Discharge Pressure Fluctuation — Pulsation, Oscillation, Cyclical Changes

Question

Accepted Answer

Condenser fan cycling pressure switch operation. Check: Intentional, normal operation in low-ambient or partial load. Fan ON drops high-side → OFF → pressure rises → ON cycles. 30 sec to 3 min cycle is normal. Cycles under 1 min mean differential is too small. What to do: 1) Adjust fan cycling switch differential — typically OFF pressure = ON pressure + 30–50 psi (~0.2–0.35 MPa). 2) Too rapid cycles accelerate compressor/fan motor wear → increase differential. 3) If cycling occurs in normal operation above 25°C ambient, suspect undercharge or coil blockage. TXV/EEV hunting — expansion valve oscillation. Check: 10–30 sec pressure oscillation (both low and high simultaneously) → suspect expansion valve hunting. High-side amplitude ≥0.2 MPa is clear. Suction line frosting area also varies in sync. What to do: 1) Refer to superheat-hunt case — bulb position/insulation check first. 2) For EEV, adjust controller PID (slow down response from default). 3) Recommend EEV for systems with large load swings (multi-evaporator). 4) Persistent hunting risks compressor liquid slugging — stabilize immediately. Worn condenser fan motor bearings — irregular rotation speed. Check: Periodic 'whir-whir' noise from fan; measure bearing vibration with vibrometer — 2–3x normal indicates wear. Touch motor case at bearing area; abnormal heat also confirms. What to do: 1) Try grease replenishment on motor bearings (only if grease nipples present). 2) If ineffective or no nipples, replace motor. 3) Worn bearings → reduced airflow → pressure fluctuation → increased compressor stress. 4) Recommend annual motor vibration inspection.

Discharge Pressure Fluctuation — Pulsation, Oscillation, Cyclical Changes

Likely causes

Condenser fan cycling pressure switch operation

TXV/EEV hunting — expansion valve oscillation

Worn condenser fan motor bearings — irregular rotation speed

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