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 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.

How to Diagnose Air Conditioner Output Cool Air Flow Too Weak, Too Cold, or Too Warm

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.

Sketch courtesy of Carson Dunlop Associates, a Toronto home inspection, education & report writing tool company [ carsondunlop.com ].

1. Check that the air register is open

2. Check for a dirty or clogged air filter: replace the filter.

Watch out: some HVAC systems have more than one air filter in more than one location. Be sure you've found all of them. For example a filter may be located at a central return air inlet grille and another may be located at the air handler itself.

Clogged Air Conditioner filters can lead to lost cooling capacity first, because the clogged filter reduces the air flow through the system, meaning that you'll feel less air flow at the supply registers than was previously present.

See AIR FILTERS for HVAC SYSTEMS for details.

3. Check for a dirty or broken blower fan: if the squirrel cage fan in the blower unit is dirty the blower may be spinning but not moving much air. If the blower is a belt-driven unit check that the belt is intact and that the blower spins.

See DIRTY A/C BLOWERS for details.

If you don't know what an "air handler" or blower unit is, or for more blower unit diagnostics

see AIR HANDLER / BLOWER UNITS

4. For Cooling Systems: Check for an iced cooling coil  inside the air handler; a refrigerant leak can cause frosting while later the when more refrigerant has been lost the result is delivery of inadequately-cooled air.

Air flow that is too slow for any reason (such as a dirty filter or dirty blower fan assembly blades) can cause first, air temperatures that are abnormally low coming out of the air conditioner, and eventually a reduction in air flow as coil ices over.

See FROST BUILD-UP on AIR CONDITIONER COILS and

also

see DIRTY COOLING COIL / EVAPORATOR COIL

5. Check the ductwork for any cause of blockage, partial blockage (crimps, excessive bends), leaks, disconnects. Blocked, crimped or disconnected air ducts can also cause loss of cool air or too little cool air coming out of supply registers.

See DUCT & AIR FLOW PROBLEMS

Reader Comment: faulty HVAC Duct Fire Damper Blocks Airflow

2016/01/22 Peter said:

No air coming from vents in your commercial building? We had a 7 ton unit that was supposed to heat and cool two big rooms and some bathrooms. Nothing was heating up those rooms. The 7 ton Rudd was new, everything was checked, even a bigger gas pipe was put to it. While the ducts at coming directly from the 7 ton were shaking with air flow, no air flow was coming from the ducts in the rooms, which were about 50 feet away.

At first we thought it was the ductwork design. It literally had about 15 turns in it to get from the roof to the building. We thought it might be leaks. Perhaps insulating the roof ductwork.

Because nothing seemed to fix the problem, and we had tenants, we spent a fortune putting in at first, temporary radiant heat, and then permanent electric radiant heat. After months of trying to figure this out, my HVAC guy finally discovered the problem. Commercial ducts have Fire dampers in them.

Fire dampers are passive fire protection products used in heating, ventilation, and air conditioning (HVAC) ducts to prevent the spread of fire inside the ductwork through fire-resistance rated walls and floors. Fire/smoke dampers are similar to fire dampers in fire resistance rating, and also prevent the spread of smoke inside the ducts.

When a rise in temperature occurs, the fire damper closes, usually activated by a thermal element which melts at temperatures higher than ambient but low enough to indicate the presence of a fire, allowing springs to close the damper blades. So, apparently with us, a fire damper in our ductwork decided to close. We easily spent thousands and thousands in emergency heating only to find out it was a defective $100 fire damper.

Reply:

Peter,

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'll add your remarks to the article as you will surely thus help other readers.

See AUTOMATIC FIRE & SMOKE DAMPERSfor details about these devices.

Other HVAC Air Delivery Problems: no air flow, air flow too warm, air too humid

NO AIR: Is there no cool air at all coming out of the supply registers at all?

Check all of the heating and cooling controls to be sure that they are set properly and working normally. The thermostat is set to COOL, FAN to AUTO or ON, HEAT to OFF, and the set temperature on the thermostat is set below room temperature (if you want cooling).

SNAFUs in the individual controls and relays and switches for the air handler, blower, compressor/condenser unit and refrigerant metering equipment will, if not working properly, lead to these same complaints.

Check the following causes of complete loss of air flow

• Are there closed supply or return registers, or a closed ductwork damper.
• Is the thermostat not calling for cooling or heating as appropriate
• Has the system lost electrical power
• Is there a blower unit that is not operating because a blower door is open (door switch shuts off the system)
• is there a condensate pan switch that has shut down the blower assembly, disconnected ductwork
• Is there a completely frost-blocked cooling coil

AIR TOO WARM: Or is there air blowing out of the supply registers but it's not cool enough?

Typical causes of too-warm air flow out of an HVAC system in cooling mode are

• Refrigerant leaks & lost refrigerant
• Air flow moving too fast across the cooling coil
• Backup heat is stuck on (discussed below)
• Insulation missing from the duct system
• Ducts routed through a hot, un-conditioned space

AIR TOO HUMID: if the air conditioning system is not dehumidifying, and presuming no one has left windows or doors open to humid outdoor air, air flow may be too rapid or the system may be over-sized.

See DEHUMIDIFICATION PROBLEMS

HVAC Control SNAFUs That Can Lead to Too-Warm Air Conditioning or Weak Air Flow

As you read below, a reader (Pete) reminded us that even when the A/C system seems to be working properly a too-warm air output problem in a heat pump system could be due to emergency or backup-heat being left in the "ON" position.

Question: why is my heat pump blowing warm air?

4/30/2014 Pete said:

I have an old unit that will not blow cool/cold air. I have had a tech out twice and I am trying to see if there are any things that I can check or ask my technician to check. The tech had the unit working properly, after his last visit, until about 2 weeks ago. At that time we had a cold-snap and I had to turn my heat back on. When I turned it back to AC it has not worked properly since.

The outside unit (condenser) is running as is the air handler. Plenty of air is blowing out of the registers but it is only about 70-71 degrees. I do not know anything about fixing ACs but I am pretty handy and I have been researching on the web.

Some of the viable options I found are a stuck or bad REVERSING VALVE on HEAT PUMPS, coolant line could be blocked or contaminated or the Thermostatic Expansion Valve could be bad. Does that sound right? Any thoughts? Also, I noticed that the high pressure line (small copper tubing, I think) coming out of the outside condenser is cold to the touch. Should that not be warm/hot? I really would like to get one more season out of the unit. PS: all filters are clean.

Reply:

With the benefit of hind-sight - your comments below, that cold refrigerant line was an important clue.

Reader follow-up: (2 hours later) Problem solved: emergency heat stuck on

Pete said:

Found problem. AC was working but the emergency heat strips were stuck on. So the cold air was getting heated. Replaced and is working. Hooray!

Fix Existing Air Conditioning Problems Before Trying to Improve its Cooling Capacity

This discussion as it applies to air conditioning or cooling systems is now found

at COOL AIR SUPPLY IMPROVEMENT

and for heating systems and heat pump systems this topic is discussed

at WARM AIR SUPPLY TEMPERATURE & IMPROVEMENT

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2020-09-22 by Tom

Question, just had and HVAC tech over. The fan is working and the freon level isn't the issue. Low air flow coming from the vents and it is cool, but weak flow.

The pipe has ice on it when running and he suspects it is a blockage/resistance to the coil. Is there a fix outside of replacing the whole unit ($11,000). Could it be anything with blocked/detached ducts? Can a blockage to the coil be fixed? Thanks.

On 2020-09-22 by (mod) - Is there a fix outside of replacing the whole A/C unit ?

I would not consider replacing the "whole unit" unless it's very antiquated;

I've seen the air flow rate double or triple simply from cleaning the blower fan - a small bit of debris greatly reduces the fan output;

And I'd really be upset if I spent $11K on a new AHU and heater or A/C unit only to find that the original problem was a crimped, fallen, or damaged air duct. So let's inspect the whole duct system with care.

If the coil is "blocked" itself that's normal and occurs as dust and debris accumulate on the coil fins (maybe because someobody forgot to install the air filter in the system); coil cleaning service is a normal and common service procedure and costs a micro-fraction of a new system.

The only rub is if the system is very old and the coil very corroded the tech may be afraid to spray on the cleaner; (Spray, wait, vacuum up the muck).

On 2020-09-06 by Kathy

Filter replaced blower motor replaced sucking air not blowing air?

On 2020-09-06 - by (mod) -

I suspect that the fan was installed backwards

On 2020-08-24 by Joey Weng

Is there any way that my couch is blocking the intake with this amount of space? Our motor has burnt out twice this summer. The repair crew said this wasn't enough space and was the cause of the motor burn out. The first motor worked for 2 years until breaking and the 2nd broke only a month after installation. Is it more likely that this is an electrical issue or is this really blocking intake?

On 2020-08-25 - by (mod) -

I suspect there's some other issue, Joey. You've got a reasonable amount of room in front of the return air inlet. A blocked filter or blocked return air duct might overheat the system. The service tech can certainly also check supply voltage and wiring.

 

On 2020-04-22 by Kristie - Only getting cold air pressure on one side of movie home

Only getting cold air pressure on one side of movie home, replaced cross over and looked down in every register vent with mirror and light and it's clean from front to back on both sides and still no pressure but the side that works blows hard. Please I'm going nutts

On 2020-04-23 - by (mod) -

Kristie

There may be a balancing damper closed somewhere - follow the ductwork from the point that the two sides of the home's air system split off - look for a damper lever or motor.

On 2020-02-23 by Calvin Hvac have code that reads airflow problem

Hvac have code that reads airflow problem

On 2020-02-23 - by (mod) -

Calvin

You're on a good diagnostic page as listed here are causes of weak HVAC air flow.

First check to see blower fan is actually working and then check that you don't have a very dirty air filter in the system. When you check those two obvious things take a look through the other causes given on this page. Let me know what you find and don't hesitate to ask follow-up questions.

On 2020-02-16 by Dan = evaporator fan motor runs very slow

System was working fine during cooling mode, we use gas heat and barely use the heat setting. when the weather started warming up, we turned on the cooling setting, but the evaporator fan motor runs very slow. side panel off and you can see the fan turning very slow, input wire has 20 volts. any help is appreciated.

On 2020-02-16 - by (mod) -

Dan

Seeing just 20VAC on a fan circuit that normally runs at 120VAC, particularly if the input voltage to the unit is found to be proper (120VAC) suggests a bad wiring connection (corroded or a loose electrical splice) or control board or relay. There could be a low-voltage short in the unit's wiring. (Look for damaged wires, corrosion, or poor-quality after-market repair parts).

But most likely is a bad fan motor.

On 2019-02-05 - by (mod) -

Question:

(May 30, 2014) SHAN SINGAPORE said:
Hi
my LG aircon out door unit
is working but no cooling in rooms.
model is H2UC186FA0
KINDLY ADVICE WHAT COULD BE POSSIBLE PROBLEM.
CAN I GET SCHEMATIC FOR OUTDOOR UNIT AS WELL.
THANKS A LOT...

Reply:

Shan

If the air handler is moving air but the air is not cold most likely the outdoor compressor/condenser is not working.

See inspectapedia.com/aircond/Air_Conditioner_Diagnosis.php

About a schematic, I too found it difficult to locate a service manual for your unit - I went to LG's support website at lg.com where the H2UC186FA0 is indeed listed, but the company says that no technical manual is available - which was a surprise.

Here is a link to the LG service manual for the H2UC series split system air conditioners

LG HVAC Systems

inspectapedia.com/heat/HVAC-Manuals-Air-Conditioners-Boiler-Furnaces-J-O.php#LG

or go directly to the LG HVAC SYSTEM SERVICE MANUYAL [PDF] at

You can contact an LG Support Representative at
1 800-822-822(National call Rate)

On 2018-05-28 by Sam Manz - AC tripped fuses - iced coil

A few days ago I had to trip a couple of fuses to install a new light in the hallway.
What I forgot to do was to shut the AC off while I was doing the work.

What happened next was that my condenser froze because I had shut down the blower while the AC was still running.

The condenser had accumulated so much ice that my duct system was about 80% blocked.

Had to start the heater to defrost the condenser and then set the temp bakdown to AC levels.

On 2018-05-28 - by (mod) -

Sam

Thank you for your report of this experience. It will certainly help other readers. Your system should be okay once you defrost of the coil. Let us know what happens.

 

On 2014-09-25 - by (mod) -

More or less true. A better tempeature check is across the air handler or cooling coil.

On 2014-09-23 by Kevin

I was told that a quick test to check the A/C was to check the temperature difference be between the vent and return. If there was not a 15 to 20 degrees difference the A/C was not operating correctly. True?

On 2014-07-23 - by (mod) -

I'll work on the topic with the email ASAP

DF

On 2014-07-21 by Erin re: new A/C

Thank you very much. I just sent those photos by email. If there is anything else I can send to give you more info please let me know. Thanks so much.

On 2014-07-21 - by (mod) -

The explanation sounds a bit unclear to me too. Use our CONTACT link to send us some sharp photos of the equipment and it's labels.

You should also have an installation and operation manual left by the installer that gives equipment properties and also a manufacture telephone number for such questions.

On 2014-07-20 by Brand new A/ C not up to par

We had our air conditioning unit replaced 4 weeks ago and it hasn't been able to maintain the thermostat temp or bring down the temp when it is higher than the thermostat is set at. The company that installed it tells me that because the unit is a new, energy efficient model, it isn't designed to try to cool the house if the outside temperature is lower than what my inside temp is set at.

My dilemma is that the outdoor temp may be 72F, my thermostat is set at 72F, but the actual internal temp of the house is 78F because it had been hotter earlier in the day and the unit had been unable to maintain the temperature at what the thermostat was set.

He came to check the unit and found that the filter was fairly dirty. He recommended we get the ducts cleaned and we have an appointment for that at the end of the week. He felt confident that this was the issue and did not see any need to investigate any further. Note we did have the ducts cleaned 3 years ago.

I am being told by the installer that the new units are not designed to bring down the temperature within the house on cooler days or evenings (ie 72F) and he tells me that I should expect it to struggle and perhaps not maintain the temperature when it is very hot out (ie 84F and up).

This sounds fishy to me and I'm wondering if I should be having someone else to come and look at it. Having just paid $230 to service our old unit, then $2500 to move and replace it with a new unit, I am reluctant to seek a second opinion that is going to cost more money unless absolutely necessary.

It is a Keeprite Unit and I have a picture of the information on the outside of the unit but I'm not sure what information from it would helpful to you.

On 2014-07-07 - by (mod) -

C Lake

I'm not sure what you refer to by "riser" installation, nor do we know what kind of AC is installed in your building. But rebooting a thermostat is certainly no solution. From just the information in your note I'm not sure what's going on - though I suspect your cooling units are missing coolant or refrigerant. You need an onsite inspection by a qualified HVAC technician or engineer.

On 2014-07-06 by c.lake

My post needs correction.
Back up with room Air conditioner, and fans to cool the bedroom in a central air condo.
It came out as "access" which was incorrect. Thanks

After a recent HI rise bldg. Riser installation, our units were removed and replaced. Some hose hookups were incorrectly placed, causing loss of air. Some AC units never cool to designated temperatures, causing intermittent shut down of cool air, not the unit. We cannot get the same cooling efficiency we had prior to the riser job. In fact, we have to keep rebooting our thermostat to coax cool air, and even then, it's not cool enough.

Our management refuses to address this problem even though many are affected, even those who had new units installed are experiencing this problem. I have to back up my central air now with a room access unit and fans. What, in your experience, do you think could have happened. It was fine before the Riser job. Thank you.

...

Continue reading at DUCT & AIR FLOW PROBLEMS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see AIR FLOW TOO WEAK FAQs - questions & answers about weak cooling or heating air

Or see these

Recommended Articles

• DUCT & AIR FLOW PROBLEMS
• SUPPLY DUCT AIR LEAKS
• SPLIT SYSTEM AIR CONDITIONERS & HEAT PUMPS. If your air conditioner or heat pump is a split system or mini split system design (using a wall-mounted cooling or heating unit)
  

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AIR FLOW TOO WEAK at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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