Rick's Free Auto Repair Advice

Evaporator freeze up

Evaporator freeze up — most common causes

Why does the evaporator freeze up in your car?

There are only three causes for evaporator freeze in your car: your cabin air filter is clogged and is restricting airflow, your AC condensate drain is clogged and water is building up around the evaporator coil, OR your AC system is low on refrigerant.

Low refrigerant charge can cause evaporator freeze up.

You might think a low refrigerant charge would have the opposite effect and make the evaporator too warm. But it doesn’t work that way. Let’s review how AC works

1) The compressor sucks refrigerant GAS from the evaporator coil and compresses it into a high pressure gas. When you compress a gas, it heats up.

2) As the heated gas circulates through the condenser mounted in front of your radiator, the airflow cools the high pressure heated gas, causing it to condense into a high pressure LIQUID. It leaves the condenser as a high pressure liquid and moves to the expansion device.

3) The expansion device (expansion valve or orifice tube) meters the flow of high pressure liquid refrigerant into the evaporator coil.

4) Once the refrigerant is in the coil, it begins to boil (the boiling  point of R-134a is -15.34°F) and change from a liquid to a gas. In a properly charged system, the amount of time the refrigerant stays in the evaporator is just long enough to change state from liquid to gas.

If the system is low on refrigerant, the gas, instead of leaving the evaporator quickly, will linger in the evaporator and continue to absorb heat. That drops the surface temperature of the evaporator coils to freezing (32°F or lower). At that point, any humidity in the air begins to freeze on the surface of the evaporator.  If this continues, the entire surface of the evaporator will become solid ice, restricting airflow almost completely. Also, if the condensate drain is clogged, water level will rise in the evaporator case and the extended contact with the evaporator fins will cause the water to freeze.

If the system has a thermostatically controlled expansion valve, the valve will detect the temperature drop to 32°F and stop the flow of refrigerant into the evaporator.  Without refrigerant flow, the evaporator ice will begin melt. Once melted, the expansion valve will again flow refrigerant and the cycle repeats.

Low refrigerant causes evaporator freezing in car

When refrigerant picks up more heat than is required for it to change from a liquid to a gas, the refrigerant become superheated.  Superheated refrigerant takes too much heat away from the evaporator and that’s what causes it to freeze.

Restricted airflow also causes evaporator freezing  in car

If the cabin air filter is clogged to the point where it reduces airflow, the low airflow stays to long in contact with the evaporator and transfer too much of its heat to the refrigerant, again causing the refrigerant to superheat, resulting in a evaporator freezing in car.

A clogged condensate drain can cause evaporator freeze up

This doesn’t happen very often because most drivers notice the sloshing water sound or water leaking onto the floor, before the water level gets high enough to freeze on the evaporator. But if the water can’t drain, it can freeze on the evaporator coils and reduce airflow.

©, 2020 Rick Muscplat

Posted on by Rick Muscoplat


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