Furnace blower fan limit safety switch installation & troubleshooting FAQs set #3:

More frequently asked questions (FAQs) about heating furnace fan limit controls, the fan limit switch purpose, operation, setting, installation, wiring, and testing of furnace combination controls, also commonly called the "fan limit switch" on warm air heating systems.

HVAC Fan Limit Switch FAQs set #3

Recently-asked questions about furnace fan limit switch controls.

These questions and answers were posted originally at FAN LIMIT SWITCH - topic home, so you may want to start reading there.

Also be sure to read FAN LIMIT SWITCH TROUBLESHOOTING. At the end of this article you'll find a complete list of air handler or fan limit control switch diagnosis and repair articles.

Photo: the interior of a typical fan limit control switch found in the air handler of furnaces - forced warm air heating systems, discussed by Ken Martin's question below

[Click to enlarge any image]

On 2021-11-22 by Inspectapedia Com Moderator

@Andy Haninger,

That suggests there could be a loose or corroded electrical connection or, though less likely, a bit of debris that fell off of a temperature sensor.

Thanks for the follow-up. I understand what you're saying.

On 2021-11-22 by Andy Haninger

Thank you for all of your help with this project. So far I've had no luck getting this limit switch out. The issue (I think) is that the hex head screws holding it in place are rounded off at the top or are really secured in place. I don't really want to try to fix what ain't (really) broke and I don't really want to put any repair money into it. I'm tired of lighting the pilot The system is 33+ years old, which is a good run. I know it's had trouble before. The blower motor was replaced 7 or so years ago

I'm going to start getting estimates on a new system that will allow me to move on to worrying about something else.

To make things more frustrating, after I poked around inside the furnace this past weekend, it's working perfectly fine. It's still wasting gas like it's 1988, but it's turning on and off as expected.

I did cut the breaker to it before poking around. Maybe it just needed a reboot?

I don't like to give up like this, so will try to follow up if I have any more info or regrets after replacement.

Take care.

On 2021-11-15 by Inspectapedia Com Moderator

@Andy Haninger,

That's right-on. That type of switch is still sold, including in generic form by other manufacturers; you'd take the old part to your local heating supplier.

In our home page for this topic

FAN LIMIT SWITCH

see the discussion titled:

Types of Fan / Limit Controls: adjustable vs. snap-disc

where we discuss CamStat and other universal replacement controls.

On 2021-11-15 by Andy Haninger

Thank you again. Here's a picture of what I believe to be the limit control switch I've been overlooking.

The model number is Stemco 430-1448 HH12ZA252C (the shadows blocked part of it, so I'm at least partly going by an Amazon listing for the part).

I'm not terribly enthused about getting a part that was "removed from a working unit and guaranteed to work" so hopefully if this is my culprit, a new part is available.

On 2021-11-14 by Inspectapedia Com Moderator

@Andy Haninger,

If you disconnect the thermostat but the heat exchanger is still hot, it's proper and normal for the fan controller to run the fan a bit longer -

The explanation is at FURNACE FAN CYCLES AFTER HEAT (link in earlier response)

When that cool-down cycle is "on-off" as you describe it may be indeed a dirty heat exchanger or air velocity issue or one of the problems and diagnoses suggested on that page, so it's worth a look.

When you can't find the fan limit control it might be because you're looking for a device that looks like the one at the top of this page while your furnace uses a smaller, less-obvious switch - a snap-disc type controller.

The home page for this topic

FAN LIMIT SWITCH can help you identify the type of fan control you have and to find it.

On 2021-11-14 by Andy Haninger

@Inspectapedia Com Moderator,

Thank you. I've read through this page and tried what I could. I have still not been able to locate my fan control switch and haven't felt comfortable enough to take out and clean the heat exchanger or blower fan.

I did adjust my pilot light slightly lower as someone else suggested that too large of a pilot could cause enough heat to start the fan. I didn't expect this to work, though, and didn't see any change.

I did recently come home to find the blower running, but no flame in the furnace. The pilot had blown out. The thermostat is a basic Honeywell and said Heat On, so it had apparently called for heat. But why was the fan on if the pilot wasn't there to ignite the furnace? There is no way the furnace got up to the temp needed to start the fan.

I set the thermostat from Heat to Off and waited for the fan to turn off. After a minute or two, it did it's usual Off, On two secs, Off, On one sec, Off. But why did it wait to turn off? No heat, no fan, right?

I've noticed that when I set the thermostat's fan control from Auto to On, I do not hear a relay in the thermostat clocking like I do when it calls for heat, but the fan does turn on and off correctly.

I'm wondering if the thermostat is failing in some way. It would at least be a fairly simple and cheap thing to try.

As another test, I've tried pulling the thermostat from the wall after a call for heat, but before the fan has turned off.

The fan kept running for a minute or two and then did the off-on-off-on-off thing.

Again, setting the fan from Auto to On and back to Auto causes the fan to turn on and off as it should.

On 2021-11-04 by Inspectapedia Com Moderator

@Andy Haninger,

Our list of Recommended Articles found above includes an article you will want to review:

FURNACE FAN CYCLES AFTER HEAT

as that includes most causes (and diagnostics) for the case you describe. You'll see that the problem could be a dirty heat exchanger, wrong fan speed, or something else;

Watch out: we do NOT recommend trying to clean or fool with the fan limit sensor itself as doing so can bend it out of calibration and make it unsafe.

On 2021-11-03 by Andy Haninger

I have an old (30ish years) Bryant gas furnace. All works fine until the fan shuts off. It then spins back up for a few seconds, shuts off, spins back up for a shorter period of time (~1s) and then back off.

I suspect the fan control switch is dirty and taking a long time to unwind as it cools, but I'm not sure where it is to confirm and identify the model in case I need to replace it instead of cleaning it.

On 2016-10-28 by (mod)

You're right, David: if the system is overheating it can crack the heat exchanger, risking fatal carbon monoxide poisoning.

On 2016-10-26 by David

My Ducane furnace is shutting down after 5 minutes of operation on high heat limit. Jumped out the switch and it ran longer. Didn't let it run to long though it may damage something

On 2016-10-14 by Lee

Have an old hallmark natyral gas furnace the just replaced blower on. Since replacement of blower motor am not able to stop blower from coming on and going off again and again. Could it be the fan settings? Please help me set the fan settings properly

On 2016-10-11 by John

Blower cycles on and off even when heat and AC are off and fan is set to auto.

On 2016-07-24 by (mod) - tapping the limit control to see if it runs: loose or corroded electrical wire connection?

Tapping the switch and seeing it run suggests that you've got a bad or shorted thermostat wire.

On 2016-07-24 by Rick

AC blower would not come unless tapped on fan/limiter switch. I replaced limiter switch, now fan will not come on unless I set fan limiter pull switch to manual(constant run) mode. The old switch did not have a push/ pull mode switch.

Do I have new problem or still the original problem, that I can now override with my newly acquired mode switch? On manual setting, fan will not shut off, even with thermostat turned to "off".

On 2016-07-07 by (mod)

Nancy this is an issue to review first with your landlord, your rental contract, and with the landlord's legal obligations - that may vary depending on where you live.

Watch out: If you believe there is an immediate life/safety issue you can and should get out of the building and call your emergency services.

On 2016-07-07 by Nancy Pierce - Bryant air conditioner, Goodman furnace - I'm getting sick - SAFETY WARNING

Im on rent my landlord has the fan blowing in the summer. The air conditioner doesnt work. Im getting dust and the smell of gas in the house and heat through thr cold air vents as well as vents heat normally comes through.

The air unit is a Bryant and the furnace is a goodman.

Something is terribliy wrong here cause im getting nauseated, my throat swells closed and i have to seek medical attn. Im afraid i wont wake one day because of the way the furnace is set up.

On 2016-02-23 by (mod)

Sorry Eric I'm not familiar with this board. I agree that it looks as if you've pinpointed a possible hardwired or jumpered 90 minute cycle setting.

I would download a copy of the owners' manual for the furnace and if necessary a separate guide for the specific control board if that is not an OEM part in your furnace.

There the options and controls will be detailed. It may be you need simply to remove a jumper.

Keep us posted.

On 2016-02-23 by Eric - Goodman B12260-08 board

I have a York Diamond 80 Furnace that come on every 90mins on the clock to pump air through the house. It usually come on about 10-15 mins then shuts itself off. Then after another 90 mins it would come on again.

I notice there is a Goodman B12260-08 board that has some jumper switch on it with number 60,90,120. I try to adjust that but does not seem there is any impact....Any ideas?

On 2016-01-16 by (mod)

Can, yes. Should? No, subverting this safety control can destroy a furnace or can kill building occupants.

On 2016-01-15 by Dee from michigan

Can you over ride a combination bower limiter switch

On 2016-01-06 9 by (mod)

Andre, please use the page bottom CONTACT link to send us photos for comment or posting.

Indeed there are several versions of fan limit switch and they do not all look alike. Several of them appear as photos in this article series but I'd be glad to see and comment on your system and some sharp close-ups of each of its controls.

On 2016-01-06 by Andre - how do I post a picture of my fan limit switch on a 1960s Bryant Quietline

Can I post a picture here? I have been trying to locate the fan limit switch on my 1960s Bryant Quietline, but it seems to look different than the ones shown on this website.

On 2015-11-26 by (mod)

Mister

Since the system worked previously and you've not changed any switch or thermostat settings, I'd replace the control.

On 2015-11-24 by misterw - mobile home unit blows only in summer position

Same problem as Michelle, got an old miller unit for mobile home, blows in the summer position and not winter, worked fine in winter position until today. Any tips\help appreciated

On 2015-10-23 by (mod) - check fan limit dial settings

Sounds as if the air flow rate is cooling down the heat exchanger / plenum to shut off the fan to avoid blowing cold air on to people who want heat; be sure the air filter is installed; check the air flow rate.

Check that the limit switch settings are to factory spec.

Don't bend the switch plate or spring or you'll force it out of calibration and may make the system unsafe.

On 2015-10-23 by Glen

I just installed a new blower fan and now the fan control starts the fan then dials back and shuts it off in abt 17 seconds. Then dials up and starts the fan again.

On 2015-10-23 by michelle

My furnace has a winter, summer, and off switch. The blower blows in the summer position but not the winter position. So it blows cool air not heat, but the blower gets real hot.

On 2015-10-17 by Anonymous

Can't say from s o little informatiom. Use the page bottom CONTACT link to send some clear photos to the InspectAPedia.com editor and we'll be able to give advice.

On 2015-10-17 by Anonymous

What is the black switch on oil miller furnace for should it be on or off mode

Question: My furnace stays on and running even when the thermostat is disconnected

(Mar 18, 2015) Armando Morales said:
My furnace stays on and running even when the thermostat is disconnected. Please tell me what's the problem and how to fix it.I think is the limit switch but I'm not sure.

Question: furnace won't turn off

(Sept 20, 2014) richard said:

My furnace turns on immediately when I switch the breaker on and will only turn off by switching the breaker back off. I put a new thermostat and new wire to it in. Any ideas?

Reply:

Sure Armando

Check for thermostat wires shorted together or a failed fan controller.

In the More Reading ARTICLE INDEX links see

• FAN WONT STOP - THERMOSTAT SWITCH
• FAN WONT STOP - LIMIT SWITCH
• FURNACE BLOWS COLD AIR

Richard

Check for shorted thermostat wires

See HEAT WON'T TURN OFFfor helpful diagnostic procedures

Question: blower fan only works on manual

(Nov 30, 2012) Fan blower works when limit swit said:

Hi There
My fan blower in my furnace will work when I push the button on the limit switch to manual but not when I pull the button out to auto. Is it possible that I need to replace the limit switch or maybe the fan motor? Any help would be appreciated.

Thanks Nick Van Amstel

Question: blower fan wont run

6/16/14 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?

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.

Nick, see the diagnostics at FURNACE FAN WONT START

Question: furnace overheats, stops before thermostat is satisfied

(Jan 23, 2014) Rusty Agus said:

My frnace overheats and stops without reaching the desired temperature as set-up in the thermostat. Technician for the company that I have a maintenance agreement with said that the cause is the plenum being undersized. I can't believe his diagnosis. Should I get a second opinion or could you tell me the real cause of the problem? Thanks.

This question was posted originally at HEATING SYSTEMS

Reply:

It's a fair question and the explanation is possible, Rusty. BUT if the system is not new, if it was not behaving this way before, then one would not expect the problem to be plenum size.

Also here's a crude rule of thumb tip: compare the cross section of both the return air plenum and supply air plenum to the cross section of the furnace heat exchanger. If they match then that's good.

Finally, I would look for some other clues that would explain the problem, such as

- reduced air flow due to a dirty air filter

- reduced air flow due to dirt on the blades of the squirrel cage fan in the air handler

- reduced air flow due to squashed, disconnected, or damaged ductwork

Question: blower won't shut off and heat is inadequate

(Jan 1, 2015) Anonymous said:
my gas furnace is not functioning properly. The blower fan is running constantly and the house temperature keeps falling, the air coming out is pretty cool.

I went to look at the filter and it seems fine, but as I was down there, I noticed that the burner keeps igniting over and over, repeatedly, but the house will not warm. What is going on?

Reply:

The blower fan may be set to ON or MAN at the thermostat or at the fan limit switch.

Normally when the thermostat calls for heat, if the fan is on AUTO, the fan won't turn on until the burner has already fired AND the temperature in the air plenum over the heat exchanger has reached or passed the FAN_ON temperature on the fan limit switch.

The fact that your burner is cycling on and off suggests a control problem, shorted thermostat wires, a loose connection or something similar. I'd ask for a heating service call.

(Jan 1, 2015) Anonymous said:
Hi Dan, the blower fan is just set on AUTO. I was suspecting, like you stated, from what all I have read here is that maybe a problem that I can't fix. Thank you for responding. I appreciate your help. This heating and air system is awful. I have to call a service person at least 2-3 times a year, and the house is only 6 years old.

Reply:

Having to make repeated service calls suggests something fundamentally wrong. I'd talk to the service manager, politely, and ask that they send a senior, experienced tech so that you can get down to a single annual service and maintenance call.

Question: blower fan won't turn off, blows cool air at end of heating cycle

26 January 2015 jeri said:
my oil furnace turns on fine and blows hot air but when its time to expel the hot air after the tempeture has been reached, it does not, it blows cool air. What the problem?

Reply:

Jeri

It sounds as if the limit switch is leaving the blower on too long after the plenum has cooled down. Check the FAN OFF temperature setting or time.

 

Question: stuck pressure switch?

If u get a pressure switch stuck open fault - Julio 8/16/11

Reply:

Julio if you are asking about a fan motor reset switch that does not reset, either there is a short in the motor or a bad switch itself. "Pressure switch" - I'm unsure what you mean.

Question: blower keeps starting & stopping at beginning & end of cycle

(Nov 13, 2012) phil said:

My blower starts and stops and starts at the beginning of the cycle and at the end as well.

Reply:

Phil,

Depending on air movement rates and heat exchanger temperatures and fan limiit switch settings, a blower can cool down the plenum at the start of a heating cycle, causing the blower fan to turn off. And at the end of a cycle when the heat exchanger is much hotter, the blower may cool down the plenum, shut off the fan, but residual heat in the heat exchanger can re-heat the plenum, turning the fan back on.

Try widening the ON OFF gap between the FAN ON FAN OFF settings on the limit switch by about 5 degrees. Do not change the upper fan limit safety setting.

Question: how to delay blower turning on to avoid blowing cold air

(Nov 19, 2012) Mark Brady said:

When heat is called for the furnace burners start and the blower comes on with no delay in a gas furnace. I have two similar furnaces and the other one has a temp delay. I've verified limit switch is turning. And have adjusted the on limit up and down 10 degrees from it's 120 F set point. It just seems like the blower comes on with the burners. And I'd like it to delay like it should.

Reply:

Mark, I agree that we don't want to blow cold air

I suspect a bad control or a wiring error.

Just check all settings at the thermostat and limit switch

TT set to HEAT, FAN AUTO

limit switch fan control set the fan override to AUTO

shut the whole system off, let it cool down, thunder it on and thermostat up. If it blows cold air immediately most likely it's time to replace the control.

Question: fan runs faster in manual than in auto mode

(Dec 7, 2012) Bill McManus said:

Had new thermastat installed last year, after which the fan always turned faster in the manual on mode than in the auto on mode. This year my heat would not come on and my plumber replaced the igniter.

I told him about the fan speeds and he said he never heard of that but replaced the fan limit switch and it still runs at two speeds, no change. He said the switch was part of the no heat problem. Does this sound right to you?

Reply:

Bill if your unit has a variable speed fan it may be that the controls are wired to only use the slower speed in heating mode. Often blowers run at different speeds for cooling than for heating.

Question: fan cycling when thermostats are OFF

(Jan 2, 2013) chris castagner said:

I have a question the fan on my furnace come on and off all the time even when i have both my thermostats cool, off , heat switches to off and the fan switch setting on there is only on and auto and they are both set to auto

Reply:

Chris I suspect a short circuit in the thermostat wiring or a control problem -

Question: Heat won't turn off

(Jan 2, 2013) Tom said:

I have a Borg Warner unit and the blower and burner keeps running. I installed a new fan limit switch but I noticed once I turned th gas back on the system reached fan on setting the fan came on and when the unit continued to run i
turned the thermostat off and the fan continued to run.

So I turned the breaker off to the fan and the burner kept on burning, it would not shut off unless I turned the pilot valve off or the main gas valve.

Also the gas valve has a 2 wire connection on it. One wire I assume is coming from the thermostat and the other wire I think should be wired to the fan load limit side. I cannot figure out why the burner will keep running no matter what unless I turn the gas off.

Reply:

Tom see FAN WONT STOP - LIMIT SWITCH

as a place to start diagnostics. Also check for shorted thermostat wires that keep calling for heat.

...

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

Or see FAN LIMIT SWITCH FAQs-4 - more-recent questions & answers on fan limit switch or control operation, troubleshooting, repair

Or see these

Recommended Articles

• AIR HANDLER / BLOWER UNITS - home
• FAN, AIR HANDLER BLOWER UNIT
• FAN MOTOR START CAPACITORS
• FAN LIMIT SWITCH
• FAN LIMIT CONTROL SETTINGS
• FAN LIMIT SWITCH INSTALLATION & WIRING
• FAN LIMIT SWITCH TROUBLESHOOTING
• FAN RUNS ONLY ON FAN-ON / MAN
• 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
• FURNACE FAN WONT START
• FURNACE FAN WONT STOP
• WOOD FURNACE FAN LIMIT CONTROL
• FAN MOTOR START CAPACITORS
• FAN NOISES, HVAC
• FAN ON AUTO MAN THERMOSTAT SWITCH
• FAN ON SWITCH ADDITION
• FAN WONT STOP - THERMOSTAT SWITCH
• NO HEAT - FURNACE

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FAN LIMIT SWITCH FAQs-3 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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