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Why is my AC blowing warm air?

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Last Updated on April 26, 2019


ac blowing warm air

It’s incredibly frustrating when you just can’t get cool on a hot day, even though your air conditioning sounds like it’s working (it’s blowing). Is it you or is it really hot in here? You check the thermostat, and in fact the temperature is soaring well above the set temp. When you place your hand over one of the AC registers, you find your AC blowing warm air.

What’s going on? Is your AC blowing warm air because of a simple issue you can fix yourself, or do you need to call in a professional?

Read on to learn about the most common causes of an AC blowing warm air, and which ones you may be able to tackle yourself.

7 common causes of AC blowing warm air

Your AC is blowing warm air because while the fan is operating, something is shutting down the cooling mechanism. Causes include simple things like thermostat settings and poor air flow, but also could be issues that can spell big trouble, like electrical problems and refrigerant leaks.

We’ll start with the easy issues you can fix, and let you know when you need the skills of a professional to diagnose and repair a larger problem.

1. Thermostat issues

Whether you’re worried about your home or workplace AC blowing warm air, the first thing to check should be the thermostat. It’s a fact: they do get tampered with, by kids accidentally hitting it with a flying object, or a co-worker turning it off because of being too cold.

Make sure the thermostat is set to COOL and the fan set to AUTO. And, of course that the set temperature is actually lower than the air temperature.

2. A clogged filter

Reduced air flow is one of the most common problems that can impede the cooling operation of your air conditioner. Strange though it may seem, your air conditioner actually needs warm air flowing over the coils in order to function properly. Your air conditioner’s filter is designed to remove dust and contaminants from the air flowing through the equipment. However, an air filter that’s clogged with dust and debris can completely block air from entering the system. When that happens, the coils can’t remove heat. So your system’s fan just blows heat back into your space.

If you haven’t had your system maintained in a while and your AC is blowing warm air, the next step is to check your air filter, and change it if needed.

Related article: Why and how to change an air filter

3. Return vents closed or blocked

Here’s another air flow issue that can result in your AC blowing warm air. Is it possible that vent grilles were closed over the winter and never re-opened? Or (this is common in offices!) do you have furniture or other obstructions blocking them?

Take a walk around your space and make sure all registers and returns are open and allowing air to flow freely.

4. Disconnected or leaky ducts

It’s also possible that you have air flow problems that you can’t see… in your ducts. If a duct has holes, cracks, or is completely disconnected, return air may not be making it back to the unit. Or cooled supply air may be getting wasted in your walls, attic or crawl space, and the AC is blowing warm air as a result.

If your ducts are accessible, you might want to take a look. It’s not uncommon for damage to occur during construction projects, or from birds or rodents getting into them. On second thought… this might be an area where you’re better off getting professionals to inspect the condition of your ductwork.

Related article: Poor Air Flow from Your AC? Check for Duct Problems

5. Loss of power to the outdoor unit

Many air conditioning systems have an indoor evaporator/air handler unit and and outdoor condensing unit. (In areas like NYC where there’s not much access to outdoor space, your “outdoor” condensing unit may be in a mechanical room). If the air handler is working fine, the fan will still blow air into your space. But without the condenser unit working properly, there will be no cooling; hence, your AC blowing warm air.

That scenario can happen when your condenser unit loses power, often due to a blown fuse or tripped circuit breaker. If you discover that your outdoor AC unit keeps tripping the circuit breaker, read this to find out what to do next:
Why is my air conditioner tripping the circuit breaker?.

IMPORTANT: Never try to diagnose electrical issues yourself unless you are a trained electrician or HVAC expert. Live voltage is not something you want to experiment with!

There is one thing you can check, however: your emergency shutoff switch. It may have been accidentally shut off, cutting the power to your condenser unit and leading to your AC blowing warm air.

6. Dirty condenser coils

One of the important functions of your condenser unit is releasing heat to the outside of your building. The condenser coil is responsible for that job. Because it’s exposed to the elements, it can get caked with dirt and grime, not to mention leaves and trash. Eventually that impedes the operation of the coils, and your system can’t cool effectively, so the AC is blowing warm air.

7. Refrigerant leak

Refrigerant is the substance that flows through your air conditioning system and (to make a long story short) makes the cooling happen. It’s a closed loop, so you should not need to add refrigerant unless there’s a leak in your refrigerant line or the coils. When that happens, your system loses refrigerant charge and cooling power. Depending on the size of the leak, cooling loss can happen slowly or all at once, but eventually your AC is blowing warm air.

Never allow anyone to add more refrigerant without fixing the leak! If you do the problem will just happen again. Additionally, if your system uses the refrigerant R22 (also known as Freon)… which is likely if it’s older than 10 years or so… that refrigerant is being phased out as per the EPA come January 2020. That means it will get increasingly expensive to fix, and you will need to think about replacement in the near future.

Find out more:

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Regular maintenance prevents most causes of AC blowing warm air

If you did find a small problem you could fix easily, congrats! Now, we bet you want to make sure this doesn’t happen again. Here’s your answer to uninterrupted comfort all summer long: preventative AC maintenance. Check it out: you’ll be glad you did!