Air Handler Blower Fan Speed FAQs

Air Handler / Blower Unit Fan Speed Q&A
Other Causes of Slow Blower Fan Speed Q&A

Air Handler Blower Fan Speed FAQs

These questions and answers about the blower unit fan speed pertain to the indoor air handler of an air conditioner, heat pump or furnace.

Q&A about the outdoor condenser unit fan speed are at CONDENSER UNIT FAN SPEED FAQs

Also see helpful diagnostics

at AIR FLOW TOO WEAK

How to adjust blower fan speed in an air conditioner, heat pump, or furnace system

How do you adjust the blower gas furnace to speed it up a little bit?

The reason is that I'm adding 300 sq feet. Also how do you open the cover of the cooling coil? - Hoa, 8/16/11

Reply: How to increase the heat output from a gas fired furnace or cool air output from an air conditioner

Hoa

If there is no removable access cover to give view of the cooling coil then a technician would have to cut away sheet metal to give access. That's not uncommon. A new cover is fabricated and screwed or taped in place afterwards.

While dual speed or variable speed fans can run at different RPMs and thus vary the delivery of air through the system, you can't speed up a one speed blower fan. But you can increase air output from the system by several measures:

- increase the return air provided to the system by adding more return ducts
- be sure your air filer is clean, not blocking airflow
- add booster fans at individual supply or return registers

As long as the total BTU requirements of the building don't significantly exceed the BTUh rating of your gas furnace (or air conditioner or heat pump system) you should be OK.

Reader follow-up:

Thank you for replying! I live in California and my concern is about the cooling system. The house I bought is 1-story, about 1500 sq ft. (originally, this house was 1100 sq ft but an extra room was added to the back of the house).

What should I do to connect the air ducts of this new room to the original-sized air ducts in the rest of the house?

Also, the outdoors AC unit and indoors furnace are close together and they are two tons. Is it ok if I replace the furnace and cooling coil with 3 tons? 8/17/12

Reply:

Hoa the duct question needs to be answered by an on-site HVAC installer with some expertise and experience - I simply can't know enough from simple text Q&A to give a solid answer to sizing, routing, placement of registers, etc.

About changing the total BTUH rating of your cooling system, I would NOT change the total BTU capacity without discussing the heating and cooling loads of the whole house with an expert.

If you make the new system too big, that is, provide an air conditioner with too much capacity for the size of the home, the unit may cool the air but it won't dehumidify properly, you won't be comfortable, and the short-cycling of the larger unit caused if an oversized unit is installed can damage the equipment.

See AIR CONDITIONER BTU CHART

How fast (in CFM) should the blower fan speed be?

How fast (cfm) should the blower fan (york ZF240N) be with a outdoor temperature of 45 degrees? Doesn't seem to be moving much air. On 2019-11-18 by Steve

Reply by (mod) -

Thanks for the question, Steve.

Take a look at

BLOWER FAN SPEED SETTINGS

where we give fan speeds in CFM for typical situations.

Then take a look at the diagnostics and solutions at AIR FLOW TOO WEAK

Blower fan starts but only gets up to half speed

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 2016-06-30 by Ron Jon

by (mod) re:

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

Blower motor speed will not change.

Blower motor speed will not change. I have a freeze-up issue with the A/C not that the refrig has been replaced.

The filter and evaporator are clean so I believe I'm not pushing enough air thru the evaporator coil in the air handler.

In testing my blower motor it seems that no matter how I wire it, it runs at the same speed (3-phase, 4-speed options).

I checked the resistance of each winding and they read in appropriate steps - but my ear tells me the blower is running at the same speed on the low setting as it is on the high.

Any ideas what would cause that? - 2017-08-30 by Skip

by (mod) -

Are you sure this is a four speed motor? The motor may be a single speed machine. Did you check the data tag?

If the motor never gets up to anything but a low speed there may be a dip switch on the control board that is not set properly; less likely but possible: there could be a bad run capacitor.

Low refrigerant can also cause coil icing that blocks airflow through the cooling coil; you might mistake blocked air flow for a fan speed problem.

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

Other Reasons a Blower Unit Fan May be Slow - FAQs

Noisy LOOSE wobbly BLOWER FAN shaft diagnosis & repair = reduced fan speed or fan output

Old Amana blower shaft is wobbly. Do these shafts running through a squirrel type blower unit start to bend or get out of balance?

If so, what do I need to do to stop the pully from looking like a warped wheel when running?; replace with a shaft?

What causes this? Finally, how do you know what the correct tension of the belt should be from the motor pulley to the fan pulley?

Many thanks in advance. I cannot get a good honest answer. (June 25, 2011) Brian H

I have 2 separate heat handler motors, one squirrel cage seems out of balance? it's noisy but not all the time? i have a tech who wants 450.00 just for the cage??? and that doesnt include the motor??? wow maybe it can just she cleaned?? he never looked just wants to prelace ir cage? (Sept 4, 2012) Jay

My a/c fan fell off! When reinstalling it should the air flow be into the unit or out? I hope we put it on the correct way! (Sept 19, 2011) pat

I have a problem with my air condition when switched off the internal fan moves free but when switched on it jammed in a certain position (Sept 29, 2012) patrick  said:

Reply: if your air handler blower fan assembly drive shaft wobbles

Brian H: if your air handler blower shaft wobbles in the bearings you need to either replace the whole blower assembly (which is what most service techs will do) or replace the bearing assembly and blower fan shaft (labor intensive too).

I'm assuming that your description of the pulley looking "warped" when the system is running confirms that the bearings are shot. But first check that the pulley is mounted square on the shaft and that the pulley is not loose.

Many fan belt pulleys include a metal key that locks the pulley to a slot in both pulley and drive shaft; the pulley can be loose, wobbly, and still turn the shaft; the pulley is tightened to the shaft with an allen screw.

Jay, check for a loose bearing or bent, damaged blower fan assembly; It should be possible to replace just the squirrell cage fan assembly and bearings if that's where the problem lies. If the tech's bid includes labor it may not be way out of line; I suspect the part is less costly than that. Ask for clarification.

Patrick

Check for a loose or damaged fan motor shaft or shaft bearing or loose connection of fan blades to the motor shaft. I suspect a damaged motor or shaft.

 

Should there be some travel on the shaft of the blower motor?

Should there be some travel on the shaft of the blower motor? On 2016-12-27 by T-bone

by (mod) - end play when moving the blower motor drive shaft

Good question T

A bit of end play when moving the blower motor drive shaft in and out of the motor - that is, along the long-axis of the shaft - is usually OK but if there is lateral play - you can wiggle the drive shaft from side to side in the motor, then the bearing(s) is (are) shot.

Undersized air ducts (reduced air flow) caused our heat pump motor to burn out?

No air was blowing from our heat pump. Techician said the motor was burned out and we need a new motor.

He said it probably happened because when they put the new heat pump in 2 years ago they didn't put new air ducts in, so our old ones are probably too small and made the motor burn out.

Could this be true? On 2017-09-13 by Kay

by (mod) -

Kay

I am not smarter than your onsite technician. But I suppose the tech thinks the motor for your air handler blower was over-worked.

On the face of it I'm surprised that a new heat pump would require new air ducts unless you were completely redesigning the system and its capacity.

Keep in mind that some techs can be smart at HVAC equipment repair but not good at communicating clearly.

Pulley fell off in Westinghouse blower unit - can't get it back on

I have an old (1965) Luxaire furnace with a Westinghouse blower. There was a terrible noise yesterday, I found the large pulley had fallen off. We couldn't get it back on.

Looks like the threading is stripped. Ordered a new pulley from Grainger. There is no threading on theirs. Grainger said, that is how they are now. I can get it lined up but it won't go on past the first thread. Any tips or ideas?

I am alone with a 4 year old & it's chilly! by Stacey

by (mod) -

Stacey

Usually a pulley is pushed onto the motor shaft and is secured with an allen screw or set screw.

But if the pulley you ordered won't slip onto the shaft where it belongs one would need to replace the shaft, motor, or blower parts.

You could try filing the shaft but I worry that if the pulley is askew the system will stress the motor or blower and lead to a new failure.

Gibson gas furnace lost blower speed

I have a Gibson kg6rc 100c-16b natural gas furnace that SUDDENLY lost RPM of blower assy and cold air stream droped dramaticaly though there is SOME stream, so fan wheel is rotating and clean. I checked it. HELP! Anonymous

Reply:

Assuming this is a direct drive unit not a belt driven fan I'd look for a failed motor, bearing, or start capacitor.