner turns off and blower still runs in short intervals until furnace has cooled. All the while no heat makes it into the house. P.S.

Fan Limit Switch was just replaced. Please help!

Reply: Some things to check:

Dirty air filter or blocked, crimped, leaky return air ducting causing inadequate air passage through the heating plenum, dirty blower fan itself not moving enough air

Loose wire connection at the limit switch or at the blower assembly

Improperly-installed fan limit switch, helix in contact with something inside the plenum or wiring error or bent control components while someone messed with the switch adjustments

Defective fan limit switch

A blower that cycles back on after turning off can also mean that air flow is the opposite (not enough air) of what I posed above. In this alternative we are seeing a too-fast, cooling down of the plenum temperature sensor (the fan limit swith helix spring) before the plenum steel itself has cooled.

A result is that the heat exchanger or plenum re-heats the air in the supply plenum thus turning the fan back on.

Use our CONTACT US link to find our email to send me photos of your system and of the control. [Photos at left and above were sent by the reader - Ed.]

Reader follow-up: 30 Jan 2015 Matteo wrote:

I’ve attached a couple photos of the furnace for you.

I've taken off the cover panels on the furnace and removed the air filter for the photos. Our conversation that I started on the site is pasted below.

The air filter is new, just changed it. The fan and probably the motor is very dirty.

I plan on cleaning it this weekend. Up till about 2 weeks ago the furnace was working fine. Of course, hadn’t been using it very much and then all of a sudden we did have it on everyday.

The wiring at the Limit switch seems fine. Followed the connections and seem alright. The switch was wired like this and worked fine until recently. But I will check to make sure the helix is not in contact with anything.

The Fan limit switch, does seem to be functioning fine. The temperature does not seem to be dropping too fast. Also, when I pull the switch to turn on the fan manually, it turns right on and runs with the same problem, 30 secs or so and off for a few minutes before cycling back on.

One thing I forgot to mention, there was a slight electrical almost burning smell right when this problem first started.

Could the problem be with the fan motor? Or maybe the fan motor heats up and loses it connection to the power until it cools back down? Thanks again! Matteo

Reply: look for a burned start/run capacitor at the blower fan assembly

Take a look at the blower itself when the unit should be starting - I see in your photos signs that there could be a pulley or belt problem: black debris is being deposited where it would spin off of the blower motor pulley.

Watch out: if your system does not have a blower door interlock switch or if you wire the switch off to watch the blower run, you could easily get fingers or a hand cut-off - stay away from an open blower compartment.

Next:

Also, when I pull the switch to turn on the fan manually, it turns right on and runs with the same problem, 30 secs or so and off for a few minutes before cycling back on.

in your note is suggestive of a fan circuit or motor problem since pulling the FAN ON switch, IF all other wiring is OK, should run the fan constantly - the limit switch won't consider temperature at all in making the fan run:

I suspect there may be a bad fan motor start/run capacitor. See if there is one you can find in the fan compartment (with power off) and if so just replace it.

 

Question: I can't find the manual FAN-ON button on my fan limit control.

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. (Feb 19, 2013) Carmen said:

Reply:

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

Check your wall thermostat to see if there is a FAN-ON switch position there. That can force the fan to run continuously when you want that condition.

 

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

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 2020-10-28 by Pam

Reply by (mod) -

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.

Blower fan runs only intermittently even if the thermostat isn't connected

My hvac squirrel cage fan comes on intermittently even if the thermostat is completely disconnected. - On 2018-04-28 by Kirk

Reply by (mod) -

First check the temperature of your furnace. The blower fan may be running to cool down the heat exchanger to prevent dangerous cracking damage to that component.

If that's not happening, then I would look for thermostat wires shorted together or a bad fan control module

 

Question: Furnace fan won't start

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 22, 2013) Erica Hartley

I installed 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. - (Jan 29, 2013) Steve

Johnstone gas furnace blower won't start

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 2017-05-01 by Lynn

Reply: where to find the diagnosis and repair when the furnace blower won't TURN ON

Erica and Steve we diagnose this condition

at FURNACE FAN WONT START

Can a failed outdoor compressor/condenser cause the indoor blower fan not to start?

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 2017-08-23 by Tom b -

Reply by (mod) - Yes in some conditions

Yes, maybe, Tom. For example, in heating mode, the blower fan won't start below a certain temperature. That feature avoids blowing cold air on people during the heating season.

That circumstance could occur IF your outdoor compressor/condenser unit is a heat pump and is operating in heating mode.

But in cooling mode, if the outdoor compressor/condenser isn't working, in a typical residential air conditioning system, the blower will run, but the air delivered simply won't be cooled.

What tells the furnace buner flame to turn OFF

what tells the flame to turn off the heating setting up stairs on the thermostat. what tells the furnace buner flame to turn off On 2017-03-17 by stan

Reply by (mod) -

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)

New Honeywell Limit switch with the brass jumper broke off by mistake

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 2017-03-09 by Dave -

Reply by (mod)

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.

Furnace Blower doesn't stop when it should until I tap on the limit switch cover

@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 2017-01-27 by Michael

Reply by (mod) -

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

 

Question: furnace burner won't re-start

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. (Jan 6, 2013) Elliott and (Feb 2, 2014) Anonymous

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.

Blower will come on but heat will not - (Feb 21, 2014) Anonymous

Reply:

Anon this sounds like the problem is not the blower but the heater - see the diagnosis and repair steps found

at

• NO HEAT - FURNACE

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

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. (Mar 31, 2014) Darren

Reply:

Watch out: 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: burner won't re-start

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. (Jan 6, 2013) Tiffany said:

Reply:

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

 

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: intermittent blower fan operation

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

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: mblower fan starts and stops

(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

Below are commented off QA

...

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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