Measured at the evaporator outlet or the outdoor unit's suction line, superheat equals actual suction line temperature minus saturation temperature read from suction pressure. It proves that all the liquid has boiled off before the compressor. Fixed orifice systems are charged to a target superheat from a chart using indoor wet bulb and outdoor dry bulb; low superheat risks floodback, high superheat means a starved coil.
Suction saturation's 42 and the line's 64, that's 22 superheat, it's low on charge or the TXV's starving it.
Clamp the probe on clean, bare suction line near the service valve and let the system run 15 minutes before trusting the number.
Also heard as
- SH
- suction superheat
Do not mix it up with
- Subcooling: Superheat is measured on vapor in the suction line; subcooling is measured on liquid in the liquid line.
Related terms
Subcooling
How many degrees liquid refrigerant is below its condensing temperature at the same pressure.
Saturation
The state where liquid and vapor refrigerant coexist, so its temperature is locked to its pressure.
TXV
Thermostatic expansion valve: a metering device that adjusts refrigerant flow to hold constant evaporator superheat.
Floodback
Liquid refrigerant returning continuously to the compressor through the suction line while the system runs.
Starved
Describes an evaporator not getting enough refrigerant, so part of the coil runs warm and superheat is high.