What Causes A Window Air Conditioner To Freeze Up And How To Fix It

It can be frustrating for everyone in the household during a hot day when a window air conditioner freezes up and stops functioning. You lose all the "coolness" and are potentially stuck with a hefty repair bill. In this post, we will explore the causes of a frozen air conditioner, the aftermath damages, and what can be done to fix it for good.


How An Window Air Conditioner Works

Before we begin, let’s have an understanding of how a window AC cools the air. It uses the standard refrigeration technology that involves expansion and condensation. When air is pumped in, the evaporator coils absorb the heat by converting the refrigerant liquid to vapor form. Cool air will be released, and the excess heat will be expelled outside by the condenser coils by converting the refrigerant gas back to liquid. This refrigeration cycle will continue until the desired room temperature is reached or the window AC is switched off voluntary or involuntary e.g. frozen.

Why Window Air Conditioners Freeze Up

You can tell if the window air conditioner has frozen when ice is formed outside the unit. You can also check by removing the air filter at the front of the AC unit and checking on the evaporator coil. If there is ice buildup near the radiator-like coil, it is frozen and poses a water damage risk.

There are a couple of reasons why Window AC would freeze up during operation as such:

1. Poor Airflow

An impeded airflow is the most common reason for a frozen window AC. The excess moisture will be absorbed by the coils and become frozen under a cold temperature. The AC will not function until the ice on the coils is completely melted.

Solution: Make sure airflow can circulate freely across the room. Since you cannot reposition the window AC, move any furniture, drape, or object that is inhibiting the window AC ventilation.

2. Dirty Air Filter

Another common cause of poor airflow is a dirty air filter. All window air conditioners come with a pre-filter that traps airborne particles like dust, dirt, hair, and debris from getting through to the coils. Without maintenance, the dirty filter will get clogged over time and blocks air from passing through. The disrupted airflow will impact cooling efficiency, force the AC to work harder, and freeze up.

Solution: Inspect and clean the air filter every 3-4 weeks when the window AC is operational. Turn off the AC and carefully remove the air filter. Gently vacuum away the trapped dust particles to avoid tearing the filter. You can also apply lukewarm water to wash away stain residues. Just make sure the filter is fully air dry before inserting it back into the window AC.

3. Dirty Evaporator And Condenser Coils

While an air filter will do a great job in capturing airborne contaminants, there will be some microscopic particles that get through and adhere to the evaporator and condenser coils. It can be a problem as moisture will form on the evaporator fins when a window AC is working. When the tiny space between fins is clogged with foreign objects, it prevents air from flowing through, and ice will form faster than the moisture can drip off.

Solution: Carefully remove the window AC from the window and move it outdoor. Take out the front panel and metal jacket to access the internal coils. Remove and clean the formed ice on the coils if there is any. Use a microfiber rag and gently wipe away dust particles on the coils. You can also use a soft-bristle brush to remove particles stuck between the fins. Spray the coils with an AC coil cleaner (available in any hardware store). If there are any bent coil fins, you can straighten them with a fin comb. Lastly, let it air dry before popping back the panel and reinstalling the window AC. Repeat this cleaning step every couple of months.

4. Low Refrigerant Level (Leak)

Uncommon, especially in a newer model, refrigerant leakage is a cause of window air conditioner freezing up. You can tell by looking at the back refrigerant bar level, or there is unexplained liquid on the ground. Alternatively, get a thermometer and place it next to the AC to check any discrepancy against the temperature reader. When the refrigerant reaches a low level, it will expand extensively, transiting liquid to gas. The lower than usual temperature causes the coils to freeze up easily, thereby affecting AC cooling efficiency.

Solution: The window AC refrigerant is designed to last for a lifetime and will rarely leak unless it is damaged. Suppose there is a leak; the best solution is to hire a licensed air technician for inspection and the estimated cost to repair. Costly it may be, many states restrict the handling of refrigerants without a special license. The HVAC specialist will refill, repair or replace the refrigerant for good.

5. Wrong Size Air Conditioner

A powerful, oversized air conditioner in a small room can lead to ice buildup as the appliance is constantly switched on and off. It prevents a consistent flow of air circulation that halt damp air from condensing and freezing on the coils. Ultimately, it will cause the AC to stop functioning and break down when the issue continues to arise.

Solution: The best way is to replace the window AC will a smaller unit. However, it is implausible due to the additional cost. Most people will be stuck with the oversized window unit for good. The sensible option is to raise the temperature on the AC unit that prevents the appliances from turning on and off.

6. Low Outdoor Temperature

A common scene during the winter season. Cold outside temperatures below 62 degrees Fahrenheit can cause a window air conditioner to freeze up. This is because the refrigerant might be frozen before it can circulate through the coils.

Solution: Do not run a window AC when the outdoor tempeature falls below 60° F. Instead, turn off the heating system, open windows, or use a portable AC if you need to cool the house.

7. Incorrect Fan Speed

Little do people know that the window air conditioner might freeze up when it runs at a low fan speed and indoor temperature. This is because in a cold environment, there is insufficient airflow to transfer the heat outdoor, resulting in stagnation in the moist air.

Solution: It’s simple. Run at a higher fan speed to ensure a constant airflow to stop ice buildup. If you feel chilliness from the higher fan speed, raise the thermostat a few degrees to counterbalance the temperature. If the loud fan noise is an annoyance, try moving the window AC to another window further behind or switch to a silent ceiling/ wall AC.

8. Clogged Drainage System

A clogged drain hole is a plausible cause of a window air conditioner iced up. Window air conditioners come with a drain hole and catch pan to expel moisture outside the house. When the drainage system is clogged, moisture extracted from the hot air has nowhere to go and freezes up internally under a cold temperature. It does not help when the outdoor humidity level is high and the AC struggles to keep indoor air dry.

Solution: When there is a dripping or water leak spotted underneath the window AC, it is a sign of a congested drain tube. Open the drain port and pump out the excess water. Clean up any residues inside the drain tube that are causing the congestion.

9. Other Broken Components (Fan/ Compressor/ Thermostats)

Suppose non of the above causes and solutions manage to fix the window air conditioner from freezing up; there could be other components that malfunction as such:

  • A bad compressor will not pump the refrigerant properly to cool the coils.
  • An inaccurate thermostat sensor will cause the window AC to shut off even though the coils continue to cool and condense moisture.
  • A broken, loose blower fan or bent shaft will create rattling noise and not spin properly.
  • A damaged electronic board with a reading error and unexpected shutdown.

Solution: Again, this is not a quick fix. You will need a licensed HVAC specialist to diagnose the damages. Often times it would be more economical to replace the entire window AC with a brand new unit than go through the repairs. Keep that in mind.

How To Unfreeze a Window Air Conditioner

When your window AC is iced up, the first thing is to turn off the unit and unplug it from the power source. Open the panel, let the ice thaw slowly, and drain out all the excess water. Do not attempt to use any tools e.g. screwdriver, to chip out the ice. Also, never use a dryer or heating device to speed up the process. You're at risk of damaging the coils and internal components.

Final Thoughts

As long as the window AC is under proper care and maintenance, you will not have to deal with icing issues for years to come. Sit back and enjoy a problematic-free and consistent cooling all year long. On top of that, you will save money on the electric bill and avoidable repair costs. We recommend cleaning the window air conditioner at least once a year. Add a few drops of motor oil on the fan blades motor before you start using it during the fall. Having an air purifier besides the window AC would help to ensure there's always cool air around.

Max Fernandez

A loving father and a dedicated reviewer for airfuji.com with more than 1000 air purifiers under his belt. Max Fernandez is also one of the million patients currently suffering from asthma. Feel free to nudge him if you have any questions.