The last thing you want when running your air conditioner is to feel it blow warm air out of the vents.
Sometimes your AC is blowing warm air because of something simple and other times it may be a sign of a more significant issue.
The most common reason your air conditioner is blowing hot air is that your outdoor unit is not running.
Whether it has no power or a faulty part, without the condenser running the air in the house will feel warm and the temperature in the house will climb.
7 Reasons The AC Blowing Hot Air
- Outdoor Unit Not Running
- Low Refrigerant Charge
- Faulty Thermostat or Wrong Setting
- Heat Pump Reversing Valve Is Faulty
- Clogged or Dirty Air Filter
- Frozen Evaporator Coil
- Ductwork Leaks or Holes
Reason #1: Outdoor Unit Not Running
The outdoor section of your air conditioner is called the condenser.
The condenser is what rejects heat from the house that’s been exchanged by the refrigerant.
If it is not running, it cannot move heat energy from the house to the outdoors. This will allow it to feel like the AC is blowing warm air.
It’s possible you have a bad capacitor or contactor, or you might have a tripped breaker preventing your condenser from running.
Luckily these problems can be fairly easy to identify and repair to get your central air back up and running.
Reason #2: Low Refrigerant Charge
A lack of refrigerant in your A/C will make it feel like it’s blowing warm.
Refrigerant, commonly referred to as Freon, is the medium used to exchange heat from your house and carry it outside where it can be rejected.
All air conditioners require a specific amount of refrigerant to function properly. You can read my full guide on how to recharge a home A/C here.
If this level is too low, it won’t have the capacity to keep the air cool. Most of the time the evaporator coil (indoor coil) will ice up and the air that makes it through cannot be cooled properly, causing it to feel warm.
Air conditioners should never need to be refilled. If your system is low, it has a leak. Leaks have to be repaired to prevent further issues from forming and to keep you from going without AC.
This problem will have to be properly diagnosed by a professional and the leak will have to be found and repaired to prevent it from happening again in the future.
Reason #3: Faulty Thermostat or Wrong Setting
A faulty thermostat setting actually happens pretty frequently. Sometimes buttons get bumped or switches get moved accidentally and your system could actually be set to “heat” or the fan could be set to “on”.
If the fan is running continuously, the air may feel warm when the air conditioner isn’t on, but it’s just circulating.
Or if the furnace is running accidentally, obviously the air from the vents will be hot.
It’s also possible to have a short in the thermostat causing it to run the furnace when the AC is on or if you have a heat pump, it may not energize the reversing valve.
Start by checking the settings on your thermostat and adjust the temperature accordingly.
If you notice the furnace is running when it shouldn’t be, it’s best to shut everything down and call a professional to take a look at things for you.
Or if you’re comfortable enough, try replacing your thermostat and see if it corrects the problem.
Read Also: Why Does My AC Smell Musty?
Reason #4: Heat Pump Reversing Valve Is Faulty
If you heat your home with a heat pump, your outdoor unit runs for both heat and AC.
There’s a valve in your heat pump that reverses the flow of refrigerant depending on what the thermostat is calling for. Sometimes this valve can stick preventing the flow from reversing.
When this happens, the thermostat could be calling for cooling but your system is actually running in heat causing your A/C to blow hot air.
Reversing valves are difficult to replace and should only be repaired by a qualified technician.
Reason #5: Clogged or Dirty Air Filter
Having adequate airflow through your coils and ductwork is essential for optimal central air performance.
When the filter gets dirty and reduces airflow, it lowers the amount of heat that can be exchanged through your coil. I encourage you to read our guide on what happens if you don’t change the air filter here.
This will cause your system to work overtime trying to continue cooling your home.
Eventually your evaporator coil may ice up and the warm air from your home will just be recirculated through your duct.
Even though there’s a big block of ice on your indoor coil, the air will still feel warm coming from your vents.
Shut the unit down and allow it to thaw. Replace the filter and that should take care of the problem for you.
Reason #6: Frozen Evaporator Coil
Whether your coil froze from a lack of refrigerant or from a lack of airflow, the result is the same.
Iced up coils will cause it to feel like your AC is blowing out warm air. Start by checking and replacing your air filter if necessary. You also may want to read my article on how to thaw a frozen unit.
Allow the coil to thaw completely before trying it again.
If it freezes up again it’s most likely from a low refrigerant charge. I recommend calling a professional to find and repair the leak and recharge your system properly.
Reason #7: Ductwork Leaks or Holes
This is more of an issue in attic systems. Ductwork in unconditioned areas has to be sealed properly to prevent heat loss or heat gain.
If you have gaps or holes in your ductwork drawing hot air, your central air can’t make up for that.
Most air conditioners are designed for approximately a 20-degree temperature drop across the evaporator coil.
If the air entering the duct is 110 or higher it will make the AC unit blow hot air from the vents.
Most of the time this type of problem can be easily rectified by sealing up the ductwork with tin tape or duct seal.
RELATED: why is my aC unit freezing up?
Frequently Asked Questions
What do I do if my AC is blowing hot air?
Check your thermostat settings and make sure they are set properly.
Replace your air filter and shut your system down to allow any ice to thaw. Turn it back on and if the problem persists you should call a professional to properly diagnose the problem.
Should I turn off my AC if it’s blowing hot air?
Yes, it’s best to shut your system down if it’s blowing hot air instead of cold air.
Some central air components can be damaged if left on while not functioning properly. Shutting it down will prevent further damage from occurring.
How do I reset my AC?
The best way to reset your AC is to turn power off at the circuit breaker and at the furnace.
Once power is restored, the unit should be reset.
What can cause my AC to blow hot?
There are several reasons that can cause your A/C to blow hot including thermostat issues, low refrigerant charge, dirty filters, frozen coils, stuck reversing valves, and ductwork issues.
Can it be too hot for AC to cool?
Because air conditioners move heat outside and exchange it with the outdoor air, extremely high temperatures can cause your air conditioner to struggle significantly.
Typically temperatures over 100 degrees are when AC units begin to lose their ability to cool effectively.