Superheat
How many degrees the refrigerant vapour has risen above its boiling point at the measured pressure. Technicians use it to judge whether the evaporator is being fed correctly, since too little superheat risks liquid returning to the compressor.
What is superheat?
Superheat is how many degrees the refrigerant vapour has risen above its boiling temperature at the pressure it is at. It shows whether the evaporator is being fed the right amount of liquid.
Superheat: how it works
Refrigerant boils at a temperature fixed by its pressure. Once all the liquid in the evaporator has boiled, any further heat picked up raises the vapour temperature above that boiling point, and the difference is superheat. Measure the suction pressure, convert it to saturation temperature, measure the actual line temperature, and subtract.
Where you meet the term
- On a commissioning sheet, where the superheat achieved at handover is recorded as the baseline for later visits.
- In a diagnosis, where superheat and subcooling read together separate a charge problem from a metering problem.
- In an expansion valve fault, because superheat is the value the valve exists to control.
How to check it
- Take suction pressure and convert it. A pressure and temperature chart, or a gauge set that does the conversion, gives the saturation temperature for that refrigerant at that pressure. Technician
- Measure line temperature at the correct point. The clamp goes on the suction line where the manual specifies, with good contact and insulation over the probe, or the reading is worthless. Technician
- Compare against the published target for the model. Targets differ between refrigerants, metering devices, and operating conditions, so the manual for that unit is the reference and not a general figure. Technician
Superheat: what it is not
- Superheat is not a temperature. It is a difference between two temperatures, one of them derived from pressure.
- No universal correct value applies. The manufacturer publishes a target for the model at stated conditions, and indoor and outdoor conditions move it.
- Superheat alone does not identify a fault. It is read alongside subcooling, because the pair separates charge from metering.
Superheat questions
Why does low superheat matter?
It means liquid refrigerant may not have fully boiled by the time it leaves the evaporator. Liquid reaching the compressor is what damages it, which is why low superheat is treated as urgent rather than as a small deviation.
What does high superheat indicate?
That the evaporator is being underfed, so the refrigerant finishes boiling early and the vapour keeps warming. Low charge and a restricted metering device both produce it, and subcooling is what tells them apart.
Can I measure superheat myself?
No. It requires connecting gauges to the sealed refrigerant circuit, which needs certification in both the United States and the United Kingdom.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE TerminologyIndustry reference definitions for heating, ventilation, air conditioning, and refrigeration vocabulary.