Formula
Measured SH = Suction Line Temp − Dew-point Temp(P, refrigerant)For piston / fixed-orifice metering. TXV systems charge by subcool, not superheat.
Pressure basis
Rounding
Target superheat
Target superheat
26.9°F
±5°F is in spec
Measured superheat (optional)
Refrigerant
How to use this calculator
Find measured superheat from suction pressure and line temperature, and the target for a fixed-orifice system.
- Select the refrigerant and enter suction pressure with the correct psig/psia basis.
- Enter suction line temperature measured near the service valve.
- Read measured superheat as line temperature minus saturation (dew point) temperature.
- Enter indoor wet bulb and outdoor dry bulb to get the target superheat for a fixed-orifice system.
Frequently asked questions
- What should superheat be?
- On a fixed-orifice system the target comes from the indoor wet bulb and outdoor dry bulb chart and can range from about 5 °F to 30 °F. A TXV system is normally 8–12 °F of evaporator superheat.
- What does high superheat mean?
- High superheat usually means the evaporator is starved: undercharge, a restriction, a plugged filter drier, or an underfeeding metering device.
- Where do I measure suction line temperature?
- On a clean section of the suction line within about 6 in of the service valve, with the sensor insulated from ambient air.