Q&A about air handler or furnace blower fan limit switch temperature settings.

This article series describes in detail the setting of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems.

FAQs on Settings for a Furnace Fan & Limit Control Switch

These questions and answers about making the proper settings on a fan limit controller or troubleshooting that control were posted originally

at FAN LIMIT CONTROL SETTINGS - be sure to read that article too.

Fix for a Broken Fan Limit FAN-ON-AUTO Switch

My manual fan-on switch was pushed in and the part that sticks out broke off when removing the honeywell cover.

So it is stuck in the fan on mode and runs constantly (unless i turn off the breaker). There is no way to get at it to get it back to auto setting.

I don't want to replace the whole limit switch if I don't have to. Is there any way to get at it from inside or behind to at least get it to the pulled out setting and being automatic again? - 2017-09-24 by betsy

Reply by (mod) -

Betsy

On most fan limit switches that include this control pushing the button IN puts the fan in manual ON mode - just as you say. If you can't hook and pull out the broken off switch stub you will want to replace the switch.

Disassembling the switch - which is not intended by its design, risks even slightly bending its temperature sensing movement that would render the switch unsafe.

Followup by Betsy

Thanks for the reply. After sending the question I had an idea to drill out the remaining plastic stub and try and hook the little bit of remaining plastic with a sharp dental tool. Amazingly it worked!

The furnace cycled on and off as it should.

And if I hadn't of thought of that, I would have tried something similar after reading your reply.

We've had snow already this year, so having the furnace working is a wonderful thing. Thanks again!

Reply by (mod) -

Well SUPER going. Especially as you managed to drill into a safety control without drilling too far and snagging it or getting zapped.

FAN switch broke in FAN-ON mode - how do I get back to Auto?

My manual override switch Honeywell broke in manual position. Is there a way I can set it back to auto? On 2017-11-28 by Ron Hupe -

Reply by (mod) -

Ron

Well I used to say no, just replace the switch, but a reader who is apparently somewhat a meticulous and careful worker, wrote to me that he drilled a tiny hole into the broken stub of the push pullswitch on the fan on switch on his limit control, then he inserted a little screw and used that to pull the button back out.

I can't officially recommend modifying any heating safety control since if you foul it up or damage it it is unsafe and could risk a building fire. Just sayin'

 

My furnace fan won't shut off

My fan will not shut off...what do I do ? - On 2011-10-04 by Shawn -

Reply by (mod) -

Shawn
First take a look at our page top photo -at FAN LIMIT CONTROL SETTINGS - if your switch looks like the one we show, check that white pull/push knob - it may have been switched to keep the fan on continuously.

Second check your thermostat - it may have a "FAN ON" switch - be sure your switch is set to AUTO or try OFF to see what happens.

And see the diagnostic and repair steps

at

• FAN WONT STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH

Finally, those steps don't work I'd call a service tech as you may have a bad thermostat or fan limit control switch.

No white fan button and the fan never shuts off.

there is no white fan button and the fan never shuts off. The limit setting is really low, can I change it? is this something I can do myself or should I call a furnace tech?

The off is at 50, on at 100 and OFF at 150. I see above it should be 90, 130 and 200 from factory. I also noted the the disc it self is not level (text doesn't read straight across it is on an angle) any advice? - On 2011-10-19 by Shanna -

Reply by (mod) -

Shanna

Watch out: if the dial on your fan limit switch is bent askew I am afraid it may be unsafe and need replacement. Mishandling the dial during setting can bend internal parts.

The modified temperature settings could also point to inexpert hands on the switch.

Because the dvice depends on an intact bimetallic spring to work, it may not safely respond to a high furnace temperature. I'd ask your heating service co to take a look and to replace the switch if appropriate.

Followup by Shanna

Thanks Dan. the dial isn't bent, it just appears as though it has been turned about 72 degrees counter-clockwise.

Will a heating company charge to look at it? Or will they come and give a free estimate?

Reply by (mod) -

Shanna

The heating company may, depending on where you live, charge for travel and have a minimum service call fee, or may charge for an estimate and then credit you back for that charge if you buy a service or repair from them.

 

Furnace blower fan cycled off too quickly: gas valve was partly closed!

Just lit the furnace, and it was cycling the blower off in less than a minute.

After reading the article FAN LIMIT CONTROL SETTINGS, I pulled the cover off to watch the limit switches. They were set right on.

What I found was, that I hadn't turned the gas full on, so after the blower kicked in, the switch was cooling down to the shutoff. A little more gas kept the the dial where it was running. - On 2012-10-07 by Warren

Reply by (mod) -

Warren,
Thank you for the helpful short on cycle diagnostic comment. I will add it to the article text.

Blower fan turns on randomly

With the thermostat set to 'off', my fan blower motor has suddenly started to randomly turn on, run for 10-15 minutes, then shut off. I replaced the thermostat thinking that was the problem, but the problem persists.

And when I say randomly, it could be several hours or several days between incidents. Is the fan limit switch the problem? (The A/C furnace unit is probably 16-17 years old.) On 2018-06-13 by mc63 -

How do I set the fan limit switches on my electric / wood furnace?

I have a wood/electric furnace with 2 high/low limit switches, the fan limit switch is set at factory settings, but where do I set the wood thermostat/ damper motor at? - On 2021-12-16 by Alfie ,-

Reply by Inspectapedia Com Moderator - set the damper according to the manual for your wood stove

@Alfie, If I understand correctly, since your alternate heat mode is electric with no concern about needing two different draft settings depending on which it source is in use.

You would set the damper according to the manual for your wood stove and that's probably model specific.

Can you tell us the brand and model of the stove so that we can look at its manual?

My guess is that your wood burner thermostat has to be set low enough to avoid dangerously overheating the furnace and cracking its heat exchanger, but just what that temperature is will depend on the brand and model heater.

@mc63: I'd look for an intermittent short between the thermostat wires.

Blower won't turn on

My blower did not come on after my system fired up, so I removed and tested the blower,it worked fine. Should I suspect my L4064A combination limit control? Is the limit L4064A and B the same? On 2011-10-31 by victa -

Reply by (mod) -

Victa there could be several problems but given that you had no trouble testing the blower assembly and found that the fan motor runs, I'd try replacing the switch. FIRST just be sure that the thermostat is calling for heat and that its wiring is hooked up.

You can confirm that by calling for heat and watching to see if the burner ignites in your heater. If it does then the limit switch should turn on the fan once the plenum reaches the turn-on temp.

See

• FAN RUNS ONLY ON FAN-ON / MAN
• FAN WONT START

Blower won't come on in HEAT mode

I have a furnace with a limit switch that won't turn my blower fan on when heat is on.

The blower works fine in A/C mode but doesn't turn on when the thermostat is set to heat the house.

I can manually turn the dial inside the Honeywell box, but when I let go, a spring pops the dial back into place and the blower turns off again. Any help would be awesome! - On 2018-02-04 by Bill -

Reply by (mod) -

Bill

Watch out you should never turn the fan limit switch spring by hand as doing so bends the internal spring, makes the switch inaccurate, and mix the control unsafe.

What's up morning completed I would watch the behavior of the switch when the burner comes on at your furnace.

If the burner is coming on and they dialed or switch doesn't respond to the temperature change and then indeed the switch needs to be replaced.

See FAN WONT START

 

Blower starts and stops in Cooling mode

I have a unit in my apt with the blower below the heater. The a/c unit is outside.

The problem is this: when I set my thermostat to "ON" instead of "AUTO", the unit works as expected for heat. As furnace goes off/on, the blower stays on.

However, when I try the same thing with the A/c, the blower shuts off briefly (for a second or two) and then restarts.

I need to fix this; I need the thermostat set to the On position at night, so the blower runs constantly and does not briefly "reset" when the a/c comes on. Help please! On 2017-03-04 by Chris -

Reply by (mod) -

Chris:

If there is a separate FAN ON switch on your thermostat you can set that to "ON" - or if your furnace blower unit uses a fan limit switch, some of those switches also have a manual fan-on switch that you can use.

Furnace blows cool air at end of heating cycle

i have a gas powered furnace horizontal in my crawl space under my house and it heats fine but it is to cold when it shuts down it cools off house.

how do it set furnace is about 4 hears old and i can not find the limit switch on it i have changed and adjusted them before is there a limit on this one that can be adjusted On 2017-01-05 by Ken -

Reply by (mod) -

Ken,

Pretty much any furnace will use a fan limit control. However some furnaces may use a control that is set at the factory and that has no adjustments.

I'm not sure what you mean by it cools off the house. But I don't think that changing the actual temperature ranges on the limits which would cure the problem that you described.

However if what you mean is that the fan continues to run at the end of a heat cycle, does blowing cold air into the occupied space, then I agree that the limit switch is not working properly. Most likely it needs to be replaced.

I have to keep pushing the reset button on my furnace

I have a Honeywell oil hotter furace and it fires up and runs gets to temp, and blower runs for what I think is fine, so I think great no problem so this goes on for 3-5 cycles and then the furace does not fire up and

I have to push the reset button it runs for a few seconds and then I have to push the reset again some times the green light comes on sometimes it doesn't , and it goes into lock out mode and I have to hold the reset for 20seconds and off we go again for another 3-5 cycles what is the problem, and do you have any segustions I'm so frustrated please help if possible and thank you. ELC On 2016-12-10 by Elc -

Reply by (mod) -

Elc,

Watch out: Do not keep pushing the reset button. On an oil-fired heating system when you keep pushing that button your risk ultimately causing a puff back explosion. You can search inspectapedia.com forPUFF BACK EXPLOSION to read details.

Several things can cause difficulty that prevents an oil burner from starting or running reliably. They're not things that a homeowner can repair. I suggest that you call your heating service company.

 

Burner continues operate and the blower fan does NOT turn on

The problem I’m having with my Honeywell Fan Limit Control is; when the thermostat calls for heat, the oil furnace burner turns on as it should – however the burner continues operate and the blower fan does NOT turn on.

The burner continues to run until the plenum temperature reaches High Limit setting of the fan limit control – at which point the furnace burner is shut off.

As the plenum temperature cools down the fan will start to run and continue to run until the low set point is reached, if the thermostat temperature is not satisfied – the burner will come back on and 90% of the time the fan will operate as it should;

however if the plenum cools all the way down and a totally new heating cycle is called for, the problem repeats.

This problem occurred with my old (20 years) Honeywell fan limit control – so I replaced the control with a new Honeywell Fan Limit Control.

The problem repeated itself with the new control. I rechecked the wiring and that was correct.

The bimetal probe is the same recommended length as the old one and does not seem to be binding. I did not change the factory setting on the control dial.

I returned the Honeywell Fan Limit Control and replaced it with a second Honeywell Fan Limit Control.

Everything worked fine for the first several cycles and then the problem repeated itself. I have a 10 year old Honeywell 7 day programmable thermostat and have made sure it has fresh batteries and it cycles off and on as it should.

As usual new filters have been placed in the furnace, the furnace has been cleaned and the blower motor seems to function as it should when I set it to manual operation with the switch on the control.

Any ideas? Thanks, Dennis On 2012-09-26

Reply by (Mod) - Daniel Friedman

Watch out: if your burner really stays on and the fan never turns on your furnace may have already been seriously damaged and would be unsafe. Overheating a furnace heat exchanger can cause cracks that leak fatal carbon monoxide gas into the occupied space. If that's your situation, leave the heater OFF.

Be sure you have working Carbon Monoxide and Smoke detectors.

I suspect you've either damaged the control or there's a wiring error.

These three articles have a rather complete set of diagnostic steps and repair tips. Please see

• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN RUNS ONLY ON FAN-ON / MAN
• FAN WONT START
  

Question: operating the limit switch FAN ON switch

My fan limit control says push man and an arrow, no white button, only an empty hole. What should I do?

I set the numbers at their correct positions but the blower still remains on for a while and then comes back on after going off. I was lucky enough to replace my blower motor on my own after ordering it online.

NO HELP FROM ANY MEN! The least of my worries was the fan limiter. Now after a month of heat I need help! Thank you (Jan 31, 2014) marie

Reply:

If your limit control has no actual operable push-pull button or switch, then it's a model that omitted that feature. The switch will still work automatically. The manual switch is an extra, not mandatory.

Be careful not to mess with, force, or bend the switch parts - doing so could make it unsafe.

How do I set the fan limit switches on my electric / wood furnace?

I have a wood/electric furnace with 2 high/low limit switches, the fan limit switch is set at factory settings, but where do I set the wood thermostat/ damper motor at? - On 2021-12-16 by Alfie ,-

Reply by Inspectapedia Com Moderator - set the damper according to the manual for your wood stove

@Alfie,

If I understand correctly, since your alternate heat mode is electric with no concern about needing two different draft settings depending on which it source is in use.

You would set the damper according to the manual for your wood stove and that's probably model specific.

Can you tell us the brand and model of the stove so that we can look at its manual?

My guess is that your wood burner thermostat has to be set low enough to avoid dangerously overheating the furnace and cracking its heat exchanger, but just what that temperature is will depend on the brand and model heater.

Question: Olsen Ultramax furnace NG that clicks on for 3 seconds

I have an olsen ultramax furnace NG that clicks on for 3 seconds off on for 4 seconds 3rd time kicks in to running. going to change the pilot assembly kit, hope that's all. runs a lot. put in a new thermostat already. hope to hear from you. (Mar 5, 2014) norm williams

Reply:

Norm,
I don't blame you for thinking about the pilot assembly as it sounds as if the unit has trouble starting.

Keep in mind that dirt or debris at the igniter or pilot could be the problem, as could a sensor, or at the thermocouple itself. Often by watching the flame at start up we can figure out what's going on. For example if a the pilot lights but then goes out I suspect that a thermocouple is bad.

 

Question: Does the Honeywell fan and limit control have any part in controlling the fan in a/c operation

Does the Honeywell fan and limit control have any part in controlling the fan in a/c operation or is that done strictly by the fan relay and thermostat? I just installed a new fan and limit control ion my furnace and the blower continues to run for 10 minutes after the a/c condenser shuts off? (Sept 22, 2014) JT said:

Reply: no but ...

JT

The fan limit switch is a heating control and does not work during cooling

except

if someone puts the switch in manual FAN ON position then the blower fan will simply keep running.

Question: Where to get a replacement fan limit switch L4064w1296 19538c

I need a replacement fan limit switch L4064w1296 19538c. It is no longer listed. I am using a dual energy system (oil/electricity)at my choice. Where can I get a replacement ? (May 27, 2015) Ron said:

Reply:

A check with the manufacturer of your heating furnace and with your heating service technician should confirm that you can use one of the more generic fan limit switches.

Or see L4064 FAN LIMIT CONTROL IO MANUALS & WHERE to BUY

 

Blower fan for wood stove cycles too often

We have a 1600EF USSC outdoor wood stove and the fan kicks on and off a lot. a minute on, a couple minutes off. We haven't changed the preset settings of 100/150/200.

What can we change so it kicks on and off less? Does this change with outside temperature? On 2011-10-08 by Anonymous -

Reply by (mod) -

Anon:

The temperature settings on the fan limit switch pertain to temperatures inside the warm air plenum in the air handler and should not be affected by outdoor temperatures. If the fan switch is cycling on and off too rapidly I suspect that either


- the switch is defective

or


- the limit switch is doing its job but you are blowing very cold air into the plenum, dropping it below the cut-in temperature quickly. This can happen if the incoming air temperature and flow rate is more than the heater is able to keep up with.

Thank you to our readers for their generous comments

On 2016-02-02 by Steve Beukema - Thank you for this info, it was very helpful.

On 2012-12-29 by Scott - this was the best answer i have ever gotten,thank you so much

On 2012-01-03 by MATT 77

Thank you,My twentyone years old furnace is working find thinks to your topic on setting the limit switch. super step by step instructions.(AND FREE)BLESS YOU!
THANKS,MATT77

On 2012-01-03 by Anonymous

Thank you,My twentyone years old furnace is working find thinks to your topic on setting the limit switch. super step by step instructions.(AND FREE)BLESS YOU!
THANKS,

On 2012-12-02 by Dgeorge@georgegems.vom

This is exactly what I needed. My heating co set at 60 and 150...very frequent short cycles.

On 2012-11-11 y Abimael Luciano

Really good info here. im studing HVACR and there is amazing information on heating systems at InspectApedia.

On 2011-12-03 by matt k.

Excellent instructions! I followed them and easily replaced the Limit Control in our late 80's Whirlpool furnace (Heil Quaker / Honeywell switch 1149505). Never touched the inside of a furnace before but your directions were spot on. Thank you.

 

...

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 these

Recommended Articles

• FAN LIMIT SWITCH - home
  • FAN LIMIT CONTROL SETTINGS
  • FAN LIMIT SWITCH INSTALLATION & WIRING
  • FAN LIMIT SWITCH TROUBLESHOOTING
  • FAN RUNS ONLY ON FAN-ON / MAN
  • FAN WONT START
  • FAN WONT STOP - LIMIT SWITCH
  • FAN WONT STOP - THERMOSTAT SWITCH
  • FURNACE BLOWS COLD AIR
  • FURNACE FAN CYCLES AFTER HEAT
  • FURNACE FAN CYCLES DURING HEAT
  • FURNACE FAN STOPS EARLY
  • ISOLATION SWITCH WIRING

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FAN LIMIT CONTROL SETTING FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to HEATING FURNACES

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