What Superheat is and How it Affects Your Car’s AC System

Learn what superheat is and how to use it to diagnose your car’s AC

Superheat is the temperature difference between the refrigerant vapor at the evaporator outlet and its saturation temperature (boiling point) at that pressure. Superheat in a car AC system isn’t a good thing. It’s an indication that the AC system is low on refrigerant. This article explains how to use superheat to diagnose your car’s AC system.

Understanding the refrigeration process

AC systems work by metering high-pressure liquid refrigerant into the evaporator located behind your dash. The refrigerant enters the evaporator as a liquid. As air passes across the evaporator fins, the air heats the refrigerant to its boiling point (R-134a, has a boiling point of -15.4°F), when it changes from a liquid to a gas.

If the system is charged with the correct amount of refrigerant, it will absorb only enough heat to cause it to change from liquid to gas. By the time the refrigerant changes to a gas, it should be exiting the evaporator and carrying a small amount of liquid refrigerant with it. Carrying some liquid refrigerant along with the gas is critical to the safe operation of your car’s AC system because the liquid also carries the oil that’s needed to prevent compressor failure.

The refrigerant gas, liquid, and oil enter the accumulator, where the liquid portion picks up heat and it tool changes to a gas.

Superheat occurs when the system is low on charge

When the system is low on refrigerant, the refrigerant changes to a gas and stays in the evaporator longer than it should. While lingering, it picks up even more heat from the cabin air blowing across the evaporator fins. Remember, the system is designed so the refrigerant removes only enough heat from the cabin air to change from a liquid to a gas.

Because it’s removing more heat than normal, the evaporator temperature drops to 32°F or lower. At that point, any humidity in the airflow will freeze on the evaporator fins. The ice will continue to build up to the point where airflow is blocked almost completely. At the same time, the low-side refrigerant pressure will drop to the point where the low-pressure cutoff switch turns off the compressor clutch. Or, on some AC system designs, an evaporator temperature sensor will shut down the system.

Superheat symptoms, low AC charge symptoms

Your AC blows cold. Then blows warm

The system starts out blowing cold air. However, as ice builds up, it blocks air flow across the evaporator. Low-side pressure drops and the low-pressure switch shuts off the AC compressor clutch. At that point, no more cooling is taking place. Cabin air blowing across the evaporator melts the ice. You notice warmer, very humid air as the ice melts. Once the ice is fully melted, you’ll feel only warm air until the low-pressure side builds pressure and the low-pressure switch re-engages the compressor clutch.

AC compressor clutch cycles on and off

The AC compressor clutch turns on and builds pressure until superheat occurs. Then, the low-pressure switch shuts off the compressor clutch. System pressures rise. The low-pressure switch engages the clutch, and the cycle repeats.

You notice a large puddle of water under your car after stopping

All AC systems drain condensation onto the pavement. But when evaporator icing is occurring, you’ll see very large puddles of water under your vehicle after you park.

©, 2018 rick Muscoplat