Rick's Free Auto Repair Advice

Superheat AC

What is Superheat?

At first, superheat can be a very confusing concept. But when you understand how AC actually works, it makes a lot of sense. In fact, once you understand what superheat is, you can use it to diagnose any AC system, home or auto.

How AC works

Refrigerant is a chemical that has an extremely low boiling point. For automotive use, many car makers use R-134a refrigerant which boils at -15.34°F. To make auto AC work, the system meters liquid R034a through a small port and into an evaporator coil. It blows cabin air across the coil, which causes the refrigerant to boil and change from a liquid to a gas. In the process of boiling, the refrigerant is actually absorbing about 40° to 60° of heat from the cabin air. By the time the refrigerant turns to gas, it should be exiting the evaporator coil and begin entering the compressor where it will be compressed into a high-pressure gas.

If the AC system is low on refrigerant you get superheat

In normal operation, the evaporator is right around half filled with liquid refrigerant. However, if the system is low on charge, there will be less liquid refrigerant in the evaporator coil. The refrigerant in the coil will absorb enough to heat to change from liquid to gas and then continue to absorb even more heat, causing the gas to become superheated.

In this situation, the evaporator coil has removed too much heat from the cabin air which causes any moisture on the evaporator fins to freeze and form ice buildup. Eventually, the ice blocks the airflow. The clutch cycling pressure switch or the expansion valve should notice the drop in pressure exiting the evaporator or the drop in temperature of the expansion valve’s bulb and cut off refrigerant flow to the evaporator. That will stop the cooling and allow the ice to melt.

Measuring superheat can tell you what’s wrong with your AC system. Click here to learn about diagnosing an AC system.

Posted on by Rick Muscoplat


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