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Picture this: Your window AC has been running fine all day. But next thing you know, you feel a lack of airflow coming out of the unit. So you pop open the front panel to see what’s going on, and find a giant block of ice frozen over the coil!
If you’ve found yourself in this situation, then there are a few things that you can do to try and get your window AC up and running again.
In this article, I’ll let you know what you should do when your window AC is frozen. I’ll also go over what you should check, and what kind of settings you should use on your window AC to help keep it from freezing.
Check airflow through window AC
The number one reason why a window AC gets frozen is because of a lack of airflow. Why does a lack of airflow cause a window AC to freeze? It’s all about temperature.
A window AC creates cold air by blowing your room’s air through an evaporator coil. Under normal conditions, the evaporator coil gets very cold—usually in the ‘40s °F.
If there is reduced airflow through the evaporator coil, there is not enough heat load for the coil to absorb. This results in the evaporator coil subcooling down to much lower temperatures than it normally would. Freezing temperatures.
If your window AC is getting low airflow, there are a few things that you should do:
- Clean the air filter
- Straighten the evaporator coil fins
- Check the fan motor for proper operation
I’ll go over them below.
Clean the air filter
When the air filter gets clogged, a reduction in airflow occurs. That’s why it’s important to clean the air filter regularly—once every month or so.
To clean the air filter in your window AC, follow these steps:
- Unplug the window AC from the power outlet. If you’re going to be working on your window AC, it’s important to remove it from its power source. You don’t want the AC turning on while its filter is removed!
- Open the filter access panel. The front of the window AC has a large plastic grille. Behind that grille is the air filter. Open the filter access panel grille to access the air filter.
- Remove the air filter from the filter slot. The air filter can be removed from the filter slot by bending it to release it from the holding tabs. While removing the air filter, handle it carefully so that dust doesn’t fall off the filter and get all over the place.
- Clean the air filter. The air filter in most window ACs is made of plastic, so you can safely rinse it out with water. If there is stubborn grime build-up, use a soapy water solution to clean it off. If you need to scrub the air filter, use a soft-tipped nylon brush.
- Reinstall the air filter. Allow the air filter to completely dry before reinstalling it. Once the air filter is dry, you’ll also be able to see if there are any dirty spots left over. To reinstall the air filter, align it with the holding tabs and snap it back into place. There is only one way that the air filter can be installed, so make sure you orient it correctly. After the air filter is reinstalled, close the filter access panel and plug in your AC. Turn your AC back on and see if the airflow has increased!
Clean the evaporator coil
The air filter isn’t the only part of the AC that can get dirty. The evaporator coil can also get dirty.
Why does the evaporator coil get dirty? In most cases, if the AC is run without an air filter, the evaporator coil will get dirty in no time at all. Or if the air filter is severely dirty, then some of that dust and grime will transfer over to the evaporator coil.
A dirty evaporator coil doesn’t necessarily indicate an improperly maintained window AC. The air filter isn’t perfect, and some small particles get through it and get trapped on the evaporator coil. This is completely normal.
To clean the evaporator coil, follow these steps:
- Unplug the window AC from the power outlet. The evaporator coil is inside of the window AC, so you want to remove the AC from it’s power source before proceeding.
- Remove the front panel from the window AC. In order to gain access to the entire evaporator coil, you’ll need to remove the front panel from the window AC. On most window ACs, the front panel is held in place by screws on the front or sides of the panel. Remove those screws and take the front panel off. If you’re unsure, check the installation manual of the window AC.
- Clean the evaporator coil. The first thing you should do when you clean the evaporator coil is remove excess dust that’s built up on the coil. If there is a bunch of fluff on the coil, go ahead and remove it by hand. Next, use a vacuum to suck out any dust and debris from the inside of the coil. Finally, if there is stubborn dirt or grime inside the coil’s fins, use a coil cleaning spray to clean the coil. Spray the coil with coil cleaner and give it 10-20 minutes to break down the grime. After waiting, rinse the coil cleaner off the coil using water. A spray bottle filled with tap water works well for this task.
- Reassemble the window AC. After you clean the evaporator coil, the next task is to put the window AC back together. Reinstall the front panel on the window AC and plug the AC back in. Turn your AC back on and see if the airflow has increased!
Straighten the evaporator coil fins
The evaporator coil is the part of the AC that gets cold. The coil consists of a copper pipe surrounded by hundreds of tiny aluminum fins to aid in heat transfer.
For one reason or another, the aluminum fins may become bent or damaged. This usually happens during installation. The fins may also become bent if someone (such as a child) goes up to the AC and deliberately bends them.
If the tiny aluminum fins are bent or damaged, there will be a reduction in airflow through the AC unit. To straighten the evaporator coil fins, follow these steps:
- Unplug in the window AC from the power outlet. You’re going to be opening up the window AC, so remove it from its power source.
- Remove the front panel from the window AC. The entire front panel will need to be removed from the window AC to access the evaporator coil. The front panel is usually held in place by a few screws on the front or the side of the window AC. It depends on the model of window AC, so check the installation manual if you’re unsure.
- Straighten the evaporator coil fins. After the front panel is completely removed, you’ll have full access to the evaporator coil fins. Using a fin comb, gently straighten out the fins. For severely bent fins, it may be helpful to straighten them out using a flathead screwdriver first, then comb afterward.
Check out this video of AC coil fin repair from Jordan U. The video shows condenser coil fin repair, but the fin repair method is identical to an evaporator coil.
Jordan starts the fin repair process by bending a severely damaged portion with a screwdriver. After that, pay special attention to the technique used when Jordan uses his fin comb. He combs very slowly and uses a side-to-side motion to gently bend the fins back into place. After he’s done repairing the fins, they look brand new!
If you’re going to be repairing your evaporator coil fins, I recommend using the fin straightener below:
Check the fan motor
If there are no airflow obstructions in the air filter or evaporator coil, the problem could be the fan motor itself.
If the air filter and evaporator coil are clear, but your window AC’s airflow is still weak, then your fan motor could be dirty or going bad.
To check if the fan motor is dirty, you’ll need to remove the entire AC from the window and open up its case. Take a look at the fan blades to see if there is any dust build-up.
There is also a chance that the fan motor’s blower wheel is damaged, so pay special attention to the fan blades to see if there are any broken parts.
If the fan blades are clean and free of damage, then it could be the fan motor itself that’s having trouble. It’s tough to tell if the motor is bad without specialized tools.
Even then, replacing the fan motor is a difficult task. Most times it’s cheaper to just replace the entire AC rather than replace the fan motor.
Check the thermostat and mode settings
In some cases, the AC thermostat’s mode and operation settings can contribute to the unit freezing up.
For instance, if the fan speed is set too low, then the coil will be more prone to freezing due to low airflow.
However, the thermostat settings alone usually won’t cause a window AC to freeze up. It’s a combination of thermostat settings and another thing—such as an oversized unit or cold outside temperatures—that causes the window AC to freeze.
Check if the window AC is oversized
An oversized AC unit will be more prone to freezing up. What is an oversized AC unit? Basically, it’s an AC unit that’s too big for the space that it’s trying to cool.
For example, if you are using an extra-large 28,000 BTU window AC unit to cool a tiny bedroom, then that AC unit is oversized.
An oversized AC unit provides too much cooling for the space that it’s trying to cool. The main reason why an oversized AC will freeze up is due to humidity.
When an AC runs, it is not only cooling your space but dehumidifying it as well. If the AC doesn’t run for long enough, then it won’t have a chance to sufficiently remove humidity from your space. This is an issue in oversized ACs—they don’t run long enough to dehumidify your room’s air.
That’s why oversized window AC’s are prone to freezing—If it is cooling a humid space, then the coil will freeze due to all the excess moisture in the air.
Cold outside temperatures cause a window AC to freeze
If you’re running your window AC when the temperature outside is below 60°F, then the AC will be more prone to freezing.
Air conditioners are designed to run when the outside temperature is within a certain range. Most window AC models can run at temperatures above around 60°F. Any lower than that and you run the risk of the AC freezing up. Or worse yet, damaging the AC’s compressor.
If you really need cooling when the temperature outside is low, just pop open a window.
Check if the window AC is low on refrigerant
The AC’s compressor is the workhorse of the unit. The compressor is responsible for making the evaporator coil cold. Basically, if the compressor is not working properly, then your window AC’s cooling will be all out of whack.
The compressor’s refrigeration circuit is a hermetically sealed system. This means that the refrigerant is factory-sealed inside of the AC unit. So you should never need to add more refrigerant under normal circumstances.
A malfunctioning compressor may cause the AC to freeze up in some circumstances. For instance, if the compressor’s refrigeration circuit is low on refrigerant, then the evaporator coil may freeze up.
If refrigerant is leaking out of the AC system, then you have a serious problem. Most window ACs are not designed to be worked on by an HVAC technician. So if your window AC has a refrigerant leak, in most cases it will be more cost-effective to replace the entire unit.
Check if there is a defective part in the window AC
There are lots of parts inside that can go bad and cause a window AC to freeze up.
Here are some parts that if damaged, can cause a window AC to freeze up:
- Control board
- Temperature sensor
- Fan motor
- Expansion valve
- Evaporator coil
- Condenser coil
If your window AC has a defective part, it’s almost always worth it to replace the entire unit rather than paying someone to replace an individual part. The labor costs of repairing a window AC are often much higher than replacing the unit altogether.