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.
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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. The location of a blower fan in vertical "up flow" or "down flow" heating and cooling systems is illustrated in additional sketches and photographs below.
The questions and answers discussed below were posted originally at BLOWER FAN OPERATION & TESTING
Please also see the diagnostic questions & answers now found at BLOWER FAN DIAGNOSTIC FAQs
On 2017-07-09 by (mod) re: blower fan starts, runs briefly, stops
I would check for these things in this order:
and if none of those seem to be the obvious problem that I would look at the additional Diagnostics in this article
FURNACE FAN WONT START
Sorry I can't pin it down more narrowly but I just don't know enough about your situation
On 2017-07-09 by Darin
Air handler fan starts then stops a few seconds later while the rest of ac system continues to run
On 2017-07-06 by (mod) problem with fan blades rubbing the case
Rubbing the case is a problem. Perhaps the blades are bent or a bearing is worn or the fan mounts loose or improper.
On 2017-07-05 by Dale
During a rotation, the blades are rubbing the case in one location.
I don't see anything obvious but the fan (blades) does not rotate freely. It stops or gets jammed but I can't see that it is hitting anything.
On 2017-07-05 by (mod) re: noises from the AC/Furnace sounds like a jackhammer
Smart to turn off the power - before a fire got started.
Could be simply a loose component in the blower compartment or a jammed pulley or fan belt. Take a look in the blower compartment for something visually obvious and let me know what you see.
On 2017-07-05 by Dale
When our ac is running, the blower on our ac/furnace unit has started to make a lot of noise (sounds like a jackhammer) along with a burning rubber smell. I cut power to the unit and now would like to understand what has happened so I can repair it or probably get it repaired.
On 2017-07-04 by (mod) re: will removing the air filter damage the blower fan?
sorry your post was temporarily blocked by punctuation that our spam filter thought was a URL
Removing an air filter itself wouldn't damage the blower fan,
BUT if the filter was in the blower compartment and you had to open a compartment door to remove the filter, then a safety door interlock switch could be open - thinking the door is not fully shut - keeping the fan off for safety. ,
Check that out and let me know what you find.
Watch out: running an HVAC system with no air filter will cause dust and dirt clogging of the blower fan itself and may harm other components.
On 2017-07-04 12:37:51.528208 by Ellen M Ames
My blower fan will not start. Really need to know if by any chance it may have to do with the fact that we removed the filter because it was dirty and we didn't have a new one to put in right away and that's when the unit stopped! Can something happen to make it stop due to no filter?
Please help. Trying to fix it ourselves.we were going to replace the motor in the fan unit of the a or handler. It won't start. Very nervous because it's July4th in South Carolina!!
On 2017-06-30 by (mod) re: can I use my furnace blower fan for an AC system?
Yes, Anon, that's a common retrofit situation: an existing forced air heating system gets a cooling coil added above the supply plenum to add A/C to the existing system.
The blower speed may need to change when cooling so if your blower is a 1-speed unit you may need to change the motor and controls.
On 2017-06-30 by Anonymous
Can i use a heater blower on a ac
Mike: I'd look for a worn fan bearing and if the blower is operated by a motor and pulley system, also for a loose pulley or bad pulley bearing
On 2017-06-22 by Mike
A/c in attic makes loud noises at end of cycle. Sounds like coming from squirrel cage off balance or bearing worn out and wobbles at end of rotation . happens at end of air flow and happens every 3 or 4 minutes. Noise
On 2017-06-15 by Anonymous
Blower motor is d/c it is not running full speed and stops and starts irratically
On 2017-06-09 by steve
I will try disconnecting the wires from the thermostat at the fan side and see what happens tonight. Thanks again for all your input
I replaced the relays, thermostat, and thermostat wire, yet it still runs. That's why this has me stumped. I even swapped out the control board. As far as I know these four things are all that it could be.
I do appreciate all your feedback Dan. Thank you for your advice and help
On 2017-06-08 by (mod) re: diagnose fan that stopped working after a lightning strike
Sounds like a stuck fan relay or shorted wires to the relay; I'd start by disconnecting the wires from the thermostat on the air handler end.
On 2017-06-08 by steve
i don't have any fan limit controls other than the thermostat. I know a lot of heat blowers have a timer or limit controller but as far as I know there is only the thermostat control for mine and even when set in of for heat or cool and auto for fan the fan still runs continuously
I have changed the thermostat and all other mentioned items and cant find anything that suggest another part to check. There is a constant 24v supply to the fan telling it to run.
On 2017-06-07 by (mod)
Did you remember to check that there is no manual on switch on the fan limit control itself?
On 2017-06-07 by steve fan stopped working after a lightning strike
i have a carrier split unit 2011year model heat pump. The blower stopped working on the indoor unit after a lightning storm. I replaced the module for the motor which is the x13 model. The module has the capacitor and everything built into it.
After replacing just the module end of the motor the blower came right on and runs perfectly except now it will not shut off unless I open the breaker. The control switch is in auto and the control is in off, yet fan continues to run.
I swapped thermostats with the downstairs unit and still have same problem. I bought new relays and replaced and still have same problem. I then bought new thermostat wiring and still have the problem.
I also swapped out the control board with unit from downstairs and still have the continuous running blower. The outside portion of the unit runs fine and cycles on and off by the temp like it should.
Is there anything else I can look at? ANY help would be greatly appreciated
On 2017-05-30 by (mod) re: blower motor won't turn on
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.
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
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 search box above 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
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
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
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?
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
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
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.
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
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 19:44:20.793398 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 13:03:32.823088 by 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
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
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 16:47:48.788422 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 GUIDES A/C / 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 21:33:12.467475 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.
On 2016-06-02 20:12:28.417760 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 09:18:38.445894 by (mod)
Check for a bad start capacitor or seizes blower fan or motor
On 2016-05-31 02:16:37.817693 by Anonymous
Blower fan hums but does not kick on
On 2016-03-28 21:33:40.575400 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 20:51:54.186521 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 21:32:22.271546 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 06:38:16.076893 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 19:26:45.998262 by (mod)
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 19:21:21.558944 by Garri
Air movement speed from grilles is not stable-pulsation. What is possible reason??
On 2015-08-26 17:56:14.118994 by (mod)
a wiring error, an open wire, open switch, blown fuse, tripped breaker, or damaged control relay or board
On 2015-08-26 16:53:14.369429 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 22:28:23.918892 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 20:59:12.275560 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 14:22:56.146972 by (mod)
Yes if a start/run capacitor has failed the motor it supports will not run.
On 2015-08-01 05:47:04.698276 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 15:39:18.184487 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 15:32:18.153052 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 21:48:26.738810 by Rick
What to look for when blower fan will not come on.
On 2015-05-24 19:51:37.864410 by (mod)
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(Apr 28, 2011) LS 3845 said: it was a good read i can use the tips every day
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Since the failure of an air conditioner to turn on, loss of air conditioner cooling capacity, reduced air conditioning output temperatures, loss of cool air supply, or even loss of air flow entirely can be due to a variety of problems with one or more components of an air conditioner or air conditioning system, after reviewing the lost air conditioner cooling diagnosis procedures described in this article, be sure to also review the diagnostic procedures at each of the individual air conditioning diagnosis and repair major topics listed just below. To return to our air conditioning and refrigeration home page go to AIR CONDITIONING & HEAT PUMP SYSTEMS.
If your air conditioning or heat pump system has lost its cooling capacity or won't start select one or more of the diagnostic articles listed below.