Deep Technical Chapters · Charge & Metering
Superheat
Suction line temperature minus saturation temperature at suction pressure. Tells you whether the evaporator is fed correctly.
Superheat is the gas temperature above its saturation temperature at the evaporator outlet. It tells you whether the metering device is feeding the right amount of refrigerant.
What it tells you
High superheat = starved evaporator (low charge or restriction). Low/zero superheat = overfeeding or floodback risk. Target is typically 4–8K at the evaporator outlet for TXV systems.
How to measure
Measure suction pressure, convert to saturation temp, subtract from the actual suction line temperature at the evaporator outlet. Measure at the right point or the number is meaningless.
Field Rules
- 01
"High superheat = starved. Zero superheat = floodback risk — stop."
Diagnosis at a glance
Symptoms of failure
- •High superheat: starved coil, low suction
- •Zero superheat: floodback, compressor damage
Measurements to take
- •Suction pressure
- •Suction line temperature at evaporator outlet
- •Saturation from PT chart
What can mislead you
- •Measuring superheat at the wrong point
- •Ignoring coil condition
