Service Department

Reduced air flow causes the evaporator coil to freeze.

Scotty McGregor
HVACOracle.com
2005-08-08

Evaporate Coil Freezing- caused by a dirty fan blade.

Plugged fan wheel

If the air filters in your furnace are not properly maintained, dirt will collect on the fan blades and evaporator coil. On this fan you can see the dirt built up on the leading edge of the blades and the curved blades fill with dirt. When the leading edges of the blades will become rounded they fail to scoop up the air. Sharply reducing the amount of air that the fan moves. As the airflow decreases, the coil temperature will drop. Falling from a normal 45%F ( 7 %C) to 32%F ( 0%C) moisture in the air begins to freeze to the coil, blocking the coil off. As the coil freezes, it will restrict the airflow reducing the airflow further. The coil eventually turns into a solid piece of ice and blocks the airflow completely.

If this happens, turn the AC off at the thermostat by raising the thermostat or setting the mode switch to off. Set the fan to the on position to melt off the ice. Watch for water spilling over and on to the floor or furnace. Turn the fan off if you see water, this will slow the melt process and hopefully more will go down the evaporator drain.

The fan and evaporator will have to be to correct the problem. If the evaporator coil is not too plugged up you can use a duct cleaning company for this. They have the large vacuum trucks and compressed air systems and can clean the fan wheel, ductwork, and evaporator coil. You can also contact a local air-conditioning company the can remove the coil and fan for cleaning. Cleaning will greatly improve the performance of your air conditioner and reduce cooling cost.

If you are a DIY'ers you can try to clean the fan with a brush and vacuum. To clean the evaporator you will need to access the air inlet side that is normally between the furnace and the evaporator. Do not try to remove the evaporator. Even moving the evaporator a little while the refrigerant pipes are connected can cause leaks. If required the fan is fairly easy to removed for cleaning. Normally a few screws is all that holds the fan in place. The wiring on some fan motors simply plugs into the motor or to a wiring harness. Most fan motors are hard wired directly into the controls. Any wiring that is disconnected will have to be carefully labelled so it can be properly reconnected. When the fan is removed you can take it outside for cleaning.

When cleaning make sure water or dirt do not fall inside the fan motor vents while cleaning.

If the fan is dirty then most likely the evaporator coil is too. The most coils are located inside the supply air plenum on the furnace. Getting access to the coil inlet can be tricky. If the evaporator coil was installed after the furnace plenum was made or a separate cased coil was used, then there should be a removable patch or cover on the side of the plenum. It may be possible to cut an access to the evaporator coil however you will need room to the air inlet for cleaning. An extreme care is needed when cutting the access to not puncture the coil. If the coil cannot safely access then it will have to be removed for cleaning. This requires the refrigerant to be pumped down into the condenser or reclaimed. After cleaning the refrigerant piping will need to be evacuated to 500 microns and a new filter drier installed after the coil is reinstalled.

After the coil and fan are clean, install a new clean air filter, which should be located on the return air before the fan and evaporator. The airflow should return to normal and the coil freezing should stop. If the coil freezing continues then there are additional problems. Such as undersized ductwork, blocked return air, over sized air conditioner, refrigerant piping and others