Questions & answers about diagnosing and fixing an air conditioner or heat pump (in cooling mode) that seems to run but only blows warm air.

Page top photo: an indoor air handler or "blower unit" in a building attic. Notice that the cover is missing? That alone can cause trouble in moving building air through the air handler.

The author trained as an HVACR repair technician and worked for three decades as an air conditioning & heat pump system inspector and repairman.

Q&A on how to Fix an A/C or Heat Pump Unit that Blows Warm Air in Cooling Mode

These questions & answers about a cooling system that just blows warm air were posted originally

at AIR CONDITIONER BLOWS WARM AIR - be sure to read the diagnosis and repair advice given there.

Photo: ceiling air supply register where you might check for cool air blowing into the room.

Heat Pump Condenser Fan won't run and Attic Blower Pushes out Just Warm Air

15 y/o heat pump condensing unit fan spins at slow speed then shuts off and starts again in ac mode. Blower in attic pushes warm vs cold air. - Craig

Reply by (mod) -

Could be a bad fan motor or motor capacitor or control board.

Take a look at CONDENSER FAN WON'T START

 

 

Excessive Supply Air Temp Cooling message displayed at compressor control

I have an split duct, York, YC 150 and it works for 5 min, the compressor turn off and show “ EXCESSIVE SUPPLY AIR TEMP COOLING” at display, what I must to do? - On 2017-11-18 by Mauricio -

Reply by (mod) - Excessive SAT (Supply Air Temperature) Control Suggestions

Mauro up, you are describing a special feature on some York systems. This explanation from the service manual should help.

EXCESSIVE SAT (SUPPLY AIR TEMPERATURE) CONTROL This feature is only available on Constant Volume units. On a Constant Volume unit, the compressors and heat stages are control by the space temperature only, either by thermostat or zone sensor.

As long as the room device is calling for cooling or heating the compressors or heating section does not control the actual temperature of the supply air leaving the unit.

The enabling of this feature adds a means to prevent low or high supply air temperatures from being delivered to the conditioned space.

If you don't have the service manual we can help you find one.

 

Question: AC blowing just hot air

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." - Ellie

Reply:

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:

• Install a new clean air filter
• Check that the air supply registers are open
• Check that ductwork has not become disconnected or crimped or crushed
• See DUCT SYSTEM DESIGN SIZE & DEFECTS where we list quite a few steps in diagnosing poor air flow out of an HVAC duct system

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

 

Question: old unit that will not blow cool/cold air. Running on emergency heat - stuck on

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, 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.

AC was working but the emergency heat strips were stuck on. So the cold air was getting heated. Replaced and is working. Hooray! - (Apr 30, 2014) Pete

Reply:

Thanks for the follow-up.

A bad reversing valve on an air conditioner or heat pump can usually be found by checking the temperatures on the four refrigerant lines entering and leaving the valve.

Heat stuck on will sure do the trick. Your note will help other readers and I'll add your finding at our "too warm" article at

AIR FLOW TOO WEAK

Also see REVERSING VALVE on HEAT PUMPS

 

Question: a/c is running and blowing cold air and then on its own starts blowing hot air

My a/c is running and blowing cold air and then on its own starts blowing hot air and cold air again this is a goodman heat pump. (May 6, 2014) steve nelson

Reply:

Steve it sounds as if your compressor/condenser is probably working since you see intermittent cool air.

So perhaps there is a thermostat or control problem that is shutting off the compressor, OR the coil is icing up.

But as you can read in Pete's comments above, another explanation for an A/C unit that cycles between blowing cool then warm air can be a control problem that is switching the emergency backup heat on - IF your system is a heat pump.

 

Question: Rudd Heat Pump AC Unit, too cold, fan unit ices up - the unit will continually run (blowing only warm/cool air)

I have a Rudd Heat Pump A/C Unit. Notice yesterday when I set the room thermostat to about 78 degrees a blue Aux Heat lite comes on.

The unit runs about 15 minutes and the C/B trips. If I keep it below 78 degrees and the lite does not comes on, the unit will continually run (blowing only warm/cool air).

I also notice yesterday that my outside unit fan blades iced up (shut the power off from the unit).

This morning I got out and knock the ice off and restarted unit. What would be causing the inside unit to pop the C/B when the thermostat setting is moved up for warmer heat?

Mike In Warner Robins,GA, email: mew123055@gmail.com (Feb 13, 2014)

Reply:

Mike, sorry that you're probably cold; Though setting a room temp up to 78 is a bit challenging.

Most likely you've already figured out that the problem is with your backup heat system - which kicks in when the heat pump can't extract enough heat from the outdoor air. Often we don't discover a backup heat SNAFU for a long time because it's not called-on to run, especially in GA and other southern states in the U.S.

Search InspectApedia for

Heat Pump Backup Heat

and you'll see our diagnostic articles for that topic.

 

Question: AC unit won't cool the house down to the thermostat setting

The temperature inside the house is 82 I set the thermostat to 75 inside temp.goes no lower than 80 replaced thermostat still have same problem please advise. June 23, 2014) from Robert 

Reply:

Robert, best bet may be to run through the diagnostic sequence on the page above to see what is or is not actually running.

Check for reduced air flow as well as too-warm air output. I suspect you need a repair tech but it would be embarrassing if we found the trouble were a dirty air filter.

 

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 condensor just put in. Help. - Ginny 5/17/12

Reply:

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:

• Install a new clean air filter
• Check that the air supply registers are open
• Check that ductwork has not become disconnected or crimped or crushed
• See DUCT SYSTEM DESIGN SIZE & DEFECTS where we list quite a few steps in diagnosing poor air flow out of an HVAC duct system

 

The air conditioner output air starts cool then blows warm air

i have a single unit the air starts cool then air warm - (June 30, 2014) bill frame

AC blowing warm air.

Hi I woke up one morning to the AC blowing warm air. One time before it was the capacitor so we changed the capacitor that didn't work so we changed the contactor.

This still did not work. However we can manually engage the contactor and the system will cool. Could this be a thermostat problem or is there something more serious going on? - 9 July 2015 Johnnetta Hamlett said:

Moderator reply

If manually engaging a contactor relay makes the A/C system run and produce cool air the problem might be the contactor relay itself, the thermostat or thermostat wiring, or a control board.

Ask for help from your HVAC service technician.

 

In Cooling Mode Air is Blowing but Not Cooling - just warm air comes out

Heat mode works fine, In cooling mode, air is blowing but not cooling.

Can reset breaker even though it isn't tripped and will start cooling again.

Noticed more frequently as outside air temperatures got cooler.
Seems to happen daily or every couple days, but all I do is reset the breaker and it works again - (Nov 26, 2014) Michelle

Moderator Reply:

Michelle

When the air handler blows warm air in cooling mode

1. check that the aux heaters are not stuck "on"

2. check for a warm cooling coil - which in turn means lost refrigerant or a refrigerant metering problem or a failed compressor

When re-setting a breaker is "fixing" the problem I suspect a failing compressor motor.

 

Heat pump could be blocked by blown in insulation causing room to overheat

What on a heat pump could be blocked by blown in insulation preventing it from opening and closing and cause a room to overheat - On 2018-01-05 by Cathy -

Reply by (mod) -

Cathy

Blown in or any insulation that gets drawn INTO a mechanical system that has moving parts can jam parts and even start an electrical fire.

Blown in or any insulation that gets drawn into or blocks air intakes can also cause equipment itself to overheat.

But that's the best I can guess since I don't know what "opening and closing" you refer to on a heat pump.

 

Why does my window air conditioner sometimes blow hot air?

i have a regular apartment window air conditioner, and some times it blow out hot air, why? - On 2011-07-08 by mary f howard

Reply by (mod) - window A/C unit blows only hot air

Mary if your window A/C unit blows only hot air I suspect that either it has lost refrigerant or the compressor is not working; a low but not totally lost refrigerant level can also cause icing on the condenser coil in the unit that would make it stop cooling.

IF the unit sometimes blows cold air and sometimes only hot air, I suspect icing and a low refrigerant level.

 

My air is not coming out cool; without calling a service tech what can I check?

It appears that my air may be loosing its cooling effect. Without calling a service tech, can I measure the cooling at a vent and compare to wall thermostat? - On 2011-07-06 by Tom -

Reply by Tn Goose: The temperature of the supply air at a register should be quite cool, much cooler than room temperature

Tom:
The temperature of the supply air at a register should be quite cool, much cooler than room temperature, so comparing it with the setting or reading at the wall thermostat won't be a very clear diagnostic. I'd start by looking at the article above where we discuss what things to check first if your air conditioner is not cooling adequately.

If none of those leads you to something obvious that a homeowner can fix, like a dirty air filter or a disconnected air duct, you'd best call a service tech.

Upstairs A/C is blowing lukewarm air

In our home we have two outdoor compressors. One for upstairs and one for downstairs. The downstairs unit is cooling fine.

However, the upstairs unit is blowing lukewarm air. The compressor is running fine there just isn't any cool air coming out of the upstairs vents. Any ideas? - On 2011-06-28 by Jennifer -

Reply by TN-Goose

To Jennifer:
If your compressor is indeed in need of refrigerant, make sure you find out why the refrigerant leaked and have it addressed before you recharge the unit with refrigerant. Otherwise, the refrigerant will leak out again over time.

To Jennifer:
Is the fan running on the compressor/condensor unit? If your compressor and fan are running fine, then possibly it's out of refrigerant.

 

On Hot Days mt A/C gets only a 5 deg temperature drop

My A/C gives 20 degree drop in temp between return and supply on coolish days but on hot days I'm only getting about 5 degree drop.

Also, the pipe from outside compressor has NO sweating on hot days (when not getting enough cooling) but sweats as expected on coolish days (when getting enough cooling). Does my 10 year old compressor unit need repair or replacing or something else? - On 2011-06-22 by Tim

Moderator Reply:

Tim, check first for cooling coil icing when the system is working hard on hot days; you might notice that not only is the temperature drop not enough but the cool air flow is more weak.

 

The refrigerant suction line is cold but air coming out of my supply registers is too warm

My ac is blowing air out nicely from the registers but the air is not cold enough. I touched the thin copper metal tube being fed into the unit and it was quite cold. The filter was clean. What could be causing the air not being cooled? - On 2011-06-21 by Joe T

Moderator reply:

Check first for a cooling coil being blocked by frost or ice

 

The unit is not producing enough cold air even after it was serviced

I had my system gased 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 trailor metal ducting? - On 2011-05-30 by Amanda -

Moderator reply:

If the problem is that air flow is plenty strong but it's not cool, see Step 6: AIR FLOW IS OK BUT AIR IS NOT COOL

If the problem is that air flow is weak see Step 4: WEAK AIR FLOW 2: NEXT CHECK THE SYSTEM DUCTWORK & AIR SUPPLY REGISTERS

 

Compressor runs but no cold air is being circulated

I had a line in the whip cut. and now the tech says I need to replace the compressor. The compressor runs but no cold air is being circulated. He also said that I would be better off buying a new unit versus replacing the compressor. The unit is five years old at the end of the month. Your thoughts? - On 2011-06-01 by frank -

Reply by (mod) - tech says I need to replace the compressor

Frank:

I can't second guess the tech who was at your site, but here are a few questions to ask:

1. What does "had a line in the whip cut" mean?

2. If the compressor is running and there is good air flow out of the supply registers but no cool air is being delivered, it could be a simple refrigerant leak that needs to be fixed followed by recharging the system.

So I'd ask your service tech why that means a compressor is needed or why a whole system replacement is needed.

3. If the compressor is running and there is POOR air flow out of the supply registers but the3 air is cool, then the problem is with the blower fan, a blocked or iced evaporator coil, or disconnected, crimped, damaged ductwork, or even just a dirty air filter.

So I'd want a more clear definition of just what's wrong so that you have confidence in the diagnosis and thus in the remedy.

 

Question: Outside AC turns on but no cool air is delivered inside. What's the cause?

Our AC outside turns on but it does not blow any air inside what can be the cause of this? - Veronica 8/2/11

Reply:

Veronica if the outside compressor/condenser is working but the inside air handler/fan unit is not, take a look at the diagnostic

article BLOWER FAN OPERATION & TESTING 

(More diagnostic article links at the ARTICLE INDEX the bottom of this page)

It could be as simple as a fan drive belt that has broken in an air handler, or a dirty air filter.

 

Air conditioner starts but no longer blows cold air

A/C starts but quit blowing cold today. - On 2017-08-18 by Russ Manuel -

Reply by (mod) -

Russ

Our article AIR CONDITIONER BLOWS WARM AIR collects our diagnostic suggestions for this problem,

or


You can go back to the diagnostic steps we give

at AIR CONDITIONING CHECKLIST FOR LOST COOLING

or go right to

Step 6: AIR FLOW IS OK BUT AIR IS NOT COOL

 

Question: no air is coming out of our supply registers

I have a Coleman unit. Had heating probs 2 years ago leading to complete computer panel replaced, evaporator coils cleaner, gas regulator replaced.

Two days ago, my A/C decided to stop working. I hear the freon flowing into the coil region, the fan outside is turning, but no air is coming out of registers. What can it be? - Cheryl 9/9/11

Reply:

Cheryl if you have no air coming out of supply registers the blower fan is not working or the duct system has become disconnected or blocked. Coil ice-over in the air handler can also block airflow. Start by determining that the blower is running and that the air filter is clean.

 

Question: AC runs but air not cool at all registers, "blown to ground" (i.e. a short circuit)

My AC was running but not blowing at every vent and air was cooler on some vents then others . Technician saw ice and then came back and did a leak test and found no leaks. Then AC worked for a day but began to turn on and off periodically . Technician came back out and AC compressor turned on and off while he was here.

He put a booster on it and cold water because it was hot and then tested something and said it was blown to the ground. Home warranty won't pay for blown to the ground diagnosis.

But it wasn't blown to the ground initially....right? Just trying to figure out if this makes sense or if warranty company is trying to get out of it. Thanks for your time. Aug 8, 2014) Jennifer

Reply:

Jennifer

If the evaporator coil is icing over the system is not operating properly and may be low on refrigerant ( a leak someone has not yet found) or may have a failing refrigerant metering device.

Iced over coils can indeed also reduce air flow - but ALL of the air flow would be reduced not just air at some registers.

So I'd also look for open, loose, leaky, disconnected ductwork.

I have no idea what was meant by a "booster" added to the air conditioner, I don't know what "blown to the ground" means, but that sounds a bit like a short circuit and maybe the addition of a start capacitor to a compressor motor.

IF the AC compressor motor is not starting normally it may be damaged, at or near end of life.

 

Question: AC unit not cooling, very hot high pressure refrigerant line

Our AC unit is not cooling, the high pressure line is VERY hot and the low pressure line is not much colder than room temp.

Last year we put in a new evap coil.. any ideas - Rusty 7/31/11

Reply:

Rusty, you need an expert on site to diagnose the problem. There are several problems that can give excessive pressure/temperature on the compressor outlet side of the system (the high side).

Sounds like a blockage, maybe clogged dryer or bad refrigerant metering device, sometimes a clogged condensing coil.

Start at

• THERMOSTATIC EXPANSION VALVE TEV INSTALL, DIAGNOSE, REPAIR

 

Question: no cool air is blowing inside the house but outside compressor unit is running

No cool air blowing inside the house. The fan running outside is blowing cool air not hot/warm air. Not sure if it's a compressor issue. - Orios 5/29/12

Reply:

Orios, if the compressor motor never runs, but the air handler unit is running and the thermostat calls for cooling, the problem is at the compressor / condenser unit. See the diagnostics under "COMPRESSOR & CONDENSING COIL, A/C" (article link at the ARTICLE INDEX the bottom of this article )

Question: 8 year old TempStar Air conditioner troubleshooting - won't cool

We had a service tech out for a inspection check up of our 8 yr old tempstar unit because we cannot seem to get the the house temp to the low 70s even when unit runs for 5+ hrs strait....
the tech said the coil needed cleaning or replaced and our compressor was not starting and that it was just humming & clicking when trying to start and recommended to us 2 options to remedy the problem...new compressor or new system and both were not cheap.

while he was testing the equip outside I was in my office and could hear and feel the vibration of the unit trying to start...

My son-in-law came over and we removed the coil and cleaned it thoroughly and then checked all the wiring on the outside unit compressor and found a green wire disconnected and stuffed into a hole (Hmmmmm!).
we reconnected the wire and fired the unit up and the compressor seemed to start with no problem ( no humming or clicking).

So, if it is 98F outside we cannot get the inside house temp below 79f,

I put a refrigerator thermometer in front of an air duct and left it there for 30+ minutes and then checked it

It read 61F.

I am at a loss of what to do.

Also the blowing air pressure does not seem to be strong. .what to do? - Doug S 6/24/12

Reply:

Doug the green wire is probably a ground that needs to be connected, but I'm doubtful that that will fix a humming compressor -

More likely the compressor start/run capacitor is bad or the compressor motor itself is bad. It sounds as if there may be some risky do-it-yourself work going on - I'd call a trained HVAC tech at this point.

...

 

Thank you to our readers for their generous comments

Thanks, you saved me $4000

Oops 68 degrees, thanks for helping me think this through Saving $4,000. - On 2018-01-03 by Anonymous -

Reply by (mod) -

Glad to assist, let us know what happens as that may help others

...

Continue reading at DIAGNOSTIC DETAILS A/C or HEAT PUMP, or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see AIR CONDITIONER BLOWS WARM AIR FAQs for questions & answers about condensate pump installation, inspection, troubleshooting, & repair and that were posted originally on this page

Or see these

Recommended Articles

• AIR CONDITIONING & HEAT PUMP SYSTEMS - home
• AIR CONDITIONING CHECKLIST FOR LOST COOLING
  • AIR CONDITIONING SYSTEM OR HEAT PUMP IS NOT RUNNING AT ALL - follow steps 1-9 just below
  • AIR CONDITIONER/HEAT PUMP IS RUNNING BUT COOL AIR IS NOT BEING DELIVERED
  • Step 1: CHECK THE OUTSIDE COMPRESSOR-CONDENSER UNIT
  • Step 2: CHECK THE INDOOR AIR HANDLER UNIT
  • Step 3: WEAK AIR FLOW 1: CHECK THE AIR CONDITIONING /HEAT PUMP AIR FILTER(S)
  • Step 4: WEAK AIR FLOW 2: NEXT CHECK THE SYSTEM DUCTWORK & AIR SUPPLY REGISTERS
  • Step 5: NO AIR FLOW AT ALL: CHECK THE INDOOR AIR HANDLER BLOWER FAN ASSEMBLY
  • Step 6: AIR FLOW IS OK BUT AIR IS NOT COOL
  • Step 7: REFRIGERANT PRESSURE & TEMPERATURE CHECKS TO DIAGNOSE LOST COOLING CAPACITY
  • Step 8: AIR CONDITIONER OR HEAT PUMP RUNS BUT MAKES STRANGE NOISES
  • Step 9: CALL YOUR HVAC SERVICE TECHNICIAN
• AIR HANDLER UNIT SWITCHES - find the blower unit power switch
• COMPRESSOR CONDENSER CONTROLS, SWITCHES - find the outdoor compressor/condenser switch
• DIAGNOSTIC GUIDE A/C or HEAT PUMP  - higher level diagnostics
• DIAGNOSTIC DETAILS A/C or HEAT PUMP - detailed diagnostic procedures
• In addition to citations & references found in this article, see the research citations given at the end of the related articles found at our suggested
  
  CONTINUE READING or RECOMMENDED ARTICLES.

Suggested citation for this web page

AIR CONDITIONER BLOWS WARM AIR FAQs at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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INDEX to RELATED ARTICLES: ARTICLE INDEX to AIR CONDITIONING & HEAT PUMPS

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