AC Blowing Hot/Warm Air: What’s Wrong? (6 Common Causes)

“When I turn on my air conditioner, it starts blowing hot air.”

Air conditioner blowing warm air is not all that uncommon AC problem. Unlike most fairly innocent issues, AC blowing hot air is quite alarming.

Essentially, the air conditioner should be blowing cold air. That’s pretty much its main job. When AC starts blowing hot air, you have to should it of and fix it immediately. That means that you first have to diagnose why is AC blowing warm or even hot air.

The air that comes out of AC or indoor air handler is cold because it passes through cooling coils (refrigerant coils). If your home AC is blowing warm it when on cold setting, it basically comes to this HVAC phenomenon:

Cooling coils are warm.

Cooling coils in a running AC are cooled by refrigerant (R22, R410A, etc.) expansions. Refrigerant can only be properly expanded (and undergo the endothermic process that produces cold) if it’s properly compressed by an AC compressor.

main reason why ac is blowing hot air when on cold
Schema of a refrigeration cycle. In most cases, AC is blowing hot air due to problems within this cycle.

The most common causes of AC blowing hot air include a faulty AC compressor and low refrigerant levels.

In this article, we will cover the 6 most common causes of why your AC is blowing hot air instead of cold air. We will also suggest how you can fix AC blowing warm air yourself (DIY), and when it’s time to call an HVAC professional.

The basic principle of every air conditioner is the same. That means these causes cover all types of air conditioners, including:

  • Central air AC blowing hot air.
  • Mini-split air conditioner blowing hot air.
  • Car air conditioner blowing hot air.
  • Window AC blowing hot air.
  • Portable AC blowing hot air.

It doesn’t really matter what kind of AC you have; all of them use freon, air filters, and a compressor.

It makes sense to go over all the things that can cause an indoor air handler to blow hot air. You will get a sense of what might be wrong with your air conditioner.

But first, let’s look at how an AC should work. Understanding that, we will be more capable to determine exactly what’s causing the AC  unit to blow warm air.

How Air Conditioner Should Work (If Cooling Coils Aren’t Hot)

Your AC is blowing cold air only when cooling coils are cold. Period.

In a normal functioning air conditioner, the air is passed over cooling coils. This cools the air and we get a cooling breeze from our air conditioner. If the cooling coils are hot, we won’t get a cool breeze; we’ll get a hot AC airflow output nobody wants.

To understand why AC is blowing hot air, we need to figure out what is making the cooling coils hot.

The combination of refrigerant and compressor is used to cool down the cooling coils. Here is how an AC is normally operating.

  1. AC compressor compresses the refrigerant; turning it from gas to liquid.
  2. This process is exothermic; ie. it releases heat. All that heat is directed outward.
  3. The refrigerant in liquid form goes through the expansion valve and turns into gas.
  4. This process is endothermic; ie. it draws heat from the surrounding. That ‘surrounding’ are the cooling coils.
  5. This cools the indoor coils down.
  6. Air is passed over these cold cooling coils and we enjoy a cooling breeze coming from our AC unit.

air handler is blowing hot air and how to fix it

When AC is blowing hot air, something in this process is going wrong. Either compressor is not compressing properly (fully), the expansion valve is having issued, or there is a general lack of refrigerant.

Let’s look at what exactly is wrong with your hot air blowing AC:

6 Common Causes Of Air Conditioner Blowing Hot Air

There are many reasons why your home AC is blowing warm air. Most commonly the cooling coils are hot due to an issue in the refrigerant cycle (low freon, compressor issue, valve issue). Less frequently the cause may be electronic in nature (thermostat issues, power issued).

We will cover all the issues that may lead to AC blowing hot air one by one. Here is a full list of the most common causes of why AC is blowing warm air:

  1. Low refrigerant levels (most common cause). Heat exchange is impossible without freon; you might have a leak in refrigerant lines.
  2. Loss of compression by of AC compressor.
  3. Clogged air filters (lowered airflow).
  4. Clogged expansion valve (insufficient refrigerant expansion).
  5. Power issue: blown fuse or tripped amp breaker.
  6. Thermostat issue: Wrong setting or inappropriate wiring.

Let’s start with the most frequent cause of AC blowing hot air:

#1 Low Refrigerant Levels Are The Cause Of AC Blowing Warm Air

Air conditioner effectively exchanges heat (turns hot air into cold air) only when it has sufficient levels of refrigerant. Refrigerant compression and subsequent expansion is the basic principle of how AC works.

When AC is blowing warm air, the most common cause are low freon levels. Due to the lack of freon, there the freon expansion (turning it from liquid to gas) is not sufficient. That means the cooling coil will not be adequately cooled; in fact, it might start to become warmer the longer we run the AC.

Why are refrigerant levels too low?

There is only one reason for freon levels being too low; a leak in your refrigerant lines.

Refrigerant lines are a closed system; no freon goes in or out if AC is operating normally. Leaks in refrigerant lines are not all that uncommon. Unfortunately, in many cases, you will need to contact an HVAC professional to fix the leak and refill the refrigerant.

If you have a bit of technical skills, you can even try to do it yourself (it’s not advised to do so, however). You can check out how to rechange a window air conditioner with freon here (window AC serves as a good example).

After the freon levels are restored, you should see your air conditioner operating normally (blowing cold air again).

#2 Faulty Compressor Leading To AC Blowing Hot Air

If your freon levels are OK, the reason for your AC blowing hot air might be a faulty AC compressor.

AC compressor has only 1 job: to compress the freon. Fully compressed freon will undergo full expansion, and will fully cool down the cooling coils.

If the AC compressor doesn’t do its job well and freon is not compressed fully, the freon expansions will also be insufficient. That means that cooling coils might not be properly cooled. In fact, cooling coils might start getting warm and even hot without sufficient freon expansion (as a result of the AC compressor issue).

Which leads to – you’ve guessed it – AC blowing hot air.

How to fix hot air coming from your AC due to a faulty compressor?

Compressors are notoriously difficult to fix. In most cases, it’s much easier (and cheaper) to just replace the faulty compressor. This is also what most HVAC professionals will do.

With smaller window or portable AC units, you can try to replace the compressor yourself. With bigger units (central air, mini-split, heat pumps), it’s better for a professional to do the job. The compressor, if you’re interested, is located in the outdoor unit.

When the AC compressor is replaced/fixed, the freon compressor will go back to normal. If this truly was the issue, you will see that once you fix it, the AC will start blowing cold air again.

#3 Clogged Filters (Not Dirty) Can Cause AC To Start Blowing Warm Air

To function properly, the air conditioner has to maintain adequate airflow. Failing to do so can lead to hot air coming from the air conditioner.

One thing that can restrict airflow severely enough are clogged air filters. If air filters are clogged, the airflow will be reduced drastically, and every part of AC will try to overcompensate for the lack of airflow (because its goal is to achieve a low temperature, as set by the thermostat).

This can lead to a number of problems with AC blowing hot air being one of them.

window air conditioner is blowing warm air causes
The expansion valve is located on the capillary tube that leads from the outdoor unit/part to the indoor unit/park.

Do note, however, that dirty filters usually only lead to AC not blowing cold air. For AC to start blowing warm air (temperature higher than indoor temperature), the airflow really has to be severely restricted. Only when filters are clogged the airflow is restricted enough for the AC to blow warm air result.

Air filters that are dirty and only need to be cleaned are not the cause of warm air coming from your AC unit.

Clogged AC filters are. In this case, the fix is simple: just replace the air filters. With a normal airflow, the AC will start working normally again.

#4 Clogged Expansion Valve Causes Hot Air Coming From AC

The job of the expansion valve is to effectively release through the refrigerant liquid and expand it into gas. When this expansion happens, it sends ‘chills’ throughout the cooling coils. That’s exactly what cools down the cooling coils.

If the expansion valve is not functioning properly, the refrigerant won’t be adequately expanded into gas, and the cooling coils won’t be cooled down. It might happen that the air conditioner will blow warm air while running.

What might be the problem with the expansion valve that leads to the hot air problem?


It doesn’t matter frequently but expansion valves do get clogged from time to time. To fix the problem, you have to unclog them; and that’s easier said than done. You might require some help from an HVAC professional.

After the clog is cleared, the refrigerant cycle should be back to normal and the AC shouldn’t continue to blow warm air.

#5 Power Issues: Blown Fuses And Tripped Amp Breakers

In addition to refrigerant cycle issues, electric issues can also cause your AC to blow warm air.

The first thing to do is quite basic; check all the outlets and cables. That’s usually fine.

One thing we might forget – and may lead to hot coming from your AC – are blown fuses and tripped amp breakers.

Both fuses and amp breakers are used to protect our AC from electrical surges. In case there is anything wrong with them – blown fuse or tripped amp breaker – you might be seeing some weird consequence. Hot air blowing from your AC is one of them.

Do check if your AC fuse was blown or if the amp breaker is tripped. Both are quite easily fixable.

#6 Thermostat Issues: Wrong Setting Or Wiring Problem

Now, this is very basic, but if have your thermostat set at a higher-than-room temperature, your AC will blow warm air. This is usually the first thing we check and re-check. In most cases, the thermostat setting is OK.

The bigger issue is with new thermostats. If your new air conditioner is blowing hot air, there is a chance that the thermostat is not wired properly. The improperly wired thermostat is quite a mess; a mess which may lead to some unusual things like hot air coming from your AC unit.

Note: This is only applicable to new or newly wired air conditioners. If your AC was working normally before, it should work normally now. The chance of a wire slipping from the thermostat socket, for example, is minuscule.

You can use this thermostat wiring guide to check if your thermostat is properly wired (blue wire or B is usually responsible for air conditioning).

Final Note

Hopefully, you can manage to diagnose and, in some cases, even fix the air conditioner blowing hot air problem now.

Here is the final note:

There is a difference between AC blowing hot air and AC not blowing cold air. If the air coming from your AC is not above indoor temperature, you might want to read about why the air conditioner is not blowing cold air here.

If you still need a bit of help, you can use the comments below and we’ll try to help you out, or call an HVAC professional for hands-on assistance.

11 thoughts on “AC Blowing Hot/Warm Air: What’s Wrong? (6 Common Causes)”

  1. Brand new home. When AC comes on it blows hot air for approx 30 seconds before cool air kicks in. Is this a problem? And why?

    • Hello Linda, if this happens when you turn on the AC (1st cycle), it’s completely normal. The refrigerant cycle needs a bit of time to start cooling adequately. If this happens in the following cycles, you might have a problem. Hope this helps.

  2. My Mom’s window GE Model #AEW08LNQ1 air conditioner mfg. 10/10. Yes, its old. Hardly been used tho. It’s blowing hot air. Is it worth having an HVAC person Come out to diagnose it? Thank you.

    Ana Tagliaferro.

    • Hello Ana, that’s about a $250 window AC unit? If it’s old, it’s worth less than $100. A visit from an HVAC person will cost you at least that. So, financially, it doesn’t really make sense. The best thing to do is to try to fix it yourself or just get a new one. Hope this helps.

  3. Hello. I bought a brand new window unit last summer. This summer it starts to blow warm air and then it will turn cool then warm air through the day. It’s not very old it’s kind of frusterating.

    • Hello Julie, if a new window AC unit is blowing warm air, the culprit is most often a bad compressor or a bad capacitor. It’s rather difficult to check for a bad compressor; calling an HVAC guy might be a good but a bit costly idea for a window AC.

  4. So have an outside unit, I will have stat set to 72, and the unit will run and get temp to 79. The next day it will be 81 then 83. Checked the vents and cool air is blowing (not hot or cold). Ive cleaned vents, washed down coils, and replaced filter. Any advice?

    • Hi Scott, it seems like you have an undersized AC unit. Or something is reducing the cooling output. Is this a new unit? If it is, it might be oversized. If it’s an older unit that was once capable of adequately reducing the temp, you should probably call an HVAC guy to check out what’s wrong. Hope this helps.

    • Hi Lyssa, it’s most probably a freon leak of a faulty compressor. For both, you will need an HVAC guy to help you out. Just in case it’s the filters, check if the filters are dirty and replace them; that might solve the issue as well.


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