Does A Frozen Evaporator Mean You're Low On Refrigerant?

Frozen evaporator coils are a common issue with home air conditioning systems. Your evaporator coils allow your refrigerant to absorb heat energy from the surrounding air and your blower then pushes this cooled air throughout your home. In the process, water condenses from the cooler air, forming droplets on the coils, and dripping through a drainage tube. 

However, your evaporator can become too cold. When this occurs, the condensation will freeze on the coils, creating an insulating layer of ice that can cause your system to lock up and potentially damage your compressor. Airflow restrictions and low refrigerant levels can cause your coils to freeze, but are these the only reasons your coils may be icing up?

The Relationship Between Pressure and Temperature

Your air conditioning refrigerant has high- and low-pressure sides sometimes called the vapor and liquid sides. The liquid side flows through your evaporator coils and the interior of your home, where it absorbs heat and undergoes a phase change into a vapor. Your evaporator coils expect a specific pressure level from the liquid refrigerant to maintain the correct temperature differential.

When the pressure level in the evaporator falls too low, the temperature near the evaporator coil will begin to drop. You may initially notice colder air from your system's supply vents, but your compressor will quickly lock up and shut down to prevent damage. As a result, you will eventually experience warmer, more humid air as the blower pushes air over the icy coils.

Many assume that their system requires a refrigerant recharge when this occurs. Note that split-unit central air conditioning systems are closed loops, so they don't lose refrigerant under normal operating conditions. Any loss of refrigerant indicates a leak in the system. However, low pressure at the evaporator coil doesn't always mean a low refrigerant condition.

How Restrictions Affect Pressure

There are generally two reasons why your evaporator pressure might be too low:

• You don't have enough refrigerant
• The refrigerant can't reach the evaporator

The latter case indicates a restriction somewhere in the system. A restriction in your refrigerant lines will prevent liquid refrigerant from entering the evaporator coils, reducing pressure and creating an icing condition. The thermostatic expansion valve (TXV) is the most likely culprit when dealing with an evaporator pressure issue, although restrictions may exist elsewhere.

If your AC is freezing up, it's best to contact a professional HVAC technician as soon as possible. Low-pressure levels at the evaporator can damage your compressor, and adding more refrigerant to a system with a restriction can worsen the issue. An experienced technician will know how to thoroughly diagnose your system to find and fix the underlying cause of your low evaporator pressure.

Contact a local AC repair service to learn more. 

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