Why Do A/C Units Freeze?

You have probably experienced the following scenario at least once with your window air conditioner or heating ventilation and air conditioning system:

Woman fanning herself with a colorful fan, appearing overheated, illustrating discomfort from high temperatures related to air conditioning issues. Outside the heat is blistering hot. The temperature inside does not seem much better. The air coming out of your Boston home window unit or HVAC system vents has diminished to a trickle.

Ice has built up on the interior of the unit near the evaporator coil and possibly on the outside of the unit as well.

Ice in Heat

Many Boston homeowners and renters are amazed when ice forms in hot outdoor temperatures, but this scenario is normal whenever the evaporator coil freezes.

The compressor in the unit sends cold refrigerant into the evaporator coil or "cold coil." Hot air fanned over the coil then brings the temperature to above-freezing as the coil absorbs the heat. If this process fails in any way, the coil freezes.

Dirt and Debris

Dirt that builds up on the coil can prevent the hot air from actually touching the coil's surface. A dirty air filter can also block airflow. Low air flow allows higher than normal condensation. The water builds up on the coil, freezes and then the coil and drain line freeze.

Blocked Vents

Any type of inadequate air flow can cause freezing. Whether you have a window A/C or HVAC unit, always make certain that the vents are free of plants and any debris that can block the air intake.

Additionally, if you have an HVAC system, make certain that you haven't accidentally blocked off or closed the majority of the supply registers.

Drain Issue

Condensation that drips off the evaporator coil can sometimes fail to drain out of the unit, build up and freeze. Make certain that the drain pan is empty and that you are not dealing with a blocked drain.

If you are using a window A/C unit, check that you installed it with the front slightly higher than the back so that the condensation water can drain out.

If you have a drain clog that you can't clear or you suspect that something else is causing the problem, it is time to call a professional HVAC technician for assistance.

Other potential serious causes for freezing include low refrigerant, one or more leaks, a failing compressor, a faulty thermostat and a collapsed duct. For more information, call our experienced Atlantic Heating & Air Conditioning technicians in your Boston, MA at (617) 566-6990.