Why Is My Air Conditioner Freezing Up? 10 Reasons and The Fix

In the summertime, there’s nothing worse than having frozen AC coils. When this happens, it will seem like no matter how high you set the thermostat, the room is still a sauna, and little to no airflow will come out of your ducts.

AC units usually freeze up because of a clogged filter, dirty coils, a refrigerant leak, an undersized or oversized air conditioner, and more. Some issues are easy to fix, while others can cost thousands of dollars.

If your AC unit keeps freezing up, it can be costly to keep running it in an effort to cool your home. But what’s causing this problem in the first place? And more importantly, how can you fix it? Please continue reading to learn the top 10 causes and their fixes.

a man checking why his AC unit is freezing up

Reasons Why Your AC Unit Keeps Freezing Up

1. Blocked Filter

frozen evaporator coil
Frozen evaporator coils on an AC unit – Source

A dirty or blocked filter is the most common reason an AC unit freezes up. If the filter is clogged, it doesn’t allow air to flow over the evaporator coils. This causes the coils to get too cold, eventually leading to ice buildup.

How to Fix

Check your filter monthly and clean or replace it as needed. Your AC filter can last anywhere from one month to a year. It depends on its thickness, MERV rating, and how much dust, pet hair, pollen, etc., it filters. The health of your HVAC system is highly dependent on filter maintenance.

2. Dirty Coils

If the coils on your AC unit are dirty, they can’t correctly absorb the heat from your home to cool it. And when they are dirty, the water builds up on the dirt instead of the condensation dripping into your drain pan (as it usually would).

Then, once the refrigerant within the coils gets cold enough, the water soaked into the dirty coils freezes, leading to an ice buildup.

How to Fix

Clean the coils with a coil cleaning spray or brush. You can find these at most hardware stores. Be sure to turn off your AC unit before you begin cleaning the coils.

To prevent the coils from freezing in the first place, keep your filter clean and change it as often as needed. You should also have an HVAC professional come out and clean your coils at least once a year as part of routine HVAC maintenance.

3. High Humidity

a person holding a hygrometer

If the air in your home is too humid, it can cause the coils to freeze. This is because air that is already saturated with water vapor can’t hold any more moisture. So, when the coils try to cool the air, the condensation has nowhere to go and ends up building up on the coils.

How to Fix

Use a dehumidifier to reduce the humidity in your home. You can also use fans to help circulate the air and keep the coils from freezing.

4. Bad Blower Motor

If your blower motor is failing, it can cause the coils to freeze. The blower motor is responsible for circulating air over the evaporator coils. If it’s not working properly, the air can’t flow, the coils will get too cold, and too much condensation will build up on the coils, which can cause them to freeze up.

How to Fix

If you suspect your blower motor is failing, you should have an HVAC technician come out and take a look. They might need to repair or replace the blower motor. At the very least, it might just need a re-oiling.

5. Inadequate Airflow

If there isn’t enough airflow over the evaporator coils, they can get too cold and freeze up. Low airflow can have several causes, such as a dirty filter, dirty coils, blocked return air vents, clogged ductwork, or a failing blower motor.

How to Fix

The best way to increase airflow is to ensure that all components that contribute to airflow are clean and unblocked. This includes the filter, coils, ductwork, air vents, and blower motor. You might need to hire an HVAC technician to clean the ductwork if it’s extremely clogged.

To ensure your air conditioning system always has proper airflow, you can use smart airflow monitoring systems, like SmartAC’s monitoring kit.

6. Refrigerant Leak

HVAC technician using a leak detector

If your AC unit leaks refrigerant, it can also cause the coils to freeze. Compressed liquid refrigerant flows through your evaporator coils inside your AC unit. Normally, warm air from your home blows across the cold coils and transfers its heat to the liquid refrigerant. At the same time, moisture from the air condenses onto the coils.

But if there’s a pinhole leak in the coils, the liquid refrigerant will escape and rapidly lose pressure. During this process, the refrigerant rapidly cools (as a byproduct of expansion). And when it comes into contact with the condensed water on the coil, it freezes.

How to Fix

If you suspect a refrigerant leak, you should have an HVAC technician come out and assess it. Refrigerant leaks are serious but common AC problem that only an EPA-certified technician can handle.

7. Clogged and Leaky Ductwork

If your ductwork is clogged or leaky, it can cause your coils to freeze. This is because the air that’s supposed to be flowing over the coils is escaping through the ductwork leaks. Or, the air doesn’t come through at all if it is blocked. As a result, the air around the coils gets too cold and causes them to freeze.

How to Fix

If you have clogged or leaky ductwork, you should hire an HVAC technician to clean it and repair any leaks. This job is best left to professionals because it involves working with potentially dangerous chemicals (like insulation) and equipment (like power tools).

8. Thermostat Issues

If your thermostat is set too low, it can cause your coils to freeze. The thermostat controls the blower motor and the temperature setpoint. If it goes haywire, it can result in the coils becoming too cold or the blower not moving enough air, leading to ice on the coils.

How to Fix

If you think your thermostat might be set too low, you should check the owner’s manual to see the ideal setting. You can also try raising the temperature a few degrees and see if that helps. If you still can’t get the coils to stop freezing, you might need to replace the thermostat.

9. Undersized AC Unit

photo of a window-type AC

If your AC unit is too small for your home, it will have to work overtime to cool your house down. This can cause many problems, one of which is freezing coils. An undersized unit will struggle to remove enough heat from the air and could make the coils get too cold and ice over.

How to Fix

If you have an undersized AC unit, you should replace it with a larger one. It is best to hire an HVAC professional because it involves choosing the right size unit and properly installing it.

Getting a new AC unit is an expensive solution to the problem. However, besides frozen coils, your AC system will never adequately cool you home and run all the time. This can lead to high energy bills and other significant issues with the AC system that can quickly become more expensive than getting a new unit.

10. Oversized AC Unit

If your AC unit is too big for your home, it will turn on and off frequently– known as short cycling. When an air conditioning unit short cycles, it reaches the temperature setpoint too quickly and then shuts off. But when it turns back on again a short time later, the coils won’t have had enough time to defrost, leading to frozen coils.

How to Fix

If you have an oversized AC unit, you should replace it with a smaller one. Just like an undersized unit, purchasing and installing a new AC unit is an expensive solution to the problem. An oversized unit will also not adequately cool your home. Instead, you’ll have some hot spots, cold spots, and high-indoor humidity, which can lead to high energy bills and other significant issues with your home and AC system.

Keep Your AC Running Right

There are many reasons why your AC coils might freeze. Some of these, like dirty coils, a clogged filter, and some causes of inadequate airflow, are easy to fix. Others, like an undersized or oversized AC unit, are more difficult (and expensive) to solve. If you’re having trouble with your coils freezing, the best thing to do is call an HVAC technician and have them take a look. They’ll be able to diagnose the problem and recommend the best course of action.

About The Author

Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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