Superheat & Subcooling Reference
What superheat and subcooling are, which charging method goes with which metering device, and the typical target values. Both are found by reading a pressure into the P-T chart to get a saturation temperature, then comparing it with the measured line temperature. The final target always comes from the equipment’s charging chart or data plate.
The two measurements
Superheat = suction line temperature − saturation temperature (from the P-T chart at the suction pressure). It is how far the vapor is above its boiling point, measured on the suction line.
Subcooling = saturation (condensing) temperature (from the P-T chart at the head pressure) − liquid line temperature. It is how far the liquid is below its condensing point, measured on the liquid line.
Charging method by metering device
| Metering device | Charging method | Typical target |
|---|---|---|
| TXV / TEV | Subcooling | Subcooling 8–15°F (10°F common); the valve holds evaporator superheat itself |
| Fixed orifice / piston | Superheat | Target superheat from the indoor wet-bulb + outdoor dry-bulb charging chart |
Typical target ranges
| Measurement | Typical range | Note |
|---|---|---|
| Subcooling (TXV system) | 8–15°F | 10°F is the common rule-of-thumb default |
| Evaporator superheat | 8–15°F | Fixed-orifice ~8–12°F; TXV-controlled ~10–15°F |
| Total (compressor) superheat | 10–30°F | Comfort A/C ~10–20°F; commercial refrigeration ~20–30°F |
Why the metering device decides the method
A TXV actively modulates to hold evaporator superheat roughly constant, so on a TXV system superheat is whatever the valve is holding — it tells you nothing about charge. What does vary with charge on a TXV system is the amount of liquid backed up in the condenser, which shows as subcooling. A fixed orifice is the opposite: it cannot modulate, so as you add charge the evaporator floods more and superheat drops — making superheat the sensitive indicator there. Match the method to the device and the number means something; mismatch it and you chase a value the system controls on its own.
Both start from a pressure-to-temperature conversion, so keep the right refrigerant's P-T chart handy — for a glide refrigerant like R-454B, use the vapor column for suction/superheat and the liquid column for liquid/subcooling. The refrigerant charge calculator runs the arithmetic.
Common questions
What is the difference between superheat and subcooling?
Superheat is on the low side: it is how many degrees the suction vapor is above its saturation (boiling) temperature — suction line temperature minus the saturation temperature from the P-T chart at the suction pressure. Subcooling is on the high side: how many degrees the liquid is below its condensing temperature — the condensing saturation temperature minus the liquid line temperature. Superheat confirms the evaporator is not flooding the compressor; subcooling confirms the condenser is producing solid liquid.
Do I charge to superheat or subcooling?
It depends on the metering device. A fixed-orifice (piston) system is charged by the superheat method — to a target superheat that comes off a chart keyed to indoor wet-bulb and outdoor dry-bulb. A TXV system is charged by the subcooling method — to a target subcooling, commonly around 10°F — because the TXV already controls evaporator superheat, so superheat can’t tell you the charge.
What is a good subcooling number?
For a TXV system, roughly 8–15°F, with 10°F the common rule-of-thumb default — but the real target is stamped on the equipment or printed on its charging chart. Treat any general number as a diagnostic starting point, not the setpoint.