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Evaporator Pressure (SST) Dropping Below Design Value — Insufficient Air or Water Flow

Likely causes

Fan belt loose or broken

Check: reduced fan RPM lowers air volume through evaporator

What to do: Measure SST and compare to design — for R-410A, design SST 5 °C = 0.88 MPa; if actual is below 0.60 MPa, severe drop requiring immediate cause investigation

Indoor fan motor bearing wear increasing resistance

Check: current rises but RPM falls short

What to do: Verify airflow — measure fan RPM (stroboscope), check belt tension (sag ≤ 10 mm), measure motor current (within ±10% of rated)

Chilled water pump impeller wear

Check: flow meter shows 20%+ drop from design flow

What to do: Verify chilled water flow — measure pressure differential across coil with pressure gauges, compare to design pressure drop. If flow is low, compare pump performance curve against current operating point

Air pockets trapped in chilled water piping

Check: bubble stagnation in low-velocity sections

What to do: Purge air from piping — manually open auto air vents or crack high-point piping ball valves to release trapped air

Chilled water strainer clogged

Check: excessive pressure loss on pump suction side

What to do: Clean strainer — verify suction pressure gauge reading, disassemble and clean strainer, reassemble and re-verify flow

Field tip

If SST drops and superheat rises simultaneously, suspect flow deficiency. If SST drops but superheat is normal, suspect refrigerant overcharge. Always read both values together to determine root cause direction.

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