Evaporator Freeze

What causes the evaporator on an air conditioning system to freeze

Scotty McGregor
HVAC Oracle
2005-07-07

There are many ways to freeze an evaporator coil. Listed below are the most common causes. The problem can also be a combination of them.

Clip on Freeze stat to prevent AC evaporator from freezing over

SPDT switch can be used to switch from high speed to low fan operation.

Variable speed control for AC Condenser fan

Frozen AC Evaporator-Low air flow from Dirty Filters

Reduced air flow is the most likely cause for coil freezing. Check the air filter and replace if necessary. The filter should be located before the fan. On residential equipment it will be between the return air duct work and the furnace or located in the fan compartment. If no filter is installed then the fan and the evaporator coil will plug dirt. The equipment will have to be cleaned and then proper filters installed.

Frozen AC Evaporator-Low air flow from a loose belt or worn pulley.

On a belt driven fan, check the pulley and belt for proper tension. A loose fan belt will slip and slow the the fan as well as wear the pulley groove. A slower fan speed reduces the air volume and can casue the coil to freeze.

Frozen Evaporator-Under size ductwork.

The AC requires a minimum of 400 CFM per ton of refrigeration, less then this will reduce the evaporator temperature and can cause the unit to freeze. Poor duct design or restricted supply and return air grills reduce airflow. Remove restrictions where ever possible to increase air flow. If you cannot correct the air flow problem a freeze stat can be clipped onto the evaporator and wired in series with the compressor (Y1 Terminal)to shut off the AC when it start to freeze over.

Frozen Evaporator-Low refrigerant charge

The pressure of the evaporator determines what the temperature of the evaporator will be. When properly charged the evaporator should be 36F-38F. If the pressure drops slightly, the temperature will drop. When the coil reaches 32F it will begin to freeze. As the unit looses even more gas the evaporator will start to rise in temperature and the cooling will eventually stop when the low pressure control opens or the compressor overheats.

Frozen Evaporator- Low outdoor temperature

Air conditioning systems are design to work when the outdoor temperature is above 65F. When the outdoor temperature falls below 65F the system can have trouble developing enough pressure to push the refrigerant through the evaporator, causing the coil temperature to drop below freezing.
On building with high internal heat loads and or large amounts of the exposed southern glass require cooling even when it's cools outside. If you need to run the compressor when it is cool outside there are steps you can take to prevent freezing.
Use a Condenser fan speed control to slow the fan as the temperature of the condenser drops. This will try and maintain the condenser at 80F and provide a steady flow of liquid refrigerant to the evaporator. Another option is to use a two speed condenser fan motor connected to a outdoor thremostat that will switch from high speed fan to low speed fan as the temperature drops.