Air conditioner air not moving (C) D Friedman Weak A/C or Heat Air Flow FAQs
Questions & answers on weak air flow from an HVAC cooling or heating system

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Air conditioner air flow too weak:

How to diagnose weak or too-warm air flow from an air conditioning or heat pump system.

If not enough cool air is provided by your air conditioner, or if the air temperature is not cool enough, or if you just can't get your A/C unit running, this article helps diagnose and correct the problem with step by step things to check and links to more detailed explanation when you need it.

This article series forms part of our series on how to diagnose an air conditioner or heat pump that is not cooling: this article explains how to diagnose and correct air conditioning problems like lost or reduced air conditioner cooling capacity, reduced or no cool air flow, reduced or no actual lowering of the air temperature, or an air conditioner that won't start.

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Weak Air Conditioner or Heat Air Flow Questions & Answers

Dirty filter blocks air flow (C) Daniel Friedman

WEAK AIR FLOW: is the air conditioner (or when heating, the heating system) blower fan unit not moving enough air, so that there is too little air coming out of your air supply registers? Here is a list of things to check, in the best order.

[Click to enlarge any image]

These questions & replies about weak cooling or heating air duct flow were posted originally at AIR FLOW TOO WEAK.

On 2017-02-21 23:02:13.636023 by (mod) - half of doublewide has weak air flow

I would get under the home and follow the HVAC ducts from the air handler onwards - guessing that as usual ducts are under the unit. You'll probably find a point at which the main line splits into two: one going to each of two halves of the home.

From that point onwards along the half of the home with poor flow you'll find the trouble: a blocked, crimped, or disconnected duct or perhaps an improperly set balancing damper.

On 2017-02-21 22:24:52.959501 by Jeff in NC

I live in a seven year old 1600 sf double wide with a heat pump . Half of the registers in the home provide excellent air flow no matter if I am heating or cooling while the other half of the registers provide little to no air flow at all . Any suggestions on what I need to check . Thank You .

On 2017-02-14 13:57:47.024577 by (mod) - weak air flow after fixing a coil icing problem

Sure: you want to check the items in the AIR FLOW TOO WEAK. article: a defective blower fan - loose belt or even just dirt on the fan blades - will drastically reduce air flow as will many other defects like an open, disconnected, or leaky duct system, improper blower motor voltage, dirty air filter, dirty cooling coil.

Either low air flow rate or low refrigerant can cause frost and ice blockage at the coil.

On 2017-02-14 08:29:31.516733 by Larry from Guam USA

Thank you so much for your reply... Even when the unit has been completely defrosted and i run the system on "on" (not cool) there seems to be no air being drawn into the coils but the blower is working and air is being passed through the vents. My concern is if the coil system is not pulling in any air on "on" mode, is this a problem?

On 2017-02-12 18:26:25.893873 by (mod)

Larry, please search InspectApedia for AIR CONDITIONER ICING to see the common causes and cures (often low refrigerant and a refrigerant leak) of the trouble you describe.

When ice blocks the cooling coil indeed the system won't be able to cool air passing through the ductwork.

On 2017-02-12 08:35:35.623168 by Larry from Guam USA

Hello there, I have a central air conditioner that froze up today... the coils and the refrigerant line always freezes up when i set it to "cool"... I also noticed when i changed my filter that it seems there is no suction coming from the evaporator; however, the blower is working and i can feel (warm) air coming through the ducts/vents. Any ideas?

On 2016-12-13 10:29:35.789914 by Paolo

Hi! I hope you guys can help me. I have a 2.4 window type airconditioning unit. Unfortunately due to the building design, the unit's condenser opens to a small 3 x 4 x 5 feet enclosed space with a grille on the other end.

That space gets really hot when the aircon is running and I'm afraid it is affecting the aircon's efficiency. Will installing an exhaust fan help with this? I appreciate any advise. Also we are renting this unit so any structural changes would not be allowed.

On 2016-12-11 03:46:50.375637 by (mod)

Have you tried checking through the suggestions in the article above? Those are my best ideas.

On 2016-12-10 14:58:04.603026 by Aushlean

Hi. I have an electric fur if in my attic. Now my masterbedroom doesn't blow a lot of air and the master bathroom and closet don't get any air. I don't see a damper and the vents are wide open, and in the summer these rooms are hot. Why? What do I do to fix it

On 2016-09-14 03:03:24.683475 by (mod)

I would review the duct system. It's hard to blow cool air up words in a tall building. You may need booster fans or register controls on the lower floors.

On 2016-09-14 02:47:20.688831 by (mod)

I would check for low-voltage, a defective control on a multi-speed motor, or failing bearings, or if your blower is belt-driven I would check for a loose pulley or slipping belt. You have already made sure that there's no blockage in the duct work, crimped or disconnected duct work, or a dirty air filter, right?

On 2016-09-10 23:40:15.121514 by Anonymous


The fan blows very strong without A/C on, but it blows very week when the A/C is on. So, my house is not cool enough. Can you tell me the possible reason, please? Thanks.

On 2016-09-10 21:25:30.121675 by Terri

Just put in a new unit and coil the blower motor is not turning fast enough which is causing the Coil and AC line to ice What now?

On 2016-09-08 18:46:56.439982 by james

We have a two story house with a new central air unit , basement is really cool , first floor not as cool and second floor not cool at all, what would you suggest

On 2016-09-01 17:25:53.265963 by Anonymous

My return air cfm is 35000 comjng.
But deliver cfm is 21000 what is the reason ?

On 2016-08-27 21:00:07.054683 by (mod)

Sounds as if there are either two problems or the blower was on but the cooling coil was ice-blocked. Very low refrigerant can cause coil icing and thus can block air flow through the system.

On 2016-08-27 14:13:14.458268 by Steve Meyers

AC out, low on Freon. Turn blower on, very low volume of air. After recharge cold air plus lots of air volume. Why did blower not circulate much air even with low freon.

On 2016-08-21 20:06:39.131355 by (mod)


If you mean that across a single outlet air grille on the wall-mounted split A/C system air comes out of only half of that area, indeed something's wrong; Usually a single squirrel cage fan rotates to move air in the system but perhaps your blower is in two parts and one of them is not operating; or something inside the unit is blocking airflow.

I'd ask the service tech to remove the unit cover, check the blower assembly motor and fan and check for blockage. Also check for ice blockage on the cooling coil - a sign of a too-low refrigerant charge or a leak.

On 2016-08-21 00:58:40.337129 by John Jones

My mini-split only blows air on one side of the unit

On 2016-08-19 17:25:31.351122 by (mod)

The air temp needs to be cooler than the room temp, but measured at the vents I am not confident that there's a single "right answer" because of significant site variables in ductwork, building conditions etc. Usually we measure the temperature drop across the cooling coil.

Search for AIR CONDITIONER TEMPERATURES to read details found at

On 2016-08-19 11:05:11.786547 by Louis Cehen

when the A/C is on, what should the air temperature be coming out of the vents

On 2016-06-26 15:37:55.387951 by zubair

my 1.5 ton ac of mistsubishi air flow is low but cooling is ok, what is the reason.?

On 2016-05-30 22:20:43.152340 by Kimberly Faris

My husband changed fan motor on central ac unit,all works, but fan on unit doesn't seem to be running at full capacity. Is it electrical? Lightening hit our electric box last week, changed fuses, all is fine now, but maybe not?

On 2016-05-15 15:27:56.735331 by (mod)

Have you

1. had actual measurements of air flow at different points in the system or all of the consultants saying all is fine just expressing opinion based on no data?

2. has anyone checked for a dirty squirrel cage blower fan - dirt on the blades will substantially cut air flow

3. check for crimped, disconnected ducts or other defects listed in the article above.

On 2016-05-15 13:31:33.727019 by Serina

We have to change the air filters about once a week. Our unit is fine and the drip line is down. Home inspector and several AC companies said it's fine but yet it's barely blowing inside. Once we even found water in the duct work. We sicked it out and checked for leaks. There are no leaks. We've even replaced sections of the duct work. Why is it still acting like it's blocked?

On 2016-03-21 20:10:24.123670 by (mod)


an icing coil could mean low refrigerant or a bad expansion valve. Call your service tech who may find a leak or a bad valve or control.

If air flow is weak and the coil is not iced, look for one of the problems cited in the article above.

A fault in the outside compressor/condenser might result in poor cooling but it won't directly affect air-flow (except for an ice-blocked coil).

On 2016-03-21 20:08:20.082781 by (mod)


Look for a disconnected or crimped duct feeding the weak air register.

On 2016-03-20 16:09:31.391671 by james moore

Carrier air handler unit, noticing lower air flow from registers. A/C not cycling normaly (staying on too long), evaporator coil icing up. Blower motor approx 8 mths old(replaced new). Coils cleaned, filter replaced, condensate drain clean and open. Removed coil access panel, ran unit approx one minute with no change to air flow.

Suspect fault in external unit as it had made loud squalking noise ( not from fan), during shutting down cycle about 1-1/2 mth ago. Questioning expanision valve?

On 2016-03-20 06:09:08.018564 by Jeff

Ac works great except for one register is not blowing as strong as the others, what should I look for?

On 2016-01-22 23:27:12.669248 by (mod)


Thank you for the important report on the big hidden cost of a defective fire damper on heating costs. While I've made the general point about looking for duct obstructions I'd not considered fire dampers, devices designed to "fail" in the closed position for fire safety.

I have added your remarks to the article as you will surely thus help other readers.

Question: My Air Conditioner won't turn off - what to do?

Carla said: My outside air unit will not turn off on its own. I have to manually turn the breaker off to turn it off. Does anyone know why or how I can fix it?


Carla if your A/C won't turn off it could be that the thermostat is set to a temperature that the system cannot reach - due to lost cooling or due to a setting below the capability of the system.

If your A/C won't turn off even if you set the thermostat to a temperature that is above the current room temp, then the thermostat or an A/C control board or switch is bad and needs replacement - in that case you need a service call from a trained HVAC Technician.


Question: My Air Conditioner (or heat pump) keeps cycling on and off too rapidly - what might be wrong? short A/C on-cycle problems

Joe said: short cycling air conditioner compressor diagnosis: I have the same problem as Mathew: my A/C compressor cycles on and of every ~10 seconds. I is about 5yr old. What can we do to fix this system. Please help. Thanks!

Matthew said: short cycling air conditioner problems: Our air conditioning compressor cycles on and of every few seconds or minutes. What can we do to fix this systems not even a year old

See SHORT CYCLING AC COMPRESSOR for details of diagnosing and repairing an AC unit that keeps cycling on and off

If your air conditioner or heat pump has the opposite problem, staying on too long, see LONG-ON CYCLING AC COMPRESSOR.

Question: Why is my A/C not producing enough cool air?

A/C not producing enough cool air - I had my air conditioning system gassed up last week - $210. ! The unit is not producing enough cold air. The unit is set on 72 and does great at nights but during the day where the temp. outside is reaching mid 90"s it's getting up to 80 in the house. Is my duct work screwed up? How do you repair trailer metal ducting? - Amanda

90 degrees outside and 83 inside with thermometer at 76. Cools off to 76 when sun starts to go down and house then gets cold. New capacitor and condenser just put in. Help. - Ginny 5/17/12

Question: AC blowing just jot air

(June 18, 2014) Ellie said: Our a/c was blowing only hot air. Call repairman and he installed wires above the fan running along what looks like a large conduit pipe with a large loop of wire sticking out above the grill. We have a Carrier Heat Pump w/ a/c and have had a lot of vibration problems and at least one annual repair. We've replaced the compressor once.

This repair looks dangerous and we're selling the house and worry this repair might not pass the home inspection. Is it common to rewire the a/c "board" out of the compressor cover?

We don't think the kid they sent out and the more experienced man who came later made a proper repair. Seems like the wires should be inside where the original wires were. We live in the mountains and there is a lot of wildlife that cause mischief. We worry an exposed wire like this would be a serious temptation. Is this kind of wiring "normal."


Amanda: if your system is not cooling there could be any of a number of problems - see the article above as a place to start. If your basic complaint is that the A/C temperature at the supply registers is cool enough but the volume of air flow is too weak, we'd start by:

Ellie , no, not common and potentially dangerous. This should not be a big cost to put the components where they belong.

Weak air conditioner air flow - is there a relationship between refrigerant charge level and weak cool air flow rate?

Les said: Weak air conditioner air flow: Our A/C was serviced two months ago and the repairman said it had a leak. $400 later it was recharged with coolant and now the ac is doing the same thing. Very little pressure coming out of vents and no cold air coming out. Does anyone know what I can do for the weekend? It is stifling!!!! See A/C Air Duct Problems

DanJoeFriedman (mod) said:

Les: A leak that was fixed by a re-charge is not as good a repair as a leak that was fixed by finding and fixing the leak - you'll just have to keep adding refrigerant.

But weak air flow out of the vents would not be due to a refrigerant leak; more likely a clogged filter or crushed or disconnected ductwork, or a blower fan problem.

Weak air Flow traced to Clogged Air Filters: Can Clogged Air Filters Affect the Room Thermostat?

(May 13, 2011) Jim said: If air conditioner filters are clogged will it cause the thermostat to shut off?

Jim: clogged A/C filters won't cause a room thermostat to shut off. The thermostat responds to room temperature. However clogged A/C filters that reduce air flow, cause coil frosting, or otherwise reduce or stop the flow of cool air into the room where the thermostat is located would mean that the thermostat would remain "un-satisfied" and should mean that the thermostat says "on" - continuing to call for cooling. See AIR FILTERS for HVAC SYSTEMS

Air Filters & Cooling System Amperage Draw: Could Removing the Air Filters Cause the Electric Motor in the Air Handler to Draw Higher Amps?

Question: got one for you. i put an ammeter on my air handler and it read 8.25 amps, I removed the filters and it went up to 9.75 lmao at the situation the amperage should have gone down. what gives here

Reply: causes of variations in electric motor efficiency and current draw measured in amps

Lost: this amps variation is beyond my expertise, but in general reducing the load on an electric motor will show up as lower amps or current draw, not higher amps. Here are two interesting explanations of amps or current variations on an electric motor that I found when researching the question:

1. Voltage variations and current draw at electric motors: If your supply voltage is varying from your power company that can show up as higher amps draw on the motor (though it's a suspicious coincidence to see it exactly when you removed the filters and supposedly reduced the load on the motor). Quoting from motorsanddrives [dot] com:

"The effect of low voltage on electric motors is pretty widely known and ... The amount of power the motor draws is roughly related to the voltage times current (amps). Thus, when voltage gets low, the current must get higher to provide the same ... To summarize the situation, low voltage can cause high currents"

2. Load variations and electric motor efficiency: A second possible source of seeing higher amps or current draw on your blower motor when you pulled out the air filters and thus reduced the load on the blower motor might be illuminated by this U.S. DOE pamphlet "Determining Electric Motor Load and Efficiency" - Quoting:

"Most electric motors are designed to run at 50% to 100% of rated load. Maximum efficiency is usually near 75% of rated load. Thus, a 10-horsepower (hp) motor has an acceptable load range of 5 to 10 hp; peak efficiency is at 7.5 hp. A motor’s efficiency tends to decrease dramatically below about 50% load."

Question: what causes warm suction lines in an air conditioner or heat pump system? Warm A/C suction line (should be cold) and zone control dampers that stopped working

JMONTE said: warm A/C suction line question: After my condenser is turned on for about 4 minutes the suction line starts to get warm to the touch. can you tell me what the problem may be


JMONTE: If the HVAC suction line gets warm, you may be out of refrigerant, or the system may be running in heating mode if it's a heat pump. See OPERATING TEMPERATURES HVAC.

Becky, If a motorized HVAC zone control damper is not opening or closing, most likely the motor has failed, or the thermostat that operates that zone control is off or set in an incorrect position. See ZONE DAMPER CONTROLS

Becky said: motorized air conditioning zone dampers not working

My house has "zone" control with dampeners to close off the upstairs over the garage room unless that thermostat is on - the room is not cooling. I have located the damper under the house. What are some causes for the damper not opening and how to repair them? condensation, motor to damper? silicon glued properly?

Question: poor air circulation & poor air temperature at my HVAC system

(Feb 16, 2014) Stephanie said:

Hello! Looking at a foreclosure property and the inspection today revealed a problem with the HVAC system. The home is 2.5 levels and includes a basement. The 2nd level (master with 2 BRs) does not have a thermostat, and also has poor air circulation- temperature is controlled via thermostat on loft/3rd level .

The inspector mentioned a 'temperate change' from vent to BR space on 2nd floor. There was no noticeable heat coming from vents on 2nd level, but noticeable heat coming from loft level. Any ideas on what is going on?

Also as an FYI- first level has 2 different thermostat/zone controls and basement has one. Are 4 furnaces total. Thanks for any help, I'm a complete novice when it comes to this stuff!


Stephanie, from just your message I can't guess at whether the problem is with
- poor original design (such as inadequate return air)
- a system maintenance or repair issue (dirty filter or blower fan for example)
- damage (a crimped or disconnected air duct)

The fact that you observe plenty of heat coming from the loft level tells us that the basic heating equipment is working and suggests focusing on a duct or register problem.

I'm glad to address your questions here, but it'd be more useful to post heating questions on an appropriate heating diagnosis page where other readers may also have something to offer.

If you search InspectApedia for

no heat - furnace

you'll see our lead article on how we approach diagnosing the problem

Question: why is there a gap under doors in my house?

(Feb 12, 2015) Tom Dalton said:
I was trying to find credible proof as to why there is a gap under the doors all through the house. I've tried explaining return air for the H V A C and ventilation for the bath exhaust fan to draw air from. My wife wants to close the gap at the bottom of the bathroom door. Any advise?



In a house with central air returns, if you make the rooms more air tight by fitting tightly-closing doors you will certainly impede the delivery of heated or cooled air from the HVAC system - it's like trying to blow air into a balloon - you can do it, up to a point. Then your lungs just aren't up to the task. (Or you burst the balloon)


(July 27, 2015) June said:
The diagnosis was done for my apartment fan/blower unit. I need a new one. What type of establishments sell them? Home Depot does not.

Also, do all plumbers install these or do I need someone who specializes in this?



HVAC suppliers


(Oct 5, 2015) Susana said:
Two floors house plus basement. In the basement ac and heating works fine but in the first and second floor I get too little air, almost nothing. We changed the filters but doesn't work. I'll appreciate any advise or idea. Thanks.


I'd look through the suggestions in the article above: we need to know the details of your duct work, location of dampers, blockages, disconnects, duct sizes, return air adequacy etc.


(Oct 13, 2015) Alex said:
my office space upstairs has 6 rooms with ac ducts. Yet a single room has little to no cool air and low air flow coming into the room, but the rest have excellent room temp. can it be because of a dirty air filter or a damaged duct?


More likely it's a duct crimp, blockage, disconnect, since the air filter is usually central (at the air handler or air return) and would affect all of the system air supply.


and for heating systems and heat pump systems WARM AIR SUPPLY IMPROVEMENT


Continue reading at AIR FLOW TOO WEAK or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



If your air conditioner or heat pump is a split system or mini split system design (using a wall-mounted cooling or heating unit) our troubleshooting diagnostics are at SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS.

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