Superheat and subcooling are two of the most useful measurements in refrigeration. Understanding them lets you charge systems accurately and diagnose faults far more reliably than pressure alone.
Superheat
Superheat is how much the refrigerant vapour has warmed above its saturation (boiling) temperature on the low side. It’s measured as the difference between the actual suction line temperature and the saturation temperature for that pressure. It confirms the refrigerant has fully boiled off to vapour before reaching the compressor — protecting it from liquid.
Subcooling
Subcooling is how much the liquid refrigerant has cooled below its saturation temperature on the high side. It tells you there’s a proper column of liquid feeding the expansion device, which is essential for capacity.
Using them to charge and diagnose
- High superheat / low subcooling — often undercharge
- Low superheat / high subcooling — often overcharge
- On TXV systems, charge to the target subcooling; on fixed-orifice systems, charge to the target superheat
FAQ
What is a good superheat value? It depends on the system and conditions — always charge to the manufacturer’s target rather than a generic number.
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