Superheat & Subcooling Calculator — HVAC Charging
Calculate superheat and subcooling from measured temperatures and pressures to charge and diagnose air conditioning and heat pump systems correctly.
- Identify the metering device Fixed-orifice (piston) systems charge by superheat; TXV/EEV systems charge by subcooling.
- Measure the suction side for superheat Subtract the saturation temperature (from suction pressure) from the measured suction-line temperature.
- Measure the liquid side for subcooling Subtract the measured liquid-line temperature from the saturation temperature (from liquid pressure).
- Compare to target Match superheat to the manufacturer’s target-superheat chart, or subcooling to the nameplate value (commonly 8–12°F).
What should superheat be on a fixed-orifice system?
Target superheat on a fixed-orifice (piston) system depends on indoor wet-bulb and outdoor dry-bulb temperature — you read it from the manufacturer’s target-superheat chart rather than a single number. In mild conditions it can be 20°F or more; in hot, humid conditions it can drop near 5–10°F. Charge to the chart value at the current conditions.
What is a good subcooling number?
For a TXV or EEV system you charge to the subcooling printed on the nameplate, which is commonly 8–12°F. Low subcooling usually means undercharge; high subcooling means overcharge or a restriction. Always use the nameplate target — it varies by equipment — and let the system stabilize for about 15 minutes before trusting the reading.
Do I charge by superheat or subcooling?
Charge by superheat on fixed-orifice (piston) systems and by subcooling on TXV/EEV systems. The metering device decides the method: a TXV actively controls superheat at the evaporator, so superheat will not track charge — subcooling at the condenser does. On a piston system there is no active control, so superheat is the reliable charging target.