Arkansas HVACR NewsMagazine March 2025

HVACR NewsMagazine March 2025

Tech News

evaporator coil. Underfeeding the coil would cause the limited amount of liquid refrigerant to boil/evaporate into a fully vapor state too early, thus allowing the vapor to rise above desired levels.

Charging By Superheat

If we have a fixed orifice metering device,

the system is typically charged by

superheat. With proper airflow, you will

compare the actual superheated vapor

measurement to a target superheat

chart.

Because the orifice is fixed or a fixed hole, the amount of superheated vapor will vary greatly with the conditions. In some conditions, it could be below 5, and it could be as high as 40 in others. You will need the return air wet bulb and the outdoor dry bulb temperatures with the chart or the app for this. While we don't set refrigerant charge by superheat in systems with self-adjusting metering devices such as TXVs and EEVs, we should still measure superheated vapor each time. With proper airflow, a TXV will open and close to maintain a constant superheat, so it's often called a constant superheat valve. As long as we have enough subcooled liquid and proper airflow across the evaporator coil, we can use superheat to determine if our TXV is feeding properly. For example, if our TXV system has a notably high superheat, we could assume that the TXV is not allowing the proper amount of liquid refrigerant to enter the When to Check Superheat

The opposite is true for a TXV that is over feeding. It will provide too much liquid refrigerant to the coil, essentially flooding it. Neither of these conditions is desirable and will not only rob the system of efficiency but could possibly damage the equipment. On some models, the TXV can be adjusted to get the desired superheat vapor. Many times, the TXV is misdiagnosed because there was not enough liquid at the TXV due to dirty screens or restrictions. Electronic expansion valves (EEVs) should also maintain superheat for us if we make sure it has enough subcooled liquid and proper airflow across the evaporator coil. An EEV uses an electronic control board and sensors to measure superheat and a stepper motor