Superheat and Subcooling: Formulas Explained
Superheat describes how much refrigerant vapor is warmer than its saturation temperature at the measured pressure. Subcooling describes how much liquid refrigerant is cooler than its saturation temperature. Both use pressure and temperature together.
Formulas and worked examples
- Superheat = vapor-line temperature − vapor saturation temperature. A line at 55°F and saturation temperature of 40°F gives 15°F superheat.
- Subcooling = liquid saturation temperature − liquid-line temperature. Saturation at 105°F and a line at 95°F gives 10°F subcooling.
These examples demonstrate subtraction, not charging targets. Use the correct refrigerant pressure-temperature data and the equipment manufacturer's procedure. The Fieldpiece digital manifold manual explains the relationship between line temperatures, saturation temperatures and the two measurements.
Why the measurement location matters
Record where the pressure and temperature were measured. A temperature taken at one location and pressure from a different part of the circuit may not describe the same refrigerant condition. Select the actual refrigerant on the instrument and follow its instructions for probe contact and pressure connections.
For blends with temperature glide, use the appropriate vapor/dew or liquid/bubble saturation reference specified by the equipment and instrument instructions. Do not substitute an arbitrary temperature from a chart.
Measured value versus target value
Calculating a value does not show whether it is correct for the operating conditions. The target comes from the equipment documentation and applicable charging procedure. Fieldpiece's HVAC Guide manual distinguishes target superheat from measured superheat and describes subcooling checks for TXV/EXV systems.
What to record before drawing a conclusion
- Equipment model, refrigerant and operating mode.
- Indoor and outdoor conditions, and whether operation has stabilized.
- Airflow or coil problems already found.
- Measured pressures, line temperatures, calculated values and the manufacturer's target.
A single value is not enough to justify adding refrigerant. Treat this as an explanation of a technician's measurements, not instructions to open a refrigerant circuit. For the surrounding concepts, see AC gauges, coil cleaning and AC troubleshooting.