Superheat and subcooling: practical guide for HVAC-R techs

Superheat and subcooling are the two most important measurements when diagnosing a refrigeration system. Mastering these concepts is the difference between a quick diagnosis and hours of guesswork.

What is superheat?

Superheat is the difference between the suction gas temperature and the saturation temperature at suction pressure.

Formula: Superheat = Suction line temperature − Saturation temperature (low side)

Normal values

• Systems with thermostatic expansion valve (TXV): 5°C to 8°C
• Capillary tube systems: 8°C to 15°C
• Systems with electronic expansion valve (EEV): 3°C to 6°C

Superheat too low (< 3°C)

• Risk of liquid slugging at the compressor — mechanical damage
• Causes: refrigerant overcharge, expansion valve open too far, low thermal load

Superheat too high (> 15°C)

• Compressor overheating — premature wear
• Causes: low refrigerant charge, stuck expansion valve, plugged filter, frosted evaporator

What is subcooling?

Subcooling is the difference between the saturation temperature at discharge pressure and the liquid line temperature leaving the condenser.

Formula: Subcooling = Saturation temperature (high side) − Liquid line temperature

Normal values

• Standard systems: 5°C to 10°C
• Systems with liquid receiver: 3°C to 8°C

Subcooling too low (< 3°C)

• Risk of flash gas before the expansion valve — capacity loss
• Causes: low refrigerant charge, dirty condenser, failed fan

Subcooling too high (> 15°C)

• Refrigerant overcharge — excessive head pressure
• Causes: too much refrigerant, undersized expansion valve

Quick diagnostic table

Symptom	Likely cause
High superheat + low subcooling	Low refrigerant charge (likely leak)
Low superheat + high subcooling	Refrigerant overcharge
High superheat + high subcooling	Restriction (filter, expansion valve)
Low superheat + low subcooling	Expansion valve too open or oversized evaporator