Air Handler Repair FAQs

Blower Assembly diagnostic questions & answers

Air Handler Blower Fan Won't Stop FAQs

Additional helpful diagnostics for this problem are at

• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH
  

Why does the furnace fan keep running even when the unit is turned OFF?

Why does my fan keep running when the unit is turned off. - On 2021-01-17 by Dorothy Spencer

Reply by danjoefriedman (mod) - Why does my fan keep running when the unit is turned off

Dorothy

I think you're asking a question about a heating system not a more cooling system.

It's normal for the blower fan to run for a brief time at the end of the heat cycle to purge the remaining heat in the air handler. That avoids the risk of overheating and cracking the heat exchanger which would make heater unsafe.

If you search this website for the phrase

Blower fan continues to run after a call for heat

Then you will see the details

Comment by Rachel:

RE-posted by Mod without advertisement

Rachel said:

@Dorothy Spencer, It may be the clutch bearing in the air conditioner pump is worn out. This part has not been replaced. The air conditioning pump needs to be replaced.

Reply by (mod) - Why does my fan keep running when the unit is turned off

Thank you for the post, Rachel

Dorothy:

If you are asking about the cooling fan in the outdoor compressor/condenser unit for your air conditioner or heat pump, see the diagnostic suggestions

at FAN, COMPRESSOR / CONDENSER UNIT DIAGNOSE & REPAIR

If you're discussing the indoor air handler fan ,take a look at the diagnostic suggestions

at A/C or FURNACE FAN WONT STOP

AC blower unit in attic does not shut off

When my outside unit shuts off the attic unit continues to run and will not shut off, even after turning the whole system off the fan on the air handler unit in the attic continues to run, any ideas what this could be? (July 16, 2015) Jodi

Reply: bad fan relay?

Jodi:

When you are "turning the whole system off" and the air handler fan continues to run

AND provided that no one has turne on a manual FAN ON switch at the thermostat or in the air handler such as on a fan limit control,

(to be sure you're not manualy forcing the fan to run see

• BLOWER FAN WONT STOP - LIMIT SWITCH
• BLOWER FAN WONT STOP - THERMOSTAT SWITCH )

THEN we think that either there is an electrical wiring or a control board or relay control switch error (you'd need an electrician) or you are not finding all of the shut-off switches.

For example turning off power to the outside compressor/condenser unit does not turn off power to the indoor air handler unit.

In other words, if you actually turn off electrical power to the unit, for sure it has to stop running or there is a wiring or switch error. In that case switch off the circuit breaker.

Some fan controls include an always-on switch as do some thermostats. Check for those.

Jodi said:

Thanks for your input - it was a bad relay switch

 

Air handler blower fan has trouble turning off

My air handler turns off but sounds as if it is struggling. The air flow isn't consistent. It is a Carrier installed in 2010.
Thanks, 2017-09-28 by Dave

Reply by (mod) - My air handler turns off but sounds as if it is struggling.

Dave,

What you describe could be caused by any of a number of problems, more than I can accurately guess with so little information. Your service tech will probably consider

Starting problems: a start capacitor for a motor that's having trouble starting or a bad controller relay

Stopping problems: a failing control board, relay switch, or possibly more subtle, a failing motor bearing

Blower fan won't stop - thermostat not even connected

Installing a rheem air handler. Attached wiring for thermostat. Without connecting the other ends to the actual thermostat and turning the power on at the unit, the fan comes on and will not shut off. After thinking there was a short in the length of thermostat wire, so changed that, twice. Still comes on with the other end not attached to anything.

After playing around with it more, I took a length of single wire off a spool, attached it to the red wire in the air handler and nothing on the other end, disconnected the green and brown wire form the air handler, turned the power on to the air handler and the fan comes on.

If I take the length of wire off the red wire from the air handler and turn the power on, the fan does not turn on. It doesn’t make any sense....I even swapped out the circuit board to see if that was some how causing it, but it still did it. Does anyone have any ideas? On 2019-07-17 by Eric W

by (mod) - the fan comes on and will not shut off

Eric

Take a look at the debugging tips starting at FURNACE FAN WONT STOP https://inspectapedia.com/heat/Blower-Fan-Wont-Start.php

On a new installation I suspect shorted wires - just as you did.

When you disconnect the TT wires right at the air handler and the fan still insists on running then check for a FAN ON switch on the limit control (assuming this is heating & cooling air handler) and check any dip switches or jumpers on the fan control board.

On a new unit my last guess is a bad control board or fan relay.

Air handler fan goes on and off even when the unit is off

My air handler fan goes on and off even when the unit is off. system works fine but when temp reaches setting on thermostat unit will turn off then fan will go on and off every couple of seconds. Only way to stop is shut breaker to handler.. Very strange. On 2017-05-18 1 by Andrew Biello -

Reply by (mod) -

That sounds like a failed control, relay switch, or shorting wires. Or possibly shorted thermostat FAN-ON wires.

See the diagnostics

at BLOWER FAN CYCLES ON INTERMITTENTLY

 

The outside A/C unit cycles on and off though the inside fan will not stop

my fan is set on auto, and the inside fan will not cycle off. the outside unit cycles on and off though the inside fan will not. It use to. It is like someone set the thermostat fan to the on position. Even in the auto position the fan never turns off. The house is cool and everything else is fine. What would cause this? - On 2011-07-06 by Colleen -

Reply by (mod) - if your indoor air handler fan won't turn off rule out a thermostat switch

Colleen: if your indoor air handler fan won't turn off, either the thermostat is never being satisfied or someone has set a switch to run the fan continuously (or thermostat control wires shorting together are causing it to run)

In addition to the "fan on" switch on some thermostats, the fan limit switch found in the air handler of many systems includes a white button that can be set to make the fan run full time. Search InspectApedia for "fan limit switch" to find the article describing that device.

Follow-up by Colleen

Thank you, I will read the InspectApedia on "fan limit switch". We thought it might be the thermostat and replaced it twice. Replacing this did not change the fan staying on problem.

Reply by Mod:

To rule out the thermostat or its wires (that could be shorted together somewhere, causing the fan to run), disconnect the thermostat wires completely at their end at the air handler or control board.

Details are at FAN WONT STOP - THERMOSTAT SWITCH

Then if your air handler is also serving as a heating system be sure that the MAN-ON switch is not set ON there.

Those details are at FAN WONT STOP - LIMIT SWITCH

Then if the air handler keeps running when it should not there's a bad relay or control board or switch setting (found on some AHU control boards).

Air Handler Blower Fan Repair FAQs

A/C has not put out cool air since the blower unit fan blade was replaced

My husband replaced the fan blade on our AC unit because it was loose and making a lot of noise.

Our air conditioner has not put out cool air since he did this. Is it possible he caused other problems? 2017-09-26 by Wanda

Mod reply:

Wanda

IF there is NO air output then I suspect there is a problem at the fan or blower unit.

But if there is air output of air that is not cool enough, working on the fan itself shouldn't cause that problem - unless by accident hubby left a wire disconnected at a control relay or board.

Rheem A/C unit blower fan motor spinning too slowly, low voltage?

CARLOS I SANCHEZ 4 July 2021

The blower motor on my Rheem package unit is spinning very slowly and the my home is not getting any air circulation.

I checked the contacting coils and got 120 on top and bottom but didnt get 24 volts in both the middle contacting coils.

Notice that the condenser was freezing up due to the fan not blowing as its suppose to. checked speed 1 and 5 and I was not getting 24 volts there as well. what could it be?

My rheem package blower motor spinning slowly and I am not getting any air circulation in my house and notice the unit freezing up in the coils and compressor.

What could be wrong?

Moderator reply:

Common causes of a too-slow blower motor include: Loose electerical connection, low supply voltage, failing motor (bearing, winding, capacitor), loose fan belt or pulley, or something else

Starting at

AIR HANDLER / BLOWER UNITS - home

you'll find a series of air handler / blower unit diagnostic steps

Also

See BLOWER FAN OPERATION & TESTING

See ELECTRIC MOTOR DIAGNOSTIC GUIDE

Intermittent blower fan failure diagnosis

I have a package unit airconditioner. The circulating fan is failing to opperate at random times. In an attempt to solve the problem, I have replaced the contact switch and capacitor. The air filter is clean. I have taken the circulating fan motor out and had it bench tested.

It tested OK. The thermostat works normally. The air conditioner cools the house well during the times that it is not failing.

The compressor fan continues to work at the times when the circulating fan fails.

There seems to be no consistent reason for the failure, as in the outdoor temperature can be hot or cool, or the machine may work normally for only 2 hours or over 24 hours.

Can anyone tell me what the problem might be, or tell me what I should try next to solve this strange problem? Any help will be appreciated. On 2011-09-06 by Amie West

by (mod) - fan controller problem

Amie

It sounds as if you could have a fan controller problem; some A/C units include controls to vary the fan speed or even turn it off; or a wiring problem, even a loose connection;

I'd review all of the wiring connections carefully for soundness and then perhaps give the manufacturer a call - for your unit model number a service department tech may know exactly what's happenbing.

Keep us posted - it'll help other readers;

by Amie West

I appreciate any idea, thanks. I have checked all the connections for continuity, but calling the manufacturer is a good idea.

I hate to call a repair man for something that I could do myself, this is just so unusual that it is hard to diagnos. I'll let you know, but if anyone else reads this and has an idea, I'd love to hear it.

 

Other AHU Blower Unit FAQs

Can a mouse in the air handler damage the blower fan?

Is it possible for a mouse to get into the air handler and cause the fan motor to get off balance and stop working?

Ours started making a loud noise and our ac tech says a mouse must have got in it and caused the fan to get off balance which caused the problem. I find it hard to believe that could happen. On 2018-05-26 by Steve

by (mod) - Yes, off-balance fan cage damages fan bearings

Steve

Yes indeed, though it's a heck of a ride for the mouse.

Once the blower and air handler unit AHU have been cleaned and perhaps sanitized, I'd check the motor bearings. If the blower fan was off balance enough it could have damaged the bearings.

Air handler control board fuses blown

Blown fuses (3A) on the air handler control board? Carrier Model CD3BXA036000AAAA made in 1995 repeatedly blows the fuse on the control board. I replaced the thermostat, disconnected one of the wires to the blower motor, but still have the problem.

The Red LED light on the board begins to flash after the fuse has blown. Any suggestions would be appreciated, I have limited electrical knowledge, but I do have a voltage meter and can generally troubleshoot or component replace. Thanks (May 18, 2014) Gary Matthias

Reply:

Blown fuses on a control board often take you to a faulty control or control board itself; check the obvious: loose or damaged wires; hard-starting motors, burnt relays, contacts.

 

Loud thumping sound after the a.c. turns off

My blower unit makes a loud thumping sound after the a.c. turns off. It sounds like something hitting against something. It sounds like air on a fan. Fan unitbwasvreplaced last year. Technicians can't find the problem. (July 24, 2014) H.Mulligan

Reply:

HM

Did you check for loose fan, bad fan bearing, or motor mounts that jiggle as the spinning components in the air handler slow down?

Buzzing noise coming from the Return Air Handler

(Sept 16, 2014) barbara holton said:

there is a buzzing noise coming from the Return Air Handler. I live in an apartment and they keep putting screws on the metal plate and the noise stops but then the buzzing returns later

. If I switch the unit off at the circuit breaker the noise stops. Do you think it may be the transformer or relay switch. Thank you

Reply:

Barbara

You're doing a better diagnostic job than your repair people. I agree that one should look for the buzzing component: often a failing low voltage transformer or a control relay.

Take a look at LOW VOLTAGE TRANSFORMER TEST for details.

Of course other vibration noises might be tracked to loose duct components or blower components, even an access panel. Those noises would also stop when the equipment is shut off.

Strong air flow buckled air handler door - makes noises

Had a new ac unit installed and have great air flow that causes the air handler door to buckle and retract when blower starts and stops making an annoying sound. Any suggestions? (Sept 26, 2014) Don Brown said:

Reply:

Don it sounds as if there is inadequate return air ducting - the air handler is air-starved. I'd look into that problem rather than just stiffening the door. Just improving the door to stop the noise is treating the symptom not the problem.

Question: how long does it take for mold to grow on the air-conditioning unit?

How long does it take for mold to grow on the air-conditioning unit? - (Oct 18, 2014) Donielle said:

Reply:

Donielle

When conditions of moisture and temperature are favorable for molds commonly found in buildings and mechanical systems mold can grow in 24-48 hours.

What is the air handler door safety switch voltage?

on the air handler door safety switch , would like to know what volts is suppling the switch and where thanks. 1 Nov 2014 Anonymous

Reply: proper blower door safety switch voltage, installation, replacement, repair

Anon,

Usually a furnace or air handler blower door safety switch carries 120 volts. The switch is connected in series between a relay in the air handler that operates the blower fan and a terminal on a control circuit board in the unit. The switch is not powered independently. It's power comes through the circuit board.

You'll be able to see this by following wires from the blower door safety switch back into the air handler.

WATCH OUT for dangerous shock hazards. Be sure all power to equipment is off before touching any switches, wiring, etc.

Replacement blower door switches are easily obtained for your air handler by brand and model via your local HVAC supplier, from the manufacturer, or often online. Some safety switches will work properly on several air handler brands and models.

I add that the blower door switch is an important safety feature that helps avoid chock or chopped-off fingers or worse. Be sure that your replacement switch is properly mounted so as to be reliably operated by the blower compartment door(s) - the switch opens- cuts off power - when the door is opened, and is depressed in to close and permit power to the blower when the door is properly closed.

Be sure that your replacement switch is properly mounted so as to be reliably operated by the blower compartment door(s) - the switch opens- cuts off power - when the door is opened, and is depressed in to close and permit power to the blower when the door is properly closed.

See BLOWER DOOR SWITCHES found also

at A/C - HEAT PUMP CONTROLS & SWITCHES

Clearance distances around air handler and heat pump

(Jan 16, 2015) Anonymous said:
I have heat pump and the air handler is in the basement what is the distance from the back of the air handler can we build a wall

Reply:

Anon:

Take a look at the installation specifications for the air handler and heat pump you installed - you'll see that the manufacturer gives allowable clearance distances both for air circulation and for access to perform various maintenance and repair tasks.

Some equipment back sides, if there are no openings or no removable panels on that side, may be permitted just inches from a wall - it depends.

Or tell us the brand and model of your unit and we can help look for the specifications.

 

Trouble cleaning the squirrel cage blower fan assembly

Feb 10, 2015) KARL BALDWIN said:

I have a squirrel cage fan cleaning problem. The fan is in a Mitsubishi, Mr. Slim MSZ-A15NA indoor unit. The squirrel cage fan can not be removed for cleaning due to problems with the installation location. Twice a year, the fan blades get caked with dirt, reducing airflow.

To clean it, I 1st have to place a large plastic sheet below the unit to catch all the drippings.

I then use Frost King Foaming Cleaner, which I spray directly onto the fan blades, from the bottom of the discharge vent as I manually rotate the fan.

After letting it soak, I then flush the cleaner and dirt out with a pump spray bottle filled with vinegar. I usually have to repeat this messy endeavor 2 or 3 times to remove the majority of the dirt build-up.

While the above procedure gets the job (partially) done enough to restore airflow, I'm trying to find an alternative spray cleaner that would do a better job. Mitsubishi engineers had no suggestions other than to point out that the build-up comes from pollution within my house.

But other than humidity, there is little else that I know of to cause the build-ups as I also have supplemental filtration - Filtreat Foam covering the air intake ducts on the unit and a pair of German made, VENTA Air Wash units placed on the floor.

My question, other than asking if you know of an alternative spray cleaner that would do a better job, is if you can offer any other better solutions. Thank you for your time.

Karl Baldwin
kbaldwindw@hughes.net
540-743-2062

Reply:

Karl

I think you are using the right cleaner and I agree that it is a lot of work and a mess.

And I agree that even a small amount of debris on the blades of a squirrel cage fan can significantly cut airflow - I've measured CFM rates before and after cleaning.

In my opinion you should not have to clean the fan very often, certainly not annually.

See the detailed procedure

at BLOWER FAN ASSEMBLY CLEANING 

I would look carefully at these sources of crud that can clog a blower fan.

- leaks in the return duct system that pick up crud from somewhere enroute

- missing, improperly fitted and leaky, or poor filtration level at the return air inlet filter

- damaged fiberglass insulation inside the ductwork

- debris generators in the air handler such as a fraying belt on a blower assembly or oil or grease leaks nearby

- leaks into the air handler return plenum or blower compartment

If the problem were an unusual source of particulate debris in your building itself, we ought to be able to see that right on the air filter.

There is a chance that a lab analysis of the air filter contents might point to an unusual particle source.

Use our email at the CONTACT link at page bottom if you want to arrange a pro-bono forensic lab examination of a dust sample from your blower fan blades and/or the filter following the procedure I document at

inspectapedia.com/sickhouse/Dust_Sampling_Guide.php

Question: sealing attic air handler leaks

If a air handler is installed in the attic, should the unit be sealed so it doesn't suck in dust particles from attic insulation? Should an HVAC company inspect attic blown in insulation if the house was built before 1975? (Feb 20, 2015) Valerie said:

Reply:

Valerie,

as long as there is adequate return air being taken from the occupied space of the home, the air handler ought not be pulling dust from the attic. Ask your HVAC techs to confirm that return air is adequate .

You can also inspect the air handler interior for presence of attic insulation dust.

But yes I'd use foil tape to seal any leaks.

Question: economical to replace air handler parts or not?

Anonymous said:
Is it worth replacing the metering device coil of the air handler vs replacing

Reply:

Anon

That depends on the equipment age and condition. It may be economical to do so. What does your HVAC service tech say?

Question: water dripping from air conditioner supply registers

Our central air system has begun dripping water at the individual room vents.

Is this a sign of pulling warm attic air into the evaporator/blower assembly and then condensing this warm air as it is being returned to the rooms of the house? (July 10, 2015) Bill Coots

Reply:

Bill without onsite investigation we can't know for sure why water is dripping out of your room vents when the AC is run.

The problem could be return air duct leaks, missing insluation, leaks into the ductwork, condensate mis-handling (if condensate can leak into nearby supply ducts), or condensate blowing into the ductwork out of the air handler because of a condensate capture and disposal problem there.

I'd take a look in the attic, first for obvious duct leaks or disconnections, and second at the condensate handling system. Map the dripping supply vents to the attic duct locations to see where to look closely.