Suction pressure at low load — distinguishing normal from faulty (low load ≠ low charge)

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Normal low load — cool ambient, night, just after pulldown. Check: Combined readings: ① Suction (low side) at ~70–85% of normal-load value. ② Discharge (high side) drops simultaneously (cool ambient lowers condensing pressure). ③ Superheat — TXV systems creep up (12–16°C), capillary systems drift down slightly (2–4°C, but not near 0°C). ④ Suction line sweat — normal level. ⑤ Always log ambient and box temperatures. Hallmark of normal low load: suction AND discharge drop together; superheat behavior depends on metering device. What to do: Action: if it's normal low-load behavior, no charging or adjustment needed. Run times become longer with often improved efficiency. Explain to the user: 'system is running gently because it's cool'. If ambient and box are normal but only suction is low, escalate to the next cause (low charge or evaporator restriction). Faulty — same low suction pressure pointing to a different diagnosis. Check: Distinguishing signals: ① Discharge normal or slightly high while only suction is low → suspect evaporator restriction, fan failure, or filter-drier clog. ② Superheat very high (>15°C) + bubbles in sight glass → low charge near-confirmed. ③ Suction line sweat absent (should normally sweat) + discharge line very hot (>90°C) → low charge + excessive compression ratio. ④ Box not reaching setpoint → load is normal but system capacity is the limit. What to do: Action: ① Clean the evaporator coil and measure fan current. ② Check filter-drier pressure drop (approximated by inlet–outlet temp difference; normal 1–2°C). ③ Verify TXV (sensing bulb location, external equalizer). ④ If low charge is confirmed: leak check → recover → evacuate → measured recharge (see prior case shortcycle-low-charge-cycle). ⑤ Compressor efficiency loss (worn valve plate) — both suction and discharge weaker than normal → consider compressor replacement.

Suction pressure at low load — distinguishing normal from faulty (low load ≠ low charge)

Likely causes

Normal low load — cool ambient, night, just after pulldown

Faulty — same low suction pressure pointing to a different diagnosis

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