Window Air Conditioner Blows Cold Then Warm – What’s Wrong?

You depend on your window air conditioner to cool the air in your room. But you may have noticed that your window air conditioner blows warm air after blowing cold air. If you're wondering why this happens, you're in the right place! We have reached out to the experts, and here's what they have to say.

When the window air conditioner blows cold then warm, it means that the compressor is not working properly. This may be due to the following: 

  • Dirty air filter
  • Frozen evaporator coil
  • Thermostat settings errors
  • Leak in the AC system 

Your air conditioner won't function at its best when it displays any of the issues we have mentioned above. Continue reading as we elaborate on why a window air conditioner blows cold, then warm, and how you can fix this problem.

An old styled air conditioner on the wall, Window Air Conditioner Blows Cold Then Warm—What's Wrong?

Why Window Air Conditioners Blow Warm Air

Window air conditioners are designed to cool a room. However, they may start emitting warm air when something goes wrong. Here are some reasons why your window air conditioners blow warm air:

Outdoor window air conditioning units on an old brick apartment building

Dirty Air Filter

When the filter of your cooling system is dirty, it becomes difficult for the air conditioner to move air through the system. This can make your AC blow warm air.

When trying to figure out why your AC isn't cooling your home as it should, the first thing you need to do is check your air filter to make sure it is clean. If you notice that the existing air filter isn't clean enough, ensure that you change it to know if the problem will be resolved.

Dirty an air filter of a window ac unit

If you have a central heating system, consider installing a water-based heat pump or an electric heat pump to reduce the amount of dust in your home.

Frozen Evaporator Coil

The evaporator coils in your air conditioners extract heat from the air in your home to begin the cooling process. Condensation is a byproduct of this activity. This is why there is a drip pan in an air conditioner and a drain to collect and move moisture out of the system.

Moisture may freeze when accumulated on the evaporator coils. When the coil is frozen, heat transfer is prevented, so there is a higher chance you will feel warm air blowing from your AC system.

To check if your coil is frozen, shut off power to your AC and open the access door to interior system components. This will give you access to evaporator coils. If you discover that you have a frozen coil, the best thing to do is to leave it to thaw. The time it will take for the ice to melt depends on the severity of the freeze; melting may take up to 24 hours. Do not turn on your air conditioner until the coils have melted.

Thermostat Setting Errors

Thermostat setting errors can make a window air conditioner blow warm air. This can happen when the thermostat is set to a temperature that is too high or too low for your home. If you are experiencing this issue, you should check your thermostat settings and make sure they are correct. Make sure the fan setting is in Auto and the Cool setting is selected.

Leak In The AC System 

There are many reasons why window AC blows warm air. Leaks in the AC system can be one of them. AC condensers are designed to blow cool air. When a leak develops in the system, warm air is blown from the unit into the room. The leak is caused by a hole in the system and it usually occurs when there is a crack or hole in the ductwork.

How Do I Make My Window Air Conditioner Cold Again?

Clean Your Air Filter

Professional electrician is cleaning the window air conditioner on the roof top of a house with his high pressure water gun

At times, it just takes a simple fix to bring your air conditioner back to working order. Your AC may be blowing warm air because you have a filter clogged with dust and dirt. A dirty filter decreases airflow and efficiency. You are advised to clean your air filters often.

Remove Dust From Your Evaporator Coil

The evaporator coil of your air conditioner is located at the back of the air filter. Typically, dust and other debris accumulate on the surface. When this happens, there is a chance that your AC won’t cool your home as usual. Before you clean the evaporator coil, you have to remove the air filter to get a better view of the coil. Cleaning your coil often may prevent your unit from blowing warm air.

Clean Your Condenser

Some part of your window AC unit is positioned outside your home, making them prone to various elements like dust, rain, pollen, and other debris. When all these things accumulate on the condenser, your unit may not be able to blow cold air. So, make sure to clean your condenser.

How Do You Know If Your Window Air Conditioner Needs Freon?

Professional ac technician is refiling the Freon into the compressor of a window air conditioner unit

Anytime you start noticing that your air conditioner unit is having issues, you may need to check if it needs freon. Here are some signs that show your AC unit needs freon:

Excess Time Before Cooling Your Home

If you have been noticing that it takes so much time for your AC to cool your home, then it could mean that it needs freon. The work of freon as a refrigerant is that of absorption of heat in the air. 

High Utility Bills

Your AC may need freon if you notice an unusual increase in your electricity bills. If your AC doesn't have enough freon, this means it has to work harder and longer before it can cool your home. The extra energy it needs to carry out this extra work may cause an increase in your electricity bills.

Warm Air Coming Out Of Your Vents 

When your system is running low on freon, it tends to start emitting warm air. You can check the temperature of the air coming out of your vent with a thermometer.

Accumulation of Ice On Your Refrigerant Line

Check your AC to see if ice has started accumulating on the copper line. If you notice any buildup, it could be that your unit needs freon. When the refrigerant in your AC is minimal, the evaporator coil becomes very cold. This may cause the moisture on the coil's line to freeze. You should call a technician to help you with this problem.

Unusual Sounds From Your AC Unit

Refrigerant doesn’t get completely finished like gas. Generally, AC units start lacking refrigerant if there are leaks.  If you notice a hissing or bubbling sound from your AC, it could mean that your refrigerant is escaping.

Can I Add Refrigerant To A Window AC Unit? 

Refrigerant can be added to a window AC unit. Adding refrigerant to your window air conditioner is a quick and easy way to get it back into working order. Adding refrigerant to your window air conditioner is not something you should do by yourself.

Environmental Protection Agency (EPA) strongly regulates the recovery and recharge of AC refrigerants. There is no need to add refrigerant except when you have a leaking refrigerant line. Recovery and recharge of air conditioner refrigerants should be done by only an EPA-licensed technician.

Pros And Cons Of  A Window Air Conditioner 

The following are the benefits and drawbacks of a window air conditioner:


  • Window air conditioners can be easily installed.
  • They are energy efficient.
  • They are less expensive compared to a central air conditioning system.
  • Availability is high.
  • A window air conditioner can be used in any room of a house.


  • Window air conditioners can be noisy and cause noise pollution around your home.
  • They can be easily damaged by strong winds and rain.
  • They can also cause mold and mildew problems due to the moist conditions next to windows.
  • Window air conditioners easily get out of balance.

To Wrap Up

There could be something wrong with your window air conditioner when it starts blowing warm air. If you notice this problem, it's advisable to check your air conditioner to find out the cause or contact a good technician to help you detect and deal with the problem.

If you have enjoyed reading this post, check out these related articles:

Does A Window Air Conditioner Pull Air From Outside?

Does A Window Air Conditioner Have To Be In A Window?

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