R-Pro › Field cases › Refrigerators & showcases
Superheat Hunting — TXV Oscillation
Likely causes
Poor sensing bulb mounting — loose contact with suction line or missing insulation
Check: Touch sensing bulb to verify tight contact with suction line, no gap. If insulation is missing or damaged, ambient air influences reading and causes malfunction. On suction lines ≥5/8", mounting on top picks up gas only → fails to read actual liquid temperature.
What to do: 1) Clamp sensing bulb at 4 or 10 o'clock position on suction line with copper clamp. 2) Apply thermal grease at contact surface (optional). 3) Wrap completely with insulation — block ambient air. 4) Lines ≤1/2" allow 12 o'clock mounting.
Oversized TXV — valve too large for system load
Check: Compare TXV nameplate ton (TR) rating with system rating. If superheat oscillates ±5°C or more per second/minute under light load, suspect oversized valve. Oversized valves over-react to minor changes → hunting.
What to do: 1) Replace with correctly sized TXV — within 100–125% of system rating is standard. 2) Some series allow swapping only the orifice (cage) — exchange smaller orifice. 3) Temporary fix: turn superheat screw clockwise 1–2 turns to reduce hunting amplitude, but not a root-cause solution.
Refrigerant migration — refrigerant moves into compressor crankcase during off-cycle
Check: If superheat reads 0–3°C for first 5–10 min after startup then gradually recovers, suspect migration. Occurs when compressor is the coldest spot — cold ambient, low-temp warehouse, or outdoor unit in winter.
What to do: 1) Install/replace crankcase heater — energized 24/7 during off, keeps compressor 11°C (20°F) above ambient. 2) Add pumpdown control — solenoid closes liquid line during off, system pumps down low-side refrigerant before compressor stops. 3) Install suction accumulator to protect compressor.
Hunting troubleshooting order: bulb mounting → valve sizing → migration. Over 90% are bulb mounting issues. Always check bulb first before disassembly.
All 565 cases work offline in R-Pro, with error codes for 62 brands and P-T data for 108 refrigerants.
Open R-Pro →Related cases
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