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Oversized Suction Line — Oil Return Failure at Low Load

Likely causes

Suction pipe oversized — refrigerant velocity below 3.5 m/s at low load causes oil stagnation

Check: Check for continuous compressor oil level drop; measure actual suction line velocity — below 3.5 m/s at low load confirms oversizing; minimum 2.5 m/s in horizontal sections

What to do: Replace suction pipe one size smaller; if replacement not feasible, consider double riser (small parallel line); set minimum frequency limit on inverter compressor

Oil pooling at bottom of horizontal pipe section

Check: Check horizontal suction pipe sections for oil pooling — visually inspect start or U-shaped sections; confirm slope toward compressor (at least 1/200)

What to do: Redo horizontal pipe slope (at least 1/200 toward compressor); install oil separator on discharge side (recovery rate 70%+) → periodic oil level checks

Field tip

For frequent low-load systems, re-check pipe size targeting min 2.5 m/s in horizontal and 3.5 m/s in vertical risers

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