Window Air Conditioner Freezing Up In Summer? Here’s What Is Wrong

A window air conditioner should not be icing up, especially in the summer. If you see your window air conditioner freezing up in the summer, you do have a problem. Finding ice in an air conditioner window unit is not something that will go away on its own. What is more, running a frozen window AC will lead to permanent damage.

frozen window air conditioner unit
Window AC freezing up in the winter is not the same as window AC freezing up in the summer.

Window AC units don’t freeze all over. The part that is usually frozen is the indoor evaporator coil. This coil is located behind the indoor air filters. If you open the window AC unit from the front, remove the filters, you will notice the ice (it can be brown ice) on the coils.

window air conditioner indoor coils covered by ice
This is window air conditioner icing in the summer. You will notice brown ice forming on the indoor coils.

This is considered as a window air conditioner freezing up and is usually the net result of a specific problem within the unit.

What do you do when you notice your window AC unit freezing up?

Don’t panic. Just shut off the window AC unit immediately. Let the formed ice on the evaporator coils melt completely. Then you have to:

  • Diagnose why AC is freezing up. You will need to figure out the root cause of what is causing the ice formation. This may include dirty coils, dirty filters, problems with the fan and fan motor, and freon leak or wiring issues.
    frozen window air conditioner in the summer due to low coil temperatures
    Here are the basic parts of the window AC unit. This sketch can serve as a good roadmap of what part of window AC may be at fault here; with a special focus on cold indoor evaporator coils.
  • Fix the freezing. When you figure out the reason for ice formation in the window AC unit, you have to fix that problem. Once you do that, the window unit should start operating normally again.

HVAC professionals deal with these situations every day (or at least once a week). They know the ins and out of how window AC units operate, and, based on this understanding, how to diagnose and fix a freezing window AC unit. Most homeowners don’t know any of this.

To help everybody who wants to repair a window AC unit icing up themselves, we will first look at how window AC operates normally. Then we will look at the 6 most common culprits for a window AC unit freezing up. HVAC professionals go one by one eliminating these common culprits, and you should do exactly the same.

Note: We will also mention when it’s better to call a licensed HVAC professional (refrigerant leak, wiring issues).

Let’s look at how window AC normally operates for us to be able to both diagnose and fix a window AC icing up:

How Does Window AC Unit Operate? (+ What Happens When It Starts Freezing Up?)

Here is the normal operating process of all window AC units:

  1. Indoor warm air is sucked into the window AC unit (fan generates this airflow).
  2. This air is passed over cold indoor coils (evaporator coils). During this process, the heat from warm indoor air is transferred to the coils, and the absorbed heat is moved outdoors via the window AC refrigeration cycle.
  3. Now cooled air is recirculated back into the room.
window air conditioner refrigeration cycle
All air conditioners operate based on the refrigeration cycle.

The key thing to understand here is what is happening with the moisture. As we know, all window AC units work as dehumidifiers as well; they reduce humidity levels by accumulating and disposing of the indoor air moisture.

Here is what happens when window AC freezes up:

  1. Indoor cold evaporator coils absorb moisture by condensation. When the warm humid air is sucked into the AC, the coils cool it down as well as remove humidity. The moisture in the air is condensed on the cold coils in much the same way as water can condense on cold windows: condensation on evaporator coils leading to icing up
  2. A) Normal Operation. Condensed water drips from the indoor coils into the drip pan (located at the bottom of a window AC unit). There it is continuously removed by a drain hose or via a drain pump.
  3. B) Abnormal Operation. Condensed water freezes on the indoor coils. In this case, the moisture freezes quickly; that’s why we see ice forming in the window AC, instead of accumulated water dripping to the drip pan.

Now, why would the water freeze on the evaporator coils?

There are only two reasons why this happens, namely:

  1. Indoor coils are too cold. If the coils are too cold, the condensate will freeze.
  2. Poor airflow. Without sufficient airflow over the coils that would support the continuous water condensation and disposal via drip pan, the coils can start icing up.

These two reasons are key to understanding how to unfreeze a window air conditioner. If you see ice formation on the coils in the summer, you have to check for culprits that either make the indoor coils too cool or impede the airflow. That’s pretty much it.

Of course, many different things can cause these two things (that lead to window AC icing up) to happen. If you pinpoint what is wrong with the window AC freezing up, you can fix it (remove the reason why window AC is freezing up) and you will again have a normally-functioning unit.

Here are the 6 most common causes (with solutions) that lead to window AC freezing up:

1. Air Filters Are Dirty (Lower Airflow Leads To Coils Freezing Up)

If you see window AC icing up, the first thing you should check for is dirty filters. This seems like the most obvious and easiest thing to check. Nonetheless, dirty air filters are the #1 reason why window air conditioners freeze up.

Most window AC units use MERV 13 filters. When you are running the AC unit, all kinds of airborne particulates can clog the filters. These include anything from dust, human hair, pet hair, larger smoke particles to mold spores, and even pollen.

If the window AC filters are clogged up enough, you will have a critical restriction of airflow. Less airflow will likely result in window AC internal coils freezing up. If you continue to run a window AC with bad airflow, you will start seeing ice formation on the coils. Most homeowners notice this ice formation only after the window AC unit stops cooling.

What to do if the window AC unit is icing up due to dirty filters?

Solution: Simple. Just take the filters out, check if they are dirty, and, if they are, clean them. You insert the clean filters back into the front of the window AC unit and turn the air conditioner on.

cleaning dirty filters may prevent window ac unit from freezing up
Process of cleaning the indoor air filters.

If the culprit for the window unit freezing up were dirty filters, you will now see that the unit is performing normally. After every hour or so, you can check if the ice is again starting to form on the coils.

If not, you have solved the window AC freezing up. If the freezing continues, the dirty air filters were not the root cause of the problem, and you should check the next culprit:

2. Fan Or Fan Motor Is Defective (Results In Bad Airflow And Icing)

Another more serious issue that can restrict airflow is a bad fan. Window AC fan has only one job:

To generate adequate airflow over the cold coils.

If the fan is not working properly (because it’s loose), you may be limited airflow. This may result in the window AC unit freezing up. A loose fan will generate a rattling or squealing noise; this is how you know something must be wrong with the fan.

loose fan can cause window air conditioner freezing up
A sound fan should not have bent blades. If the blades are bent, the fan might be loose or it doesn’t spin quick enough.

The fan is powered by the fan motor. If something happens to the fan motor and the fan is not moving the air, you will definitely see condensation on the cold coil turning into ice. The tell-tale sign of a defective motor is a lack of airflow. If the window AC is on but it’s not blowing air, you know the fan is not spinning.

Solution: In the case of a loose fan, you can reset the fan. This will involve opening the window AC unit, taking the fan out, and putting it back in. You should make sure that the fan is not loose anymore. If it is loose (one or more of the blades have an unnatural curve), you will have to replace the fan.

In the case of a broken fan motor, you will most likely have to replace the motor. Once a fan motor stops working, it’s very hard to make it work again. Do check if all the connections (wires) are adequate contact; this is one of the few options that may not lead to not needing to replace a window AC fan motor.

After you sort out the fan issues, the window air conditioner should stop freezing up (if the fan or fan motor were the culprits, of course).

3. Dirty Indoor Evaporator Coils (Less Heat Exchange Leading To Icing)

The refrigerant gas is continuously providing coolness to the indoor evaporator coils. In a normal situation, the warm airflow across these coils will continuously warm the indoor coils (coils absorb the heat) and we will get cold airflow.

Now, if the indoor coils are dirty, this heat exchange will be impeded. That means that coils will become colder and colder. Once they get cold enough, however, we will see that the condensation on the coils will start to ice up. In some time, the coils will be covered by ice.

Note: Dirt, hair, even pet fluff, and mold spores can absorb onto the coils. This is how the coils get dirty. Coils have superb heat exchanging properties; accumulated dirt less so.

Solution: Clean the evaporator coils. Cleaning the coils is quite an intensive job and requires quite a lot of tools such as a screwdriver, rags, gloves, AC coils cleaner, bristle brush, household cleaners, and so on.

Once the evaporator coils are clean, they can again exchange the heat effectively. This will prevent the window air conditioner to gather ice on the coils.

4. Freon Leak (Lower Refrigerant Levels)

This is usually the case with older window AC units freezing up. Refrigerant lines should always be intact; you don’t have to recharge the refrigerant. However, if you see window air conditioner icing up, the root cause may be a leak in the refrigerant lines.

With less freon, the ability of the window AC unit to adequately provide cooling output will be impeded. If the refrigerant leak is substantial, it will lead to window AC freezing up.

Note: Alternatively, a broken compressor can show the same symptoms as a refrigerant leak and lead to icing as well. In that case, you have no choice but to replace the compressor or just buy a new unit.

Solution: Fix the leak and recharge the window air conditioners. This is much easier said than done. In most cases, you will have to call a licensed HVAC technician to recharge the refrigerant lines. In very limited cases (and with quite a bit of technical skills), you can recharge a window AC unit yourself using this manual.

In any case, make sure you find the leak and seal it. If you don’t seal the leak, recharging the window AC refrigerant lines will not help in the long run. That’s because the new refrigerant gas will continue to leak from that hole and you will have the window AC icing problem all over again.

When an HVAC technician fixed the freon leak and recharges the window AC, you should see the unit work normally again, without the fear that it will start gathering ice again.

5. Running The Cooling Mode In Low Temperatures (Below 58°F)

If the outdoor temperature is too low, running a window AC unit in cooling mode can result in the unit freezing up. This is quite easy to understand; the input air temperature is already low and you extract additional heat via the refrigerant cycle.

This will cause the indoor coil temperature to fall too low, and the result will be icing.

Note: You can run a window AC unit on the heating mode in low temperatures, of course. For that, you will need one of these window AC units with heat function (combo units).

Solution: The good thing here is that there is nothing wrong with your window unit. You just can’t run it on cooling if the outdoor temperature is below 58°F. Just shut off the window AC unit and you will get rid of the icing problem immediately.

6. Wiring Problems Causing Window AC To Ice Up

In limited cases, you will have to check the wires as well. If by now you haven’t diagnosed the right reason why a window AC unit is freezing up, the only thing you have left to check are the wires.

There are many parts of window AC units that can be inadequately wired, including:

  • Temperature sensor. If there is something wrong with the temperature sensor, the unit can start to excessively cool the indoor coils which lead to icing.
  • Control board wiring.
  • Inadequately wired fan motor. This can lead to restricted airflow.
  • Wrongly wired run-start capacitor.
  • Wrongly wired compressor.

Some of these wiring you can check for yourself. Nonetheless, it’s recommended that you call a licensed professional in this case.

checking wiring if window ac unit is freezing up
A licenced HVAC professional will use amp-meter to correctly wire a window air conditioner.

All in all, if you go through all of these culprits, you will eventually find the reason why your window AC unit is freezing up. Then you can fix it yourself or call a licensed HVAC professional.

We hope you find all of this helpful. If you haven’t found the answer, you can use the comments below and we’ll try to help you out.

Thank you.

2 thoughts on “Window Air Conditioner Freezing Up In Summer? Here’s What Is Wrong”

  1. Window AC units are one of the most important equipment in any home. They provide comfort and cool air during the hot summer months and help keep people warm during the colder winter months. But unfortunately, window AC units can also be one of the sources of wiring problems that can cause them to freeze up.

    Window AC units are wired using either a 3-prong or a 2-prong plug. However, some older homes may only have 3-prong plugs, and some newer homes may only have 2-prong plugs. If your home has a 2-prong plug, you must purchase a conversion kit to use your window AC unit with an electrical outlet.


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