Causes of Window AC Freeze Up and How to Unfreeze?

why does my window ac freeze up

A window air conditioner that is frozen up will not cool as it should. If you continue to run a frozen up window AC, it can cause permanent damage to the unit. Here’s what you need to know about a frozen window AC – its causes and solutions. 

How Can I Know My Window AC is Frozen? 

The part of a window AC that freezes up is the evaporator coil. The evaporator coil is located in the front part of your AC unit – the half of the AC that is inside your room. This radiator-like coil will be right behind the air filter, and is usually completely covered by the filter. If you remove the filter and see ice on all or part of the evaporator, that is a frozen window AC unit. 

Window AC Coil frozen

What Happens When an AC Freezes Up? 

During normal operation, warm room air is drawn into the front of the AC by a fan in the rear of the unit. As this warm air passes across the cold fins of the evaporator, some of the heat from the room is transferred to the refrigerant inside its coils and will be sent into the outside atmosphere through the condenser on the back. At the same time, the air that was cooled by the evaporator is recirculated and sent back into the room. 

Just as moisture forms on a cold beverage container when it sits on a counter, moisture will form on the evaporator fins when a window AC is operating. This moisture is supposed to drip into a pan on the bottom of the unit and run through channels to drip outside your window to the ground below. 

BUT –  under certain conditions, the very cold evaporator will begin to form ice faster than the moisture can drip off. Before you know it, the evaporator will be completely iced over and the unit will not be able to blow cool air back into your room. 

How to Unfreeze a Window AC – Do’s and Don’ts 

When your window AC is frozen up, turn off the unit, unplug it and allow the ice to thaw and drain out of the unit normally. It won’t take long.

Do not use any tools to chip at the ice. Do not apply heat to try to speed things up. You could cause water to drip into your room or even damage your AC. Simply leave the front of the unit open and allow it to thaw slowly. 

What Conditions Can Cause the AC to Freeze Up? 

Here are the top reasons for a frozen window air conditioner and how to solve the problems when possible without having to take it to a pro for repair or buy a new one.

1. Poor Airflow – Dirty Air Filter 

The number one cause of a frozen window AC is an air filter that is dirty. Normal dust and dirt in the air, as well as pet hair, are the main things that clog filters. This will restrict the normal airflow through the AC unit and can result in a frozen evaporator. Clean your air filter regularly to prevent this from happening. Follow the instructions in the user’s manual that came with your unit.

You can quickly check if this is the issue. Take out the filter. Is it really dirty? If it is, turn the AC on to the highest cooling setting, something like Hi Cool. Let it run for a few minutes. Do you hear the compressor kick on? Does it start to cool? Clean that filter, and you should be good to go. 

2. Poor Airflow – Dirty Components 

The air not only needs to flow through the filter, but also through the fins of the evaporator in front and finally through the fins of the condenser in the back. These fins are very small with very little space between them. They can easily become clogged with dust and pet hair so that air cannot flow freely through the entire unit. If the fins are dirty, clean them carefully with a soft rag, a brush or with compressed air. Take caution to not bend or flatten them, as this will reduce their efficiency.

Pro Tip: A small, soft-bristle paint brush (half-inch to inch wide) is a nice tool for cleaning window air conditioner coil fins! 

3. Blower Fan – Broken or Loose

Is the fan making a rattling noise? Squealing? If so, this could be your issue! 

The blower fan is what keeps the air moving through your window AC. If the blades are broken, air will not be moving as it should. If the fan is loose on its shaft, it will not be spinning at the proper speed. A broken fan blade must be replaced. A loose fan may be secured by tightening a small set screw on the collar of the fan. Check the fan for excess dust or dirt as well. 

4. The Outside Temperature Is Too Cool 

Outside air that is too cool will cause the entire system to be operating at a temperature that can quickly result in a frozen evaporator. Do not run your AC when the temperature outdoors is below about 60° F. If you need cooler air in the house at that time, it’s better to open a widow or two than to try to cool it with your window AC. 

5. Refrigerant Leak 

If the coils and air filter in your window air conditioner are clean and in good condition, but the unit still won’t cool, this could be your problem.

The refrigerant in a window AC is inside a closed system and should last the lifetime of the unit. It is never supposed to be “used up” or leak out of the system. If there should be a leak, the unit will not perform the way it is supposed to, and one symptom of this could be freezing up. 

A refrigerant leak is a serious problem and needs to be addressed by a licensed HVAC technician. Many states restrict the handling of refrigerants to those with a special license. If your window AC has a refrigerant leak, your unit is probably at least 10 years old, and the best option might be to consider replacing it with a newer model. Consult your HVAC tech for an estimate of the cost to repair a refrigerant leak. You’ll probably discover that it isn’t worth repairing the unit. 

6. Defective Component 

There are several internal components in a window AC that can fail over time that could cause the unit to keep running, but form ice on the evaporator. Some of these are the temperature sensor, electronic control board, run-start capacitor, blower fan motor and others. These are all conditions that are probably best diagnosed by a licensed HVAC technician.

There are window air conditioner troubleshooting tutorials on YouTube, and they might help; other times, they’ll waste your time when what you need is a pro fix or a new window AC. 

Window ac untis can last for 8-12 years. If your unit is too old, replacing the current one is a better option. New ac units will have a higher efficiency rating which can lower your energy bill over time. 

Keep your Window AC from Freezing Up

With proper care and maintenance, a window AC unit should function properly for 10+ years. Install it and maintain it according to the manufacturer’s instructions. That means cleaning the filter regularly and taking off the cover to clean the coil fins and internal components at least once per season.

If you start using your window air conditioner in spring and it is needed well into fall, then plan the thorough cleaning twice per season.

When you take care of your AC, you should enjoy cool, comfortable air in your room while avoiding the problem of a frozen window AC.

Written by

Rene has worked 10 years in the HVAC field and now is the Senior Comfort Specialist for PICKHVAC. He holds an HVAC associate degree and EPA & R-410A Certifications.

Leave a Comment Protection Status