HVAC blower fan testing & diagnosis FAQs:

Questions & answers on diagnosing and repairing HVAC blower fans in the air handler.

This article series explains how to inspect and test a heating or air conditioning indoor air handler blower fan that is not working. We also discuss fan fan squeaks and noises. The blower fan assembly is the green component in the page top illustration from Carson Dunlop Associates The Illustrated Home.

Heating or Air Conditioner / Heat Pump Blower Fan Testing & Diagnostic FAQs

The questions and answers discussed below were posted originally

at BLOWER FAN OPERATION & TESTING - please be sure to read that collection of diagnosis and repair steps.

The blower fan is located inside a horizontal air conditioning unit in many home air conditioning systems, especially when the air handler is located in an attic or crawl area.

Click to enlarge any image]

On 2019-04-13 by (mod) - blower set to run continuously is shutting off

Carol

IF no one changed the switch to keep the fan on continuously and now it's running intermittently I'd think that there is a loose or corroded wiring connection, or a failing limit control, or a failing blower motor.

Which switch is set "ON" - at the fan limit controller? or at the thermostat? That tells your tech where to start checking wiring.

On 2019-04-12 by Carol

The blower on our furnace is set to run continuously. Lately it starts and then runs a few minutes or sometimes seconds and then completely turns off.

This happens in the evening hours when the temperature in the house is higher than what is set on the thermostat. We have had two furnace guys look at and they don't know what is wrong. To be fair, it wasn't turning on and off and on when they were here.

On 2019-02-16 by (mod) - blower fan won't stop running

Please see the diagnostics at

FURNACE FAN WONT STOP

On 2019-02-16 by Walter

Blower fan will not stop running with everything OFF

On 2019-02-11 by (mod) - furnace blower fan won't start-up

Chris see the diagnostics at

FURNACE FAN WONT START

Please, as you posted this question on several pages, help me manage the thousands of questions posted by readers by posting questions just once.

On 2019-02-11 by chris

fan will not come on in m furnace checked for a reset switch on motor there isnt one

On 2019-01-13 by (mod) - Lennox G26q3-75-1. Fan blower won't stop

JM

Be sure to review the diagnostics at FURNACE FAN WONT STOP

The problem could be as simple as a switch setting on the fan limit control in the air handler/furnace OR at the wall thermostat.

On 2019-01-13 by JMed

Have a Lennox G26q3-75-1. Fan blower won't stop, even after heat is reached, burner shuts off, and air temp cools. Does not react to fan 'off' at thermostat, or even thermostat removed.

Replace fan limit switch as readings suggested - same result. Operates just like fan 'ON' at thermostat - no matter what.

On 2017-05-30 by (mod) re: blower motor won't turn on

Bob

If there's power to the fan motor terminals but the motor never runs I suspect a bad START RUN CAPACITOR or a bad fan motor - a tech would probably try swapping in a replacement capacitor.

See FURNACE FAN WONT START

On 2017-05-30 by Bob

blower motor will not turn on

On 2017-05-19 by Anonymous

Common causes a bad control board. If it were the relay the fan wouldn't run on manual on

On 2017-05-19 by Tom

In my carrier weathermaker 58 . The blower isn't coming on when t-stat is set in auto. A/C comp. Starts up but no fan. Same with heat. Everything starts up normally except no blower. I switch t-stat to fan on and blower works along with a/c comp.

And system works normal except fan doesn't cycle because it's in fan on mode. The control module was smoked after a nearby lighting strike and has been replaced. I think it's the blower control module but before I install I'd like to find away to prove that's it. Any help would be much appreciated. Thx!

On 2017-05-11 by (mod)

We are glad to try to assist Dan.

It's always difficult to guess at repair advice for a system that we can't see and when we're not sure quite what is installed. Please let me know what you find as that will surely assist other readers.

On 2017-05-11 by Dan C

Thank you for your time.

On 2017-05-11 by (mod) diagnosing an air handler that does not use a fan limit switch

Dan C:

Air Handlers that use a gas or oil fired heat source would use a fan limit control switch.

An Electric heating system used as backup for heat pump would not require that control. If that's your situation then I suspect there is a corroded electrical connection or a bad control board or relay in your air handler.

On 2017-05-11 by Dan C

I am very sure this system has no fan limit switch. The schematic does not show a limit switch.

The fan connects directly into the control board. I believe it is controlled by a P&B relay on the control board. The system will work properly most of the time after power down. I have also removed the G wire from the thermostat at the control board and it continues to run.

On 2017-05-11 by (mod)

Use the on-page search box at the top or bottom of this page to ch ind our article series FAN LIMIT SWITCH to see locations and sppearance

On 2017-05-11 by Dan C re: can't find the fan limit switch

I cannot find the fan limit switch. The plenum is above the blower and does not have an access panel. I assume it would be located in a position to gain access. My air handling unit has the blower motor and control board/transformer above the evaporator and the plenum is above that level.

 

On 2017-05-11 by (mod) re: blower runs on after the thermostat is satisfied

Ok, I hope I've finally got it. The blower fan runs on after the thermostat is satisfied, blowing cooler air.

A brief heat extraction run-on is normal, typically a minute or less. Many minutes of running-on points to a bad fan limit switch control

On 2017-05-11 by Dan C re: get no heat after set temp is reached but fan runs on

I do not receive heat after the temp is reached that was set on the thermostat. The air coming out of the vents is not warm, but ambient temp air.

Heat pump as stated shuts down after thermostat temp is reached, but the fan continues to run with no heat coming out of vents.

On 2017-05-11 by (mod)

I meant to suggest that if you continue to receive heat from the system and the blower fan is thus operating after the outside compressor condenser has turned off then you are probably running in backup heat mode.

In heat mode if the heat pump can't provide heat the backup or aux heat has to work, else you'll get no heat but the blower may run - depending on unit controls.

On 2017-05-10 by Dan C aux heat doesn't work

The auxiliary heat does function.

On 2017-05-10 by (mod)

Check to see of the backup heat is operating

On 2017-05-10 by Dan C blower fan runs on continuously after set temp is reached on a heat pump system

I have a heat pump system. When the thermostat is in the heating position, the blower fan continues to run after the desired temp is reached and the outside heat pump unit stops

. The thermostat will continue to show heat1 in process after the heat pump has stopped and the temp has been reached.

When I reduce the temp on the thermostat the fan will stop.

I have removed the wire from the thermostat at the control board and the fan continues to run after the desired temp is reached and outside unit is off.

I have turned power off at the air handling unit for a reset and system will function as normal.

Later on will return to the same problem. I have a feeling the control board is not functioning as desired. Please help. Thanks

On 2017-04-23 by (mod) re: blower turns on for no reason

Sounds like a bad control board

On 2017-04-23 by Michael Adams

When the temperature on the thermostat reaches 85-86 my blower will start by itself

. Once this happens I am unable to start the compressor until the blower shuts off. We have to turn on the air before it reaches 85-86 in order for the system to run properly. Any thoughts?

On 2017-04-03 by (mod) re: compressor blower diagnosis

Nagi

There may be a problem with the compressor or the fan motor start/run capacitor. Or by now the blower motor is seized.

I would leave the equipment turned OFF to avoid risk of an electrical fire.

If the compressor fan or air handler unit fan runs slowly look for low voltage, a bad run capacitor, or a failed bearing that is seizing

On 2017-04-03 by Nagi Tanaka Laleg

I have a common unit air conditionner which run so well but time to I realised that when compressor start it get more load on power. And it stop the compressor one time so I chk the compressor but ther blower runs slow rotation.

Then it stop as well so I chked the power voltage its normal 415 supply but could not start only click on and off with out tripping any breaker or fuses. What will be causes mow because the blower motor only heat up without running.?

On 2016-12-09 by (mod) re: blower fan runs in wrong direction

Rob:

Yep that happens, usually after electrical component damage.

See ELECTRIC MOTOR RUN DIRECTION for details.

On 2016-12-08 by Rob

1986 carrier blower motor wants to spin in wrong direction

On 2016-08-16 by (mod)

Your tech might check
- power connections
- controls and switches
- the motor start/run capacitor

On 2016-08-16 by Stan

Blower motor dont run motor spins free it is a york

On 2016-07-22 by (mod) inadequate air movement

John,

Assuming you're using the same air handler, the problem is more likely to be in a control that is setting fan speed.

Another problem source might be ice-blocking of the cooling coil. And you checked for a dirty air filter and crimped or disconnected air duct, right?

See AIR FLOW TOO WEAK

On 2016-07-21 by John C

The two-state blower works fine with my furnace, moves hot air just fine. I get very little air movement with my central air conditioning though, just the gentlest of air flow. Utterly unacceptable. Any ideas as to what is wrong?

On 2016-07-15 by (mod) re: fan motor cuts out then turns back on on hot days

Bruce

Indeed some electric motors include an internal thermal overload switch. Your HVAC tech might want to check the actual motor temperature against the data on the motor's data tag. There you'll see the expected operating temperature range. If the motor's temp is within spec then there may be a failing bearing or a low voltage problem.

On 2016-07-14 by Bruce

My inside unit is in a closet in the garage. On a hot day, fan cuts out for a minute or two and then turns back on. Is there a fan motor temp switch somewhere? Or is it a sign of a future bad motor. During not so hot days, will not have a single problem . Thanks

On 2016-07-11 by (mod)

I can't diagnose the problem from your brief e-text, Donald, it sounds as if you need an onsite service technician.

But you can start by comparing the set temperature on the thermostat to the room temperature. If the cooling system can't cool the room enough (or if you are heating, heat it enough) to reach the set temp. the system will keep running and the compressor/condenser may cycle on and off while the fan runs continuously.

On 2016-07-11 by Donald Brzezniak

A/C is set to Auto and the unit outside and AC Compressor shuts off meanwhile the Air Handler runs constantly unless I adjust the temperature on Thermostat and then Air Handler will shut off. Thank You in Advance.

On 2016-07-07 by (mod) what happens if the fan motor was installed upside down ?

NO, but if you're blowing air the wrong way the system clearly won't work.

On 2016-07-07 by Anonymous

If a fan motor on top of the compressor was installed upside down(sucking air in instead of blowing out) would that make you use more power???

On 2016-06-30 by (mod) re: blower fan starts but only gets up to half speed

Ron

Sounds like a bad run capacitor or perhaps a failing fan bearing or motor. But first confirm that the voltage level is normal and that there are no corroded or loose wiring connections.

See FAN MOTOR START CAPACITORS

On 2016-06-30 by Ron Jon

Blower fan seems to start up fine but only gets to half speed. Unit is just under a year old, has no noises or anything, clean filters. Any ideas ? Thanks.

On 2016-06-28 by (mod) re: one air handler works improperly if the second unit is also running

One

You are describing a control or wiring problem that we can't debug by e-text; but an on-site experienced HVAC tech will be able to trace wiring and controls to see what's happening; There's most likely

- a control failure
- thermostat wires shorted
- a wiring error

- some odd combination of switch settings

On 2016-06-28 by One furnace blower will not start if second furnace blower is running.

I have a home zoned for 2 furnaces. The furnaces sit side by side and share a Healthy Climate Media Air Cleaner filter unit, ie the returns from both zones go through the filter unit and then into the furnace blowers which deliver the air to the appropriate zone.

The units are a Lennox G60UHV two speed and a G40UH single speed. Both units are on the same circuit and indeed plugged into a single duplex receptacle. The units were installed in about 2000 and have functioned perfectly until a few years ago.

At that time, first on a random basis and now more consistently the following symptom occurs: The larger unit and blower always works well and comes on when initiated by the thermostat for both heating and cooling.

The smaller unit works well when the larger unit is off. But, the smaller unit’s blower will not come on when the larger unit’s blower is on. Neither in AUTO or ON will the blower come on in full force, but it sometimes turns very slowly.

At this point it also turns on the air conditioning compressor. Other times the blower will not turn on at all, in this case the compressor usually does not come on. To get the smaller unit to work, I have to turn off the larger unit. Then the smaller unit’s fan will function normally.

Once it is on and up to speed, I can turn on the larger unit and it does not affect the smaller unit. But if the smaller unit should reach temperature and be turned off by the thermostat when the larger unit is on, it will not come on again until the larger unit is turned off.

From reading on line, I would guess that this is a starter capacitor problem. I suppose the easiest test would be to just replace the fan capacitor on the smaller unit and see if it then works normally. Any other suggestions would be appreciated.

On 2016-06-25 by (mod) re: blower won't run in AC mode

This sounds like shorted thermostat wires or a control/relay problem in the blower unit.

On 2016-06-25 y Jon

My blower fan runs until start my a/c. It starts if I turn on my heat and if I turn it on. So it was running I turned my switch to cool and it shuts off. What could be the problem?

On 2016-06-19 by (mod) re: loud hissing scraping noises from the blower fan

Sounds like a worn blower bearing or a squirrel cage fan assembly that's come loose and is rubbing against something in the air handler;

If your blower is pulley driven, also check for a loose belt or pulley.

On 2016-06-19 by Ron con

My blower motor in air handler is making a loud hissing/scraping sound. What could that be?

On 2016-06-09 by (mod) re: noisy blower on top of a block of units that never stops

Review the system operation with an experienced HVAC tech to determine first if it's operating normally. Something as simple as loose parts can cause a horriffic noise.

On 2016-06-09 by Karen odonnell

What can one do in regards to a continuas very noisy large blower on top of a block of units, never stops!

On 2016-06-08 by Mik

I called the technician. The guy checked gas in heat pump, cleared filter, everything was ok. I paid 300$ but still have the problem.
If I force for heat i'm getting strong heat from the furnace.

If I start just fan, I'm getting small heat coming (winter or summer).

My guess for backup heat all elements should run.

I'll call for the service but like to know what should be fixed.

On 2016-06-07 by (mod) re: backup heat not working

Mik,

Your HVAC service tech will check to see what kind of backup heat is working - IF your home is using a heat pump; For example if it's electric backup heat it may be that some of the heating elements are not working. I don't have any information about what you have installed, so more guessing is more likely to be misleading.

On 2016-06-07 by Mik

Thanks
What should I check?
The heat is not strong
In heating mode when the furnace works it is much more heat coming out
Thanks for advice

On 2016-06-06 by (mod)

Your system may be running in backup heat mode.

On 2016-06-06 by Anonymous

How I'm getting hot air during winter time without heatpump running?

On 2016-06-05 by (mod)

Mik that sounds normal: if the A/C compressor is not running but you run the fan, you're simply circulating warm air around your home; and if some of your AC ducts run through a hot attic or other hot space, the initial air that comes out may be quite hot.

On 2016-06-05 by Mik

The problem is not heat pump
When the pump is off I'm still getting hot air when I start fun Summer or winter

I didn't faind any air leak.

On 2016-06-03 by (mod)

Mik, See DIAGNOSTIC GUIDE A/C or HEAT PUMP where we discuss "Air Conditioning Checklist for Lost Cooling: What to Do First in Air Conditioner / Heat Pump Tests"

Then search our site for AIR CONDITIONER NOT WORKING Search InspectApedia for LOST COOLING or AIR CONDITIONER NOT WORKING to read typical causes & cures

On 2016-06-02 by Mik

Hi, I'm getting hot air when blower fan get started.
Heat or cool, or just fan, the air is heated.

The heatpump is ok, gas pressure is normal.
Any help advice will be appreciated.
rgds, MK

On 2016-06-02 by Anonymous

Attic blower air flow is normal until the attic gets very hot (summertime). Then, it slows down. Let the blower cool off, then the air flow is normal again/

On 2016-06-01 by (mod)

Check for a bad start capacitor or seizes blower fan or motor

On 2016-05-31 by Anonymous

Blower fan hums but does not kick on

On 2016-03-28 by (mod)

If your same attic blower assembly is used for cooling, distributing cool air to the building interior, the A/C won't work with the blower turned off.

And if you called for cooling, it's possible that the cooling coil in the air handler would quickly ice-up as no air would move across it.

On 2016-03-28 by Anonymous

My fan in the attic runs continuously as it is for the heat. If I turn the breaker to off will it hurt the AC unit?

On 2015-09-24 by (mod)

The thermostat's FAN ON switch should only turn on the fan so long as the thermostat temperature setting (in cooling mode) is above the room temperature. If that's not happening I suspect a wiring or control board or relay defect.

On 2015-09-23 by Pjespo

I recently had my blower motor replaced and my AC is working great but now when I just want to turn on my fan to circulate air, it blows ice cold AC air. Did wires get crossed on my furnace contol board? Any suggestions on a fix?

On 2015-09-14 by (mod)

Garri

First take a look at the blower fan operation for

- voltage variation

- loose wiring

- a defective control that is cycling the fan on and off intermittently

- loose belt on a belt driven fan

- loose pulley on a belt driven fan

- failing blower motor or blower assembly bearing

On 2015-09-14 by Garri

Air movement speed from grilles is not stable-pulsation. What is possible reason??

On 2015-08-26 by (mod)

Sounds like:

a wiring error, an open wire, open switch, blown fuse, tripped breaker, or damaged control relay or board

On 2015-08-26 by Sharon

Our blower motor was not working properly so we changed it and the capacitor now it's not getting power. The old one had power just wouldn't spin. What could it be?

On 2015-08-15 by (mod)

Ask your professionals to check the fan speed, air movement speed, against the AC unit's specifications. You may be seeing excessive condensate blow-off.

On 2015-08-15 by Mary J

Condensation dripping into bottom of return behind filter. Drips straight down off coils, never makes it to the drip pan. Soaked up more than 4 cups of water. Professionals have cleaned coils twice. Did not make any difference. Unit seems to be operating properly. Very cool. Thanks for any help.

On 2015-08-01 by (mod)

Yes if a start/run capacitor has failed the motor it supports will not run.

On 2015-08-01 by DNeeds Help

Payne AC unit- Outside fan works fine but inside blower fan does not come on. Hear somewhat loud noise from capacitor. Would this stop the inside Blower fan from operating?

On 2015-07-24 by (mod)

Sounds like a control board or thermostat wiring problem. If the problem is new your tech will perhaps check the blower control board and relay.

On 2015-07-24 by Robert

My blower works to recirculate air or when "fan" setting is on but shuts off when the A/C unit starts.

On 2015-06-19 by Rick

What to look for when blower fan will not come on.

On 2015-05-24 by (mod)

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies

Reader comments & Kudos:

(Apr 28, 2011) LS 3845 said: it was a good read i can use the tips every day

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