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FREEZING-UP– a condition where the indoor coil and sometimes the connecting lines, become covered with ice. This is generally accompanied by a loss of airflow from the registers. There are two basic reasons for an air conditioner to “Freeze-Up”:

Low refrigerant – When an air conditioning system has a leak and loses some of its refrigerant, it may begin slowly freezing at the inlet of the indoor coil. In mild weather conditions, a “low-charge” unit may go un-noticed since runtimes are relatively low. However, when the outdoor temperature gets high and the air conditioner has to run longer to cool the space, the ice build-up can cover the entire coil and spread all the way to the outdoor unit.

Low airflow – The outdoor unit of your air conditioning system pumps a fixed amount of cooling to the indoor coil. This cooling must be offset with an equal amount of heat absorbed from the air in your home. This heat is absorbed as the indoor blower moves air across the indoor coil. If there is insufficient air delivered across the coil (more cooling than heating), ice will form on the indoor coil and can spread all the way to the outdoor unit. Low airflow can be caused by a dirty air filter, a dirty indoor coil, closed or blocked air registers, a bad blower motor, or improperly-sized ductwork.

View Air Conditioning Performance Test PDF

Both conditions have the same final result, so it may be difficult to determine the cause. However, there are some telling signs.

An air conditioner with the correct amount of refrigerant and airflow will drop the temperature of the air 15 – 18 deg. F. as it moves across the indoor coil.

An air conditioner with low airflow will experience decreased air velocity from the registers and the temperature drop will be more than normal (colder air). If you experience these symptoms, check the system for causes of low airflow – clogged air filter, blocked or closed registers, loose blower belt, etc.

When a freeze-up is caused by low refrigerant, it is usually accompanied by normal air velocity from the registers but a warmer air temperature. A lower than normal temperature drop (warmer air) may indicate low refrigerant charge. A qualified service technician is recommended to check or add refrigerant.


In either case, when the indoor coil becomes frozen, the air conditioner must be turned off in order to thaw. Turn the thermostat “System” switch to the “Off” position. You may want to turn the “Fan” switch to the “On” position. This will continue to provide warm air to the frozen coil and speed the thawing process. Watch the Video.

* Be aware that as a frozen coil thaws, there may be an excessive amount of water that will not necessarily go out the condensate drain. Take necessary water precautions – especially with a unit in an attic.

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