Understanding evaporator icing and reduced air flow

Learn what causes evaporator icing and why it blocks airflow through your ducts

The evaporator is a crucial component of a car’s air conditioning system. It absorbs heat from the cabin air and moves it to the condenser outside the cabin. However, under certain conditions, the evaporator can freeze the condensation from humid air, causing ice to build up on the evaporator fins to the point where the ice completely stops airflow. At that point, your AC shuts down, and you get little to no airflow from the ducts. I’ll walk you through the conditions that cause evaporator icing and explain what you can do to fix the problem.

A Low refrigerant charge is the #1 cause of evaporator icing and reduced airflow

You might think a low refrigerant charge would have the

This image shows evaporator icing in a home air conditioning system. But the concept is the same in an auto air conditioning system where there is evaporator icing

opposite effect, making the evaporator too warm. But that’s not how refrigeration works. Here’s how a low refrigerant charge causes evaporator icing.

If the system is low on refrigerant, the low charge causes the liquid refrigerant to linger in the evaporator longer than normal and continue to absorb heat.

That drops the surface temperature of the evaporator coils to freezing (32°F or lower). At that temperature, any humidity in the air condenses and immediately freezes.

If this continues, the entire surface of the evaporator will become solid ice, restricting airflow almost completely.

A clogged AC condensate drain can also cause evaporator icing and reduced airflow

If your condensate drain is clogged, the water level inside the heater box can rise to the point where it touches the evaporator coil, where it then freezes.

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

Restricted airflow from a clogged cabin air filter can cause icing

When air can’t flow freely over the evaporator coils, the temperature can drop too low, causing moisture to freeze. This can be due to a clogged cabin air filter or debris in the evaporator case.

A faulty thermostatic expansion valve or evaporator temperature sensor can cause icing

There are three ways to control evaporator temperature: 1) a Thermostatic expansion valve, 2) an Evaporator temperature sensor, or 3) a pressure sensor located on the accumulator.

This is what a remote-sensing thermostatic expansion valve looks like

• A thermostatic expansion valve senses the extremely cold temperature and shuts off the refrigerant flow into the evaporator. To learn more about thermostatic expansion valves and how they work, see this post.

• An evaporator temperature sensor reports the freezing temperature to the HVAC controller which then commands a variable output compressor to move to the idle mode to stop compressing refrigerant.

• A pressure sensor on the accumulator senses low pressure after the evaporator and opens the circuit to the compressor clutch, stopping it from compressing refrigerant.

A fault in any of those devices can result in evaporator icing and reduced airflow.

Understanding how superheat causes evaporator icing

1) The compressor sucks refrigerant GAS from the evaporator and compresses it into a high-pressure gas, and pushes it into the condenser located in front of your car’s radiator. When you compress a gas, it heats up.

2) As the heated gas circulates through the condenser, 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. The evaporator is filled about halfway with liquid refrigerant in a properly operating AC system.

4) Once the refrigerant is in the evaporator, it begins to boil (the boiling point of R-134a is -15.34°F). The blower fan pushes hot cabin air across the evaporator fins, and that hot cabin air causes the liquid refrigerant to change from a liquid to a gas. In a properly charged system, just a portion of the liquid refrigerant changes state from a liquid to a gas.

However, when a system is low on refrigerant, more liquid refrigerant in the evaporator removes heat from the airflow, and the gas removes heat from the cabin air as well. In other words, the refrigerant absorbed enough heat to change from a liquid to a gas, and then the gas absorbed even more heat.

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

Restricted airflow also causes evaporator icing

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 transfers too much of its heat to the refrigerant, again causing the refrigerant to superheat, resulting in an evaporator freezing in the car.

A clogged condensate drain can cause icing

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.

Excessive moisture in the refrigerant can cause ice to form in the expansion valve, causing it to malfunction

If you’ve had a leak in the AC system, ambient air can enter, and that air may contain moisture. Moisture travels through the system with the refrigerant. If the expansion valve is near the freezing point, the moisture can freeze inside the valve, causing it to stick.

Is evaporation icing harmful to your AC system?

Evaporator icing reduces cooling performance and can damage the compressor and even cause complete system failure if left unchecked.

©, 2020 Rick Muscplat