Questions & answers about why does the air blower fan or furnace fan run only if the thermostat is set to FAN-ON and your system is in HEAT mode? Why won't the fan work if the thermostat is set to FAN AUTO?

This article series explains the use, setting, and adjustment of the FAN ON- AUTO button or switch or the FAN ON-MAN switch on room thermostats.

FAQs: Heating System Blower Fan ONLY Runs if on FAN-ON or MAN

These questions & answers were posted originally at BLOWER FAN RUNS ONLY ON FAN-ON / MAN - HEAT where we list the possible causes of "Fan Wont' Run on AUTO", some more-likely and most quite easy to test.

Be sure to review the diagnostic and repair steps given on that page.

On 2023-03-17 by Carin - Fan work in "auto" when the system is making heat but not when switched to "on"

Fan work in "auto" when the system is making heat but not when switched to "on" which is my preference.

On 2023-03-18 by InspectApedia Editor (mod) - Fan work in "AUTO when the system is making heat but not when switched to FAN ON

@Carin,

Are you switching the fan to ON at the thermostat or at the Fan Limit Control at your furnace or air handler?

If not the latter, go look at the fan limit control switch to see if it includes a FAN ON button.

On 2022-12-23 by Douglas - Goodman - GMP075-3 fan won't run on AUTO

My Goodman - GMP075-3 works fine with the a/c. But turn on the furnace and the fan doesn’t blow air on auto but does when in the on position?

On 2022-12-24 by InspectApedia (Editor) - fan won't run on auto if the heater is cold

@Douglas,

Does the heat source, such as a gas or oil burner or electric heat turn on? The fan won't run on auto if the heater is cold.

See the diagnostic steps at

NO HEAT FURNACE

On 2022-12-24 by Douglas

@InspectApedia (Editor), The heater comes on and cycles according to what the thermostat is set. Just the fan only runs in the on position then it runs continues. It’s a propane.

And yes the heater comes on in the auto position.

On 2022-12-24 by InspectApedia (Editor)

@Anonymous,

OK. That's helpful.

So the diagnostic steps above in this page apply. Have you tried going through each of those those

On 2022-12-06 by Eric - electric furnace will only run in the fan ON position

My electric furnace will run in the fan on position but not in the auto position change thermostat didn't help could it be a fan relay switch are a are a limit switch need help please

On 2022-12-06 by InspectApedia (Editor) - fan runs when switch on manual but not auto

@Eric,

Well you have certainly found the right page to diagnose and fix the problem of your fan not running on auto.

The fact that the fan runs when you put the switch on manual tells us that the problem isn't the fan or blower motor itself nor the fan relay. But there are other possible control problems.

Take a look through the diagnostic steps given above on this page and let me know how that works for you.

On 2022-11-15 by Alex - furnace not coming on when heat is needed

Hello, when I wake up I notice that my electric furnace is off even though the inside temp is well below the temp on the thermostat. I then use the thermostat to put the blower on ON mode and it turns on just fine. The blower also runs fines when I turn it on using the switch on the blower itself.

After letting the blower run on ON mode for a bit, I then put it back on AUTO and then it works fine. Sometimes I also turn the thermostat on and off before putting the bower on ON mode. I would appreciate any tips. Thank you!

Oh, and I can hear the clicking sound coming from the thermostat after I turn the thermostat off and then back on hoping that would help the blower come on, but it doesn't unless I have done the FAN ON thing I described in my post. Thanks again.

On 2022-11-15 by InspectApedia (Editor) - don't keep restarting the system when it's turning itself off

@Alex,

We don't know what kind of fuel or hating furnace you have installed, it could be oil, electric, or gas. But if it's not turning on it's most likely the case that it needs to be cleaned and serviced. Have you had a service call?

Watch out: don't keep restarting the system when it's turning itself off as you may create a safety hazard, particularly for gas or oil fired equipment.

On 2022-11-16 by Alex

@InspectApedia (Editor), Thank you for that. It is an electric furnace (Intertherm). Bought the place not long ago; the furnace is a bit old.

On 2021-06-11 0 by Michael - The AC fan won’t start - bad capacitor?

The AC fan won’t start originally, I replaced the dual capacitor with same specifications. It was working fine for a couple fan start and fail again. Gave it a manual push and fan start and will auto start itself for a couple times. Then the fan fail to start on its own again.

Was the new capacitor bad too or it was some other problem?

On 2021-06-11 1 by mak.church (mod) - Was the new capacitor bad?

@Michael,
This section at the link below may help you diagnose and fix your problem:

12 REASONS THAT AN AIR HANDLER, A/C OR FURNACE BLOWER VAN DOES NOT RUN

Let us know if you have additional questions.

Question: furnace fan runs on manual but not on AUTO

I have ac/heating system that worked fine till last year when I had to replace the condenser then during that summer the thermostat went bad and had to change it.

The AC part works fine on auto.

Both the the condenser and the fan/ blower go on and off at the same time when required.

Now the issue is with the heating part. The boiler works on auto but the fan/it does not. I have to either manually put it on or off or let it run non stop.

What could be the issue? - A.M. by private email 2022/09/23

Moderator reply: Check: thermostat, wiring, then the control board & relay

First thing I would check would be the thermostat wiring.

If the thermostat is properly wired at both the thermostat and at the control on your heating and air conditioning system,

and if the wires themselves are not damaged,

then I suspect that there is a control board or relay failure in your air handler such that in heating mode it's not turning on the fan.

Reader follow-up:

They did check the wires ( and broke a brand new thermostat in process) and the wires still seam connected right.

Also the blower/fan does work on auto when AC is on. I am not sure if I am explaining right.

When system is on cooling, both the condenser and the fan work on auto, they both go on when the temp is higher then what is requested and they both go of when such temp is reached without me doing anything manually except lowering the temp when needed.

When system is on heat and both the boiler and fan are set on auto the boiler starts and reaches the temp required but the blower does not start and that makes the boiler to shut down without reaching the temp in the house ( I am assuming that the boiler has a sensor by the blower that makes it shut down, I am not a tech but just a very curios homeowner).

Last winter I had to put the heating system manually on and off or leave the blower on On and boiler on Auto but this would make the blower run even when reached the temp.

Are you suggesting that they should check if the wires on the wall?

I very confused and frustrated homeowner.

Moderator reply: fan wont' run in AUTO mode: eliminate the simple problems first

Please understand that when I'm trying to help you from a remote place and I can see and know so little about your system we are flying blind.

When you say the thermostat wires were checked I don't know what the people did. The procedure that I describe would at least rule out the possibility that the wiring is incorrect.

The fact that your system worked until you change the thermostat suggests that the problem is either the thermostat or the thermostat wiring.

Of course it could be something else but we should eliminate the simple and obvious things first.

We will post thus discussion at, and ask you to read

FURNACE FAN RUNS ONLY ON FAN-ON / MAN - [you are now on this page]

So that we and others can best help you please continue this discussion by posting any further questions or follow up comments there

Reader follow-up:

I totaly understand. I appreciate what ever the help you can give me is more then what the people I hired are doing.

Will look into it and if it comes to it replace the thermostat again

Moderator reply:

We don't know what your on-site service people actually have done to diagnose this problem.

Check the thermostat wiring - means that the thermostat fan control wires have to be properly connected for it to run on both AUTO and MAN settings

Check the thermostat wires themselves - means that a thermostat wire might be correctly-connected at the thermostat and correctly connected at your heater, but if wires are broken apart or shorted together anywhere in their routing the system won't work as it should.

We diagnose this problem by disconnecting wires at both the thermostat and at the heater, then we use a test meter to see if the wires are either not continuous or if they are shorted together.

Those are easy and quick and inexpensive tests that any trained service tech ought to be able to do.

Other causes of the problem you describe are given completely above on this page. Take a look as that may help you make sense of what your service technicians tell you.

Keep us posted on what happens - what you find will help other readers.

On 2021-03-22 by Anonymous - so how do I find part numbers for my fan control - don't like my local service people

I thought with what it says on these pages, someone had a book to look up part numbers. if there was a local person, that's where i would have started.
first winter after buying house,

i called a local person to come and do a seasonal cleaning and tune up.

first guy messed it up, someone else had to come out. that didn't fix it either. they sent out someone else, who really only showed me how to adjust the limits. didn't even suggest replacing it.

that was 2015. it's been limping along, makes it warm enough. just runs too often. and before you suggest a new machine .

maybe i'll just wait until may and take it and try to order online.

everyone wants to be helpful, but if you only have reiteration as an offering, it's better to try something else. like recommending a parts site .

wish your hour and a half included travel time .

that's how long it would take to get to the place i got the inducer assembly and back.

so, what's next, if this isn't the problem?

On 2021-03-23 by (mod) - need help identifying the right fan limit control model for replacement - which temperature sensor probe length

@Anonymous,

So you don't have the manual for your furnace, right?

You can probably download it at MANUALS & PARTS GUIDES - HVAC - home - Master Index to All Brand Names - HVAC manuals, wiring diagrams, installation and repair guides.

With brand and model no. Usually a parts supplier is the easiest step. Here we're focused on diagnosis and repair, not exhaustive parts x-referencing.

Still, in my OPINION, it's pretty easy to simply remove the existing fan limit controller, and if you don't find, inside it, a label with its model number, you can still identify the brand (often Honeywell), and its trivially easy to measure the length of the temperature sensor probe directly, then re-install the unit, if necessary, until you have its replacement.

Details of removal or replacement of this heater safety control, along with some safety warnings, are

at FAN LIMIT SWITCH INSTALLATION & WIRING

My illustration shows a Honeywell Fan Limit control with a 5-inch probe. [Click to enlarge any image]

The types and brands and models of fan limit controls are organized here beginning at

FAN LIMIT SWITCH

You could also give Sid Harvey a shout as that vendor stocks every version of the control.

Watch out: I'm not sure - as your comments are anonymous - which furnace brand and model we're discussing, and thus am not even sure we're talking about a conventional fan limit control like the one shown above, or alternatively a snap-disc control or limit switch like the one shown below.

If you know your furnace brand and model number, even without the IO manual for it, any search for parts or controls will pop up websites that list the specific limit control used by your heater.

On 2021-02-24 by Bob - Consolidated Industries "Quatro", Model No. MAA 100 NR4R troubleshooting

My circulating blower doesn't come on when set to "auto" in furnace operation.

It is a Consolidated Industries "Quatro", Model No. MAA 100 NR4R, 100k BTU/hr, with a July 1993 start-up.

It's completely original and hasn't ever been worked on. Everything else works correctly. Everything. This includes the air conditioning working correctly when the blower is set to "auto" at the thermostat.

I have been heating the house by running the blower continuously, setting it in "manual" at the thermostat. I began doing this instantly when I heard the burners come on but the blower did not follow.

Attached are the wiring diagram and the unit exploded view. Using Consolidated's terminology, on the exploded diagram:

Part 6 = primary limit switch, which is downstream of heat exchanger.

Part 7 = auxiliary limit switch, on the blower housing

Part 8 = pressure switch, near the induced draft fan

Part 14 = burner limit switch, near the burners

Many thanks to reader J.R., who uploaded the Consolidated model HBA manual! Finally, I found something that tells me what normally turns the blower on when it's set to "auto". The blower is started by TIMER. (It's in the Sequence of Operation on that manual's page 34.)

Later I saw a sticker on my unit that says the blower comes on 20-30 sec. after the burners fire up.

The 1st thing I did was a continuity check of the 'primary' limit switch (I would call it a high temp. shutoff switch) – with it's wires disconnected and the unit cold. It's closed. It has no resistance.

2nd, I decided that I can check all 4 of the aux. and burner limit switches at once. See attached detail pic, "auxil limit wiring".

With the unit energized (but off and cold), there should be +24 volts DC at terminal "W" of my thermostat, right? There is actually -4 vDC. Also, there is no continuity with external ground, and it's a very high ohm reading.

This makes me think the 4 switches in that circuit are all closed, and the problem is in the control box, but I haven't opened it yet. Do you see anything wrong with what I've done? Any suggestions about what to look for next?

On 2021-03-22 - by (mod) -

@Bob,

Try again to post drawings or circuit diagrams using the "Add Image" button - you can post one image (like a .gif or .jpg or .png file) per comment but of course as many comments as you like.

Regarding "My circulating blower doesn't come on when set to "auto" in furnace operation."

As that tells us the fan itself works, I agree that we're looking at a bad wiring connection or relay or on occasion a bad fan limit controller

On 2021-03-22 by Bob - The fan limit control comes in 5", 8", and 11" lengths. which one do i want?

right, thanks for the confirmation.

Still, the "standard" comes in 5", 8", and 11" lengths. which one do i want? how can i tell? as i mention, i can see no numbers on the module. taking it out more than once, is something i'd like to avoid.

On 2021-03-22 - by (mod) -

@Bob,

If we are talking about the length of the temperature sensor that inserts into the plenum, you'd want to match the original. Too long might touch metal and fail to work; too short might not be adequately in the air path.

If the Heil manual doesn't specify, and if you can't find a model number on a sticker or inside the cover of your limit control you'll need to shut down the heat and take the control to your supplier for a match.

On 2021-03-22 by Bob - swapped the control to see if that fixes the fan problem

right, like i said, in the first comment, cover isn't available. and taking it out means being without a furnace for the time it takes to get a replacement. it is getting into the 50s during the day, some days, but this is still march, and this is rural wisconsin.

the control is a guess as it is, the time to get a new one and put it in, and maybe find out, it's not the problem . . .

thought these comments would be responded to by a professional. other diy'ers have experience to compare, but i need help. so far, all emails, comments, calls, are hitting the same wall . . . kind of like google, you used to get a gazillion different responses, now you just get the same 6 over and over, again .

On 2021-03-22 - by (mod) -

With all respect, I am a trained heating professional with field experience, and we really want to be helpful, offer exact details when we can, but we are completely unable to make up a wild guess that could be dangerous for you.

I'm trained on and have fixed numerous heating systems and controls, but please keep in mind that nobody can see your system and all of its conditions from a bit of text.

In nearly 50 years of building mechanicals systems inspection, installation, troubleshooting, and repair, I have found that virtually 100% of the time that I've been to a site after someone described the problem, I've found important information that determined the repair needed that wasn't in the original description; it's not a difference in intelligence but one of experience and eye.

Sometimes a reader, quite understandably, just "wants the right answer" but the safe and professional truth is there is no "safe, right" answer that is forced to rely on incomplete information and guesswork.

That's behind my comment to you that the length of the temperature sensor is important and involves life safety issues.

If you have a local supplier who has fan limit controls in stock, one can but hope and trust your building can survive an hour and a half (of outdoor temperatures hovering at 50 degF) of removing an old control, taking it to the supplier, getting the exactly correct new one, and installing it properly.

On 2021-02-10 - by (mod) -

I am making up this speculative answer because quite frankly I don't know for sure what's going on with your fan. But it's possible that when you try to turn it on manually you're starting the fan motor that is worn or damaged and has trouble starting up when hot.

I say that because usually a humming electric motor means that it has a bad start capacitor or the motor bearings or other Motor Parts have failed

On 2021-02-09 by Ed - Fan will run on auto but not on manual.

I'm having the complete opposite problem. Fan will run on auto but not on manual. It makes a 'humming-ish' sound but won't kick over. Any suggestions?

On 2020-12-30 by Bmabry - Electric split unit trane the fan comes on in the on position but not in automatic.

Electric split unit trane the fan comes on in the on position but not in automatic. Heat or cool isn't working on the condenser or evaporator . Unit is 2 years old with a smart honeywell t-stat.
Any clues??

On 2020-12-30 - by (mod) -

BM

Probably the cause of your Trane unit fan coming on only in manual ON position is one of the causes listed above on this page.

Most common is that the heater itself is not working. Check its breakers first.

That's because if the heater never warms up then the fan controller won't automagically turn on the blower fan.

Let me know what you find.

On 2020-09-29 by Pamela Williams - Can't set thermostat on auto cool

Thermostat always set auto fan and auto cool. Can't set on auto cool for some reason. Why?

On 2020-09-29 - by (mod) -

IF nothing was changed or replaced in your equipment or thermostat then I suspect either a failed thermostat connection, thermostat wire, or control board at the air handler.

...

Continue reading at BLOWER FAN RUNS ONLY ON FAN-ON / MAN - HEAT or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• BLOWER FAN OPERATION & TESTING - indoor HVAC air handler / blower unit fan
• CONDENSER FAN WON'T START - outdoor HVAC condenser unit cooling fan
• FAN ON AUTO MAN THERMOSTAT SWITCH
• 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 STOP - LIMIT SWITCH
• FAN WONT STOP - THERMOSTAT SWITCH
• FURNACE FAN CYCLES AFTER HEAT
• FURNACE FAN CYCLES DURING HEAT
• FURNACE FAN STOPS EARLY
• FURNACE FAN WONT START
• THERMOSTAT WON'T TURN OFF - heat won't turn off

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BLOWER FAN RUNS ONLY ON FAN-ON / MAN - 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 HVAC THERMOSTATS

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