My Air Conditioner is Frozen!

You are a responsible homeowner, but you walk outside one day and you have to admit to yourself, “My air conditioner is frozen!”  How can it be? It’s the middle of June and you have an ice buildup. 

Don’t worry. It could be a simple fix and with a little more troubleshooting, we can see if you really do need to call an HVAC service company.

The first thing to do is get to your thermostat and turn the system off. Leave the fan on so air will continue to blow and melt the buildup of ice.


First of all, it is important to understand how this is happening. It’s all due to a principle in thermodynamics called the Joule-Thomson Effect. This principle declares that when a gas expands, its pressure and temperature will decrease. When the gas is compressed, the temperature will increase.

This is the underlying principle upon which an AC unit is created. An HVAC system works when the evaporator coil expands refrigerant making the coil colder. As the blower moves air through the system, the hot air encounters the cold coil and the refrigerant absorbs the heat.

This refrigerant is then sent outside to the condenser where upon it is compressed. The refrigerant then releases the heat to the atmosphere. Cycle after cycle of this action keeps your indoor atmosphere at your acceptable comfort level.

Essentially, your AC unit is freezing because something either chemical, electrical, or mechanical has changed and is inhibiting the Joule-Thomson Effect. Some action within the system is causing the coil to cool down more than is necessary and ice is the result.


There are basically two reasons why you are seeing ice on your AC unit. Something is causing the refrigerant to expand more than is required, thus creating ice on the coils. Or, something is interfering with the transfer of heat from inside the home and into the HVAC system.

An HVAC unit will freeze if the refrigerant levels are low. Low levels of refrigerant cause the refrigerant that is available to expand more than is necessary. This excessive expansion causes a larger decrease in temperature. As the coil continues to drop to a lower temperature than is present in the atmosphere, it causes the available moisture to freeze into ice.

If there is an inadequate volume of air flow across the coils, there will be an insufficient amount of air from which to take the heat. As this sequence of events continue, the temperature will continue to go down causing the AC unit to freeze.

A failure that is electrical or mechanical can cause a breakdown of the thermodynamic systems working within the unit. Any failure in these areas will stop any heat exchange or cooling going on. HVAC systems are classified as closed systems. They have a definite amount of internal pressure that keeps the system working properly. If there is a clogged filter, a refrigerant leak, or a decrease in blower motor/fan cycling, then a unit can freeze.


Here are some things you can do before you make the call.

  • Inspect your air filter and change it if it is wet, dirty, or clogged. A dirty air filter can cause a decrease in air flow and a decrease in air temperature. This decrease can cause the system to freeze up.
  • Take a look at your evaporator coil and see if it needs a good cleaning. Any blockage of air flow will cause the coil to freeze up.
  • Grab a flashlight and inspect your ducts. These areas can get dirty and cause a decrease in air flow. They can also have leaks at junctions. Seal your ducts and keep them clean.
  • Make sure all the vents in your home are not blocked. Again, the restricted or decreased air flow can create a situation that causes the unit to freeze.


After you have done all these things and you still have an ice buildup. It’s time to call the professionals.

If you have refrigernat leaks, duct leaks, incapacitated motors, or electrical/mechanical issues, then your HVAC provider will have the skillset and equipment necessary to inspect and repair these things.