What Causes Air Conditioners to Freeze?

Frozen Air ConditionerYou may be in disbelief if your air conditioning unit looks like it is the middle of winter while it is 85 degrees outside. But, one of the most common questions we get asked is what causes air conditioners to freeze? The simple answer is usually a lack of proper maintenance on the system. We’ll explain in more detail in the rest of this article.

Technical Reason Your Air Conditioner Froze

The science behind why your air conditioner is full of ice has to do with the way your air conditioner cools. To work properly, a refrigerant liquid (often referred to by the brand name Freon) cools a part of the air conditioner called the evaporator coil. That coil provide the cold air for your home. The actual process is a bit more complicated, but you can learn about right here on our website.

How Your AC Unit Works

Learn How Your Air Conditioner Works

When the system isn’t fully operational, it can cause your coils to become too cold and bam, the whole system ices up due to the Joule-Thomspon effect. Some really smart people have put together a long wikipedia page on this, if you want to really get into the specifics.

What’s Wrong with My AC Unit?

The two reasons your AC unit may be freezing up are that


  1. Your refrigerant is expanding more than it is supposed to (and thus getting colder than it is supposed to).
  2. There is an airflow problem so hot air is not warming the coil back up and keeping it at proper temperature.

The trick to fixing a frozen air conditioner is finding what is causing either low pressure or airflow problems. This can take a bit of troubleshooting.

Ice Caused by Refrigerant Pressure Problems

Frozen Air Conditioner unit

Ice on ductless mini-split A/C unit.

Most of the pressure problems that can cause your A/C to become a frozen block of ice are due to damage to the coolant system.

  • Coolant Leaks. Too little coolant can allow extra expansion…and a below zero evaporator coil.
  • Kinks in coolant lines that cause a pressure change
  • Clogged Filter Dryer

On occasion there is a non-mechanical problem. Running your Air Conditioner when it is below 62 degrees outside can create an out of balance system and eventually ice. This is most likely to happen when in the spring or fall when it has been pretty warm outside during the day. Especially with modern insulation, your house will trap a bunch of hot air inside and may turn on your A/C even though the temperature dropped rapidly outside after dark.

Frozen A/C Units Due to Airflow Issues

Dust Clogged Air Filter

Dust-Clogged Air Filter

Problems with airflow commonly causes air conditioners to freeze up.

Occasionally, lack of airflow is caused by a broken part, such as a blower fan failure.

Most of the time, airflow problems are caused by lack of maintenance.

  1. Dirty Air Filters blocking air flow to your system. Please change your air filters regularly and pick wisely.
  2. Dirty Evaporator Coils block airflow. We check for this during annual maintenance visits.
  3. Sometimes the ducts can be excessively dirty due to previous problems in the home.

Avoiding Iced Up Air Conditioners with Maintenance

Many problems with A/C units icing up can be avoided by changing air filters regularly and having an annual maintenance checkup. During that visit the technician will check for potential mechanical problems and coolant issues that could lead to an A/C freezing up. He will also check the evaporator coil to determine if it needs cleaning.

Learn More About Planned Maintenance

Need Repairs?

If you are in our service area (Charlotte, NC and Rock Hill/Fort Mill, SC) and are experiencing a frozen air conditioner, please contact us to schedule a service call.

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