Abnormal Superheat — Low / High Diagnosis

Question

Accepted Answer

Superheat too low (<5°C / 9°F) — risk of liquid refrigerant flooding compressor. Check: Attach clamp thermometer to suction line + read low-side pressure on manifold gauge. Convert pressure to saturation temperature via PT chart → measured temp − saturation temp = superheat. Normal range: TXV systems 8–12°C (15–20°F), capillary systems 4–7°C (8–13°F). What to do: 1) Inspect TXV sensing bulb position — clamp at 4 or 10 o'clock on suction line, wrap with insulation. 2) Adjust TXV superheat screw clockwise (raises superheat) by 1/4 turn only — wait 30 min after each turn to stabilize. 3) If overcharge suspected, recover and recharge by weight. Liquid slugging causes immediate compressor valve failure — stop unit immediately. Superheat too high (>15°C / 27°F) — undercharge or TXV restriction. Check: Calculate superheat using same method above. Simultaneously check liquid line sight glass — bubbles/flash gas indicate undercharge. Discharge pressure also runs low (mimics low-load operation). What to do: 1) Leak check first — use electronic detector or soap solution on all flares, brazed joints, service valves. 2) After repair, evacuate 30 min + recharge by weight. 3) If TXV restriction suspected, disassemble/clean or replace — always replace filter-drier together. 4) Never just top off refrigerant — adding charge without leak repair violates regulations and guarantees recurrence.

Abnormal Superheat — Low / High Diagnosis

Likely causes

Superheat too low (<5°C / 9°F) — risk of liquid refrigerant flooding compressor

Superheat too high (>15°C / 27°F) — undercharge or TXV restriction

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