This article gives simple first step diagnostics if your air conditioner or heat pump (in cooling mode) seems to run normally but the air it blows out is warm when it should be cool.

These are steps that can be taken by a building owner or occupant before calling an HVAC repair technician.

Page top photo: an indoor air handler or "blower unit" in a building attic, supported on 2x4's set into a condensate overflow drip pan. Notice that the cover is off?

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

Fix an A/C or Heat Pump Unit that Blows Warm Air when in Cooling Mode

First let's be sure we've identified the cooling problem correctly:

1. If the air coming out of your air conditioning system supply registers has good flow but the output air temperature is not cool enough, continue below
   
   at AIR CONDITIONER's SUPPLY AIR IS NOT COOL ENOUGH - your are on the right page - below on this page we'll give a couple of easy things that you should check before calling your air conditioning service technician.
2. If the air flow coming out of your air supply registers is weak or none, a different and more appropriate diagnostic procedure to follow begins
   
   at Step 2: CHECK THE INDOOR AIR HANDLER UNIT and continues at
   
   at Step 3: WEAK AIR FLOW 1: CHECK THE AIR CONDITIONING /HEAT PUMP AIR FILTER(S)
3. If you have a different problem with your air conditioner go to our starting point
   
   at DIAGNOSTIC GUIDE A/C or HEAT PUMP 

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Air Conditioner Air Flow is OK but Supply Air is Warm not Cool

While the most common cause of warm air out of an air conditioner is loss of refrigerant, the problem could be as simple as an improper setting on the controls for your cooling system.

Step 1: check the following:

• Your room thermostat is calling for cooling - it's in COOL mode, not OFF and not HEAT
• At the thermostat, the temperature is set to below the actual room temperature, and that
• You can feel air coming out of the supply air registers (so we know the air handler blower fan is working)

  Step 2: check for common causes of warm air coming out of the system supply duct registers, in order of probability.

  If the air conditioner or heat pump runs but in cooling mode the air supplied is too warm, there are several things that will cause warm air discharge from the supply registers of an air conditioning system or from a heat pump system in cooling mode including the following:

  • The compressor/condenser is not running at all.
    
    First check that electricity is turned on to your outdoor compressor/condenser unit. (A pair of these is shown in our photo above).
    
    Remember that there will be at least two power switches: the circuit breaker or fuse in your electrical panel (for the circuit powering the outdoor unit) and a service switch right at or close to the outdoor compressor unit.
    
    If you are not sure where these switches are to be found, then see
    
    ELECTRIC PANEL INSPECTION - find your electrical panel and check the breaker or fuse powering the compressor/condenser, then check the outdoor
    
    COMPRESSOR DISCONNECT SWITCH.
    
    Once we know the unit has electrical power and if you've observed that the outdoor unit is not running at all, and if we're sure that indoors the blower door and condensate overflow switch are not keeping the system OFF
    
    then see COMPRESSOR / CONDENSER DIAGNOSTICS
  • The compressor runs but refrigerant has been lost due to a leak
    
    see REFRIGERANT GASES & PIPING - home
    
    and GAUGE, REFRIGERATION PRESSURE TEST
  • The compressor runs, refrigerant charge is OK but the refrigerant metering device or TEV is jammed
    
    see Step 7: REFRIGERANT PRESSURE & TEMPERATURE CHECKS TO DIAGNOSE LOST COOLING CAPACITY below.

  If both indoor air handler and outdoor compressor are running and there is good air flow but the air is not cool, you need help from a service technician.

  S/he will check for problems such as lost refrigerant or a stuck refrigerant metering control (thermostatic expansion valve or TEV)

  GAUGE, REFRIGERATION PRESSURE TEST - how to check refrigerant levels

  THERMOSTATIC EXPANSION VALVE TEV INSTALL, DIAGNOSE, REPAIR - check for a stuck TEV that prevents cooling or heating

   

Schematic above showing a heat pump in "summer mode" or cooling mode is provided courtesy of Carson Dunlop Associates, a Toronto home inspection and education company.

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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Air conditioner starts but no longer blows cold air

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

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

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

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. - On 2019-07-28 by 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

Keep me posted, Craig.

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

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: 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

(May 6, 2014) steve nelson said:

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.

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?

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

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.Thanks

(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.

 

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.

On 2011-06-01 by Eric - air blows out but it's not cool

Hey , everything is running and air is blowing out all vents but no cool air, no coil icing, no water dripping in pan, and have a clean air filter. Do i just need a recharge of coolant?

Moderator reply:

Eric:

If air is blowing out at a good rate but is not cool, see my comments to Frank just above.

 

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

Or see this

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

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Citations & References

In addition to any citations in the article above, a full list is available on request.

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