Air Handler Blower Fan Won't Stop - Other FAQs

Indoor AHU or Furnace Blower Fan that Will Not Stop

Other Air Handler Blower Won't Stop FAQs

After upgrading my themostat the fan runs continuously

Recently upgraded thermostat to digital. Now fan continually runs and heat is warm then turns cool.

And the fan won't stop blowing. What did I do wrong? On 2017-11-11 by CJ

by (mod) - Check for mis-wired or shorted thermostat wires

CJ

If those heater problems have happened since the installation of the new thermostat I suspect
- mis-wiring
or
- thermostat wires accidentally shorted together

Saul:

Reasons that a blower won't turn off when the thermostat-set-temperature has been reached are given in the article above starting at https://inspectapedia.com/heat/Furnace_Fan_Wont_Stop.php

by CJ

If they were accidentally shorted together what do I need to do? I verified wiring and appears to be correct. I do know that we relocated the thermostat and had to add some thermostat wire to existing in order to reach the spot we wanted.

Reply by (mod) -

CJ

If testing confirms the thermostat wires are shorted together, then you either find and repair the short or run new thermostat wires

 

Furnace fan comes on as soon as power to the electric furnace is turned on

Electric furnace blower comes on as soon as I turn on power at fuse box. If I take out fuses at furnace the elements will heat up. As soon as i put fuses back in fan starts and elements goes out. What could it be? Thanks On 2017-12-28 by mike

Reply by (mod) -

Mike
Since on most forced warm air heating systems the blower does not come on until the plenum has warmed up, and since your system turns on immediately, I suspect that there is either an improperly set FAN ON switch that is forcing the fan into manual on position, or the limit switch is defective

Will the furnace be damaged if the blower fan doesn't turn off?

How long before the furnace starts to get damaged if the fan doesnt shut off. - On 2018-03-07 by Jim

by (mod) - No

Jim

Running the fan continuously should not damage the furnace, but it's true that some fan motors may be designed for continuous operation and others not

I figure that if a furnace was provided by the manufacturer with a manual FAN ON switch on the limit control then the designer intended that the fan could be operated continuously.

Where is the blower fan motor switch found?

My blower motor will not stop running unless I turn thermostat down about 10 degrees below temp reading. Where is motor limit switch located? Could it be a faulty thermostat? Thanks. H. Thomas

(mod) - how & where to find the fan limit control switch

Hershel

See FAN LIMIT SWITCH and you'll fin dphotos of the control that give its typical location.

Because the limit control switch has to sense the air temperature inside the furnace it will always be mounted on the furnace, typically on or close to the supply air plenum, thus on the upper are of most furnaces (some oil furnaces locate the control lower in the unit).

The switch will usually be under or behind the furnace cover that's removed to give access to the furnace burner and controls,

or


On some furnaces you'll see the fan limit switch mounted on a supply air plenum (chamber) above the burner and heat exchanger section - that is, at the start of duct work that carries warm air into the building occupied space.

Coleman furnace fan switches tiself from AUTO to MANual

I have an old coleman furnace. The fan wont shut off after cycling. I have replaced the limit switch and fan switch with a universal. I have it set to auto and it starts and cycles fine. During the cycle, for some reason, it will automatically switch from auto to manual thus keeping the fan from shutting off. Any ideas? Thanks, Paul On 2019-01-09 by paul

by (mod) - fan switches tiself from AUTO to MANual

Paul

Thanks for a stump-the-chump question: I am puzzled too. I suspect a bad electrical connection or bad control.

Try going through the fan limit control switch diagnostic procedures at

FAN LIMIT SWITCH TROUBLESHOOTING

and let me know how that works out.

ALSO be sure to confirm that none of your thermostat wires are shorting or touching one another nor loose at their connectors.

Blower fan won't stop and can 't find OEM relay switch - they want me to buy a whole new furnace

Blower fan keeps running and contractors tell me they cannot find the manufacturers original part to replace the failed one and they want us to buy an entire new unit, can anyone help? On 2019-07-17 by Paul

On 2019-07-17 by (mod) - service tech says can't get a replacement control board for my air handler / blower

Paul,

Let's start by identifying your furnace blower unit brand and model; a photo of the unit data tag would be helpful. From that we also want the unit's age, and the country and city where the unit is installed.

Then you can contact the manufacturer to ask their advice. It ought to be possible, at worst case, to replace the blower controller: circuit board and relays; it would be a surprise for the manufacturer to say you have to replace the entire unit. Also there may be third party generic parts that will work.

Suspect bad circuit board causing blower to run continuously

So, hope you can help. I have a blower fan, that does not shut off. This is the same situation on the heat side AND the cooling side. NOTE: When heat or AC itself shuts off, fan slows down, but does not fully stop running. When heat or AC kicks on, it blows at full normal speed.

New issue, did not used to be there, for years. Oil fired furnace/AC unit. Checked all thermostat wiring, no issues. High temp switch checked and replaced( was probably fine). No change. Unit: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

It has been suggested to me that there may be a "cold weld issue" on the circuit board and to replace it. I want a second opinion before I spend the $100+ on a new board and spend the time replacing it. Any help appreciated. On 2019-07-03 by Doug Ziebro

Reply by (mod) - Replacement Blower Fan Relay Switches:

Could be a failure on the control board or on a replaceable fan relay switch. See if your unit has a separate fan relay.

When the system fan should be OFF your tech will check to see if the relay is dropping power to the fan. I'd like to actually do some tests before replacing the board.

Supco relay used as an A/C fan relay switch.

An replaceable electromechanical relay is usually a physically separate device with a visible, moving relay part that you could even push down with a finger (don't unless power is off). There's none in your photo.

Or it's labelled "relay" - here's a general purpose Supco relay used as an A/C fan relay switch.

Goodman Amana Furnace Fan Relay B1370751 B13707-51

And as another example here is a Goodman Amana Furnace Fan Relay B1370751 B13707-51

 

by Doug Ziebro - Bad Fan Controller: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

Thanks for the input. I appreciate it. Silly question...how do I know if I have a replaceable fan relay switch? I know I have a " continuous low speed fan switch", but that is always set to "off".
For a point of reference, I have worked on motorcycles and cars, including rebuilding engines.

I have also fully serviced the beckett oil heater(burner) side of this unit. So Hopefully, I will able to troubleshoot this fan switch. Thanks for the help.

Overheat at resistors on Fan Controller: Carrier 361AAN036075/ Control board Honeywell ST9103A1002

Again, thank you. I was able to replace the board, and luckily, the swap solved the problem. It is amazing to hear the AC shut off, and the fan stop as well.

We have gone for almost a year, having to manually hit the power kill switch, on the out side of the unit. to get the fan to shut off.

The beauty of the internet and the sharing of information, is a truly wonderful thing. Timing is great too, because we are supposed to be getting a 100 F+ heat wave here in NJ this week.

I have attached a picture, with an area circled, where I think the failure point potentially was. It seems slightly burned. Rear side of board looked fine.

One the new board, where the those two resistors are, is a large square block style ceramic heat sink. Makes me ponder of this was a recurring issue, spurring Honeywell to update this control board. Thanks again.

Above: Carrier 361AAN036075/ Control board Honeywell ST9103A1002 - On 2019-07-17 by Doug Ziebro -

Reply by mod - replacing control board fixes problem with blower fan that would not stop: signs of overhreating on control board

Doug,

Thank you so much for the photo, the follow-up, and the detailed look at the browning that suggests overheating on the Honeywell fan control board in your air handler - that information will certainly help other readers.

Working together makes us smarter.

Furnace fan won't stop running, is the Burner Locked Out?

Furnace fan won't stop running. Owners Manuel says it's locked out. What does that mean was running fine until we changed filter On 2018-11-21 1 by Kristine -

Reply by (mod) -

Locked out usually means that the system burner has shut off on safety because the burner would not ignite.

That doesn't sound like your problem. Check through the Diagnostics giving in FURNACE FAN WONT STOP

Blower Fan is ON and will not turn off. Control board or control board switch problem?

I am having a strange issue that I can't seem to get to the bottom of. Hopefully someone can help!

Blower Fan is ON and will not turn off.

Heat - OFF, Fan - AUTO = Fan stays ON

Disconnect ALL thermostat wires from Fan Control panel = Fan Stays ON

Replaced Fan Control Panel with New = Fan Stays ON

Wacked Blower Motor with BFH = Fan Stays ON - On 2017-12-05 by BenC

Reply by (mod) -

First change the thermostat fan setting: Set fan to AUTO,

then if the fan still keeps running:

- Check the limit switch

- Look for shorted control wires or relay

Followup by BenC

My furnace is an old Armstrong Series III 80 FYI

I now have replaced Limit switch. No Change.

Troubleshooting, I unplug Heat from Fan Control. Then Unplug either Roll Out or Limit. Blower stops, Induc fan turns on.

Replace lead for Roll Out/Limit, Induc turns off and no blower. 2-3 min later

Blower fan starts.

If I do the above sequence and plug in Blue to Heat after shutdown from Roll Out/Limit, blower fan starts with everything off.

Suspect motor or safety control causing blower fan not to work as it siould

I believe the problem is the motor. I have not confirmed this yet. But I have the Heat unplugged from motor control. When I trip the roll out or limit, blower shuts down. Meaning motor shuts down cause board goes into fault.

When time clears blower starts again. There is constant 120 from the board but blower should only start when heat or cool tells it to. With heat unplugged, the only 120 is thru the 120 and com. Which is normal. If it only shuts off when this is faulted and power is cut from these lines, and wires from fan control to blower ohm good, must be short internal on blower motor. On 2017-12-16 by BenC

Lennox blower keeps running, no heat.

Same problem here. Lenox constant blower no heat. no voltage to thermostat. Jumped limit and rollout limit.{they are in series} thermostat was powered and called for heat....for five min. then back to fan only. Do you think it's a circuit board problem? 29 degrees outside on a Sunday On 2017-12-10 17:09:17.594523 by Phil

Reply by (mod) -

Phil and Ben,

I agree that when the control board shows a fault light when the fan turns off that's diagnostic.

But could it not be a different fault - that is not the motor - that the control board is sensing, and in response to that fault it is turning off the motor?

I agree too that if the control board continually feeds power directly to the motor then there's a control problem. But if the board voltage is actually going first to a safety control like the fan limit switch then that's a different situation.

Do you have and did you check the wiring diagram for your unit?

When a blower fan won't stop I wouldn't expect a bad motor to be the direct cause, more likely it's a control or relay.

Worried that my furnace keeps igniting - possible fire hazard

I think my air off and the fan off but my furnace keeps lighting up. I'm worried it with catch fire. - 2019/11/04 Misty Lawhon

Reply by mod:

Misty

I can't tell what's happening with your furnace from your brief text: is the thermostat calling for heat?

Watch out: In any event if you have the slightest worry that the heater is unsafe then

You should shut down the system then called for repair;

No Manual Switch on our Honeywell Fan Limit Switch: Should we Keep our Thermostat Fan switch set to AUTO?

If our Honeywell fan limit switch does not have the auto/manual push/pull button for the fan, then is it safe to assume that it is always set for auto?

Our limit switch does not have the hole for the button in the cover, but when opened, it looks like the image above except there is no white button.

Maybe it broke off? (we just bought the house and moved in so don't know).

I read another comment from someone who said that not all models have the button, but our's keeps running well after the temperature is reached.

Any ideas on how to make sure the fan limit switch is set to auto? - On 2020-10-19 by Ed

Reply by (mod) - yes

Ed

Essentially, yes. IF your limit switch has no MAN-ON switch (found as a white push-pull knob or as a small metal slide left-right lever) then yeah that control is in automatic mode.

However some thermostats also include a FAN ON / AUTO / switch that can also control the fan. I'm showing that in the thermostat switch photo just above.

If your fan NEVER turns off regardless of the thermostat setting then I suspect either one of the problems listed

at
FURNACE or A/C BLOWER FAN WONT STOP

OR

one of the situations described at FAN WONT STOP - THERMOSTAT SWITCH

I turned off my A/C off but cool air is still blowing out of registers

I turn my ac off but the cool air was still running .what can cause this problem - Anon On 2012-07-19

Reply by (mod) - AC is off but cool air keeps coming out

I'm guessing either your fan is in manual ON mode or there is a problem with the blower fan relay or control circuit.

See BLOWER FAN WONT STOP - THERMOSTAT SWITCH

and A/C or FURNACE FAN WONT STOP

Furnace fan runs continuously, smell of burning plastic

Fan continuously runs on furnace, replaced the circuit board in March which fixed the issue for about 2 months, back to continuously running.

We did smell what seemed like burning plastic that we couldn't pinpoint, but seemed to be about when we noticed it was running again.

Could there be an issue with the electrical panel because it seems to keep burning out? On 2017-05-25 by Megan Fan

Reply by (mod) - F

Megan,

I agree that it at least appears that there is an external problem hurting the circuit board.

A power surge or a shorted component, or dust, water, debris can also short a board.

Electrical panels don't usually initiate a problem, more likely a damaged or corroded or defective component fails to protect against an overcurrent on a misbehaving circuit.

Watch out: for electrical and fire safety, I'd ask for help from a trained service technician. Your service tech will also check for excessive current draw through the control board, for example due to a failing relay switch or failing electric motor.

Fan sometimes kept running more than one hour after burner has been automatically shutdown

My fan sometimes kept running more than one hour after burner has been automatically shutdown, sometimes the fan stopped just a few minutes after the burner is shutdown.

Is this a problem with the fan limit switch which should tell the blower to turn off when the residual heat in the furnace is dropped to below the temperature setting? Or is just malfunctioning? On 2016-12-22 by Anonymous

Reply by (mod) -

Running the fan for an hour after the burner has shut down means something's wrong. I suspect a bad wire connection or a failing limit control.

The fact that the problem is intermittent also suggests the same diagnosis. If there's no obvious loose wire or loose connection I'd ask for help from a service tech.

 

My furnace blower stops only when is in emergency heat

Anon:

by (mod) -

The blower and furnace should run until the thermostat is satisfied.

If the thermostat is not satisfied in aux heat that may be the trouble.

In other words, your thermostat is calling for heat and is not satisfied until you turn on your backup or emergency heat system. That tells us that you're probably using a heat pump in HEAT mode and outdoor temperatures are too low for it to suffice.

It sounds as if the control that should be turning on back up heat automatically is not working.

Ask your service technician to check that control.

Backup electric heat in air handler shorted and made the fan keep running

Pull electric elements out of furnace and make sure none are touching the frame of the furnace.

I found this condition on a goodman that had the elements and the fan running all the time. - On 2016-01-09 by Larry

by (mod) - check the backup electric heating elements in the furnace of a heat pump system

Thanks Larry for the good suggestion: a fan control wire or even possibly a thermostat wire shorted to ground (the metal furnace or blower compartment body) should be on our checklist.

Blower turns on and blows cool air even when it's set to OFF

My furnace comes on and runs cool air (we don't have AC) even when I have it in the OFF position of <heat off air> and the auto/ manual fan is in the auto position.

I have the temperature set all the way down to 50 degrees. Where do we start looking? - 2015-08-18 by Mdk 8998

by (mod) - shorted thermostat wires

Check for thermostat wires shorted together

Disconnect the TT wires at the air handler; if it stops running then either the thermostat wires (TT wires) are shorted together or the thermostat itself is defective.

Fan Limit Switch was Stuck ON - I fixed it myself

After lighting my furnace, the fan kept running and would not stop. After several hours of troubleshooting and following the schematics. I traced it down to my fan limit switch. I removed the switch and found it to be stuck at the on -150 position.

I cycled it by hand a few times and took a wire brush to the base. it had some build up causing it to not release to the off position. After cycling it the switch ended up going back to the off position. I installed it and my furnace now works fine.

Great pictures on this site made it possible for me to fix it myself. Thanks!! On 2014-11-28 by RAY G

Reply by (mod) - Ray's Fan Limit Switch Problem Troubleshooting Tips

Ray you did a steller job of debugging a fan limit switch problem.

And thanks for the nice comments.

But still you should really replace the switch.

Watch out: The fan limit control switch is particularly sensitive to being forced out of calibration, especially if someone forces or pushes on the rotating dial parts - the result could be unsafe as the furnace could over-heat, risking fire, damage, or fatal carbon monoxide poisoning.

Sorry but while you did a marvelous repair I'm left ... worried.

by Ray G

Thanks for the input Dan, I have a new switch on order as I type this. and am making sure the fan runs for a bit after the furnace shuts off, in order to cool the heater box down before shutting off.

by (mod)

Quite right, Ray. That fan run-on interval after the burner shuts off is important to avoid cracking a heat exchanger.

Blower fan keeps running until I tap the Fan Limit Control Switch

The fan starts ok, but keeps running until I tap the limit switch. Have to do this each time or fan keeps running. Is the limit switch bad? - On 2014-10-06 by Anonymous

Reply by (mod) - fan stops only if I tap the limit switch

Anon

I'd check for a loose wiring connection first, then if needed, replace the switch.