Understanding Superheat in a car AC System: A Complete Guide

How To Use AC Superheat To Diagnose Your Car’s AC

Superheat is a critical concept in the functioning of a car’s air conditioning (AC) system. It plays a significant role in ensuring that the refrigerant is properly utilized to cool the cabin efficiently. Understanding superheat and how to measure it can provide valuable insights into the health of the AC system and help diagnose potential problems. This article will explore what superheat is, how it works in a car AC system, and how to use superheat readings to diagnose issues effectively.

What is Superheat?

Superheat refers to the amount of heat added to the refrigerant vapor after it has changed from a liquid into a gas, before it exits the evaporator coil.

Superheat is measured in degrees, and it’s crucial because it ensures that the refrigerant leaving the evaporator is entirely in a gaseous state. This is important for the following reasons:

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. The system meters liquid R-134a through a small port and into an evaporator coil to make auto AC work. 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 absorbing about 40° to 60° of heat from the cabin air. When the refrigerant turns to gas, it should exit the evaporator coil and enter the compressor, which will be compressed into a high-pressure gas.

If the AC system is low on refrigerant, you get AC superheat

In normal operation, the evaporator is filled about half-way with liquid refrigerant. As mentioned above, hot cabin air blows across the evaporator, causing the liquid refrigerant to boil and change into a gas. However, if the system is low on charge, there’s less liquid refrigerant in the evaporator coil. That refrigerant will absorb enough heat to change from liquid to gas and then the gas will absorb even more heat, causing the gas to become superheated. That’s referred to as AC superheat.

Measuring your car’s AC Superheat

To measure superheat, you’ll need to obtain two key pieces of data: the low side pressure and the temperature of the refrigerant line just as it exits the evaporator.

• Determine the Low Side Pressure— Connect a set of AC gauges to the low-side service port of the AC system. The low side is typically the larger line and is located between the evaporator and the compressor. With the AC running, note the pressure reading on the gauge.

• Find the Corresponding Saturation Temperature—Use a pressure-temperature (P-T) chart for the specific type of refrigerant in your system (e.g., R-134a, R-1234yf). Match the low-side pressure reading with the corresponding saturation temperature.

• Measure the Refrigerant Line Temperature— Use a digital thermometer or thermocouple to measure the temperature of the refrigerant line right after it leaves the evaporator.

• Calculate the Superheat— Subtract the saturation temperature (from the P-T chart) from the measured line temperature. The result is the AC superheat value.

For example, if the low side pressure is 35 psi, and the corresponding saturation temperature is 40°F, and the measured line temperature is 50°F, the superheat is 10°F (50°F – 40°F = 10°F).

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