Abnormal Subcooling — Low / High Diagnosis

Question

Accepted Answer

Subcooling too low (<3°C / 5°F) — undercharge or poor condenser efficiency. Check: Attach clamp thermometer at condenser outlet liquid line + read high-side pressure on manifold. Convert pressure to saturation temp via PT chart → saturation temp − measured temp = subcooling. Normal: TXV systems 5–10°C (9–18°F), not applicable to capillary. What to do: 1) Check liquid line sight glass for bubbles — bubbles + low subcooling = confirmed undercharge. 2) Leak check, repair, evacuate 30 min, recharge by weight. 3) Clean condenser coil — dust/oil layer reduces heat exchange. 4) Verify condenser fan RPM and airflow. Subcooling too high (>15°C / 27°F) — overcharge or liquid line restriction. Check: Check high-side pressure simultaneously — if condensing saturation temp exceeds normal 17–22°C (30–40°F) over ambient, overcharge. Touch liquid line; if filter-drier inlet/outlet temp difference >5°C, drier is restricted. What to do: 1) If overcharge confirmed, use recovery machine to remove charge — target subcooling 5–10°C and normal high-side. 2) If filter-drier restricted, replace (never reuse a saturated drier — moisture re-releases). 3) Inspect TXV/EEV inlet strainer — copper dust/flux can block. 4) Document overcharge cause (previous tech overfilled) for record.

Abnormal Subcooling — Low / High Diagnosis

Likely causes

Subcooling too low (<3°C / 5°F) — undercharge or poor condenser efficiency

Subcooling too high (>15°C / 27°F) — overcharge or liquid line restriction

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