Apprentice answer

Why does a fan coil drain pan overflow if the P-trap is dry or unvented

3rd YearVAVs, Fan Coils & Equipment ConnectionsCheck first

A condensate drain has to overcome unit pressure and maintain drainage. Prime/check the trap, venting, slope, and manufacturer trap detail before water damages ceilings or equipment.

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Plain-English answer

A fan coil or AHU drain may not drain just because PVC is connected. Unit pressure can hold water in the pan if the trap is dry, shallow, unvented, or piped wrong.

Check the manufacturer trap detail, pressure condition, slope, cleanout, vent arrangement, and whether the trap has water seal. Prime and correct the drain only with the approved layout, then watch the pan drain under actual fan operation.

Ask Foreman

The FCU/AHU at [tag] is overflowing because the condensate trap appears dry/air-locked. I checked trap, slope, and venting. Do you want the drain corrected per manufacturer trap detail?

Verify before acting

Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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