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Apprentice answer

Why would a large return plenum wall oil-can or buckle inward when the air handler starts

3rd YearVAVs, Fan Coils & Equipment ConnectionsCheck first

Large flat plenum panels can buckle under negative pressure when they are not stiff enough. Stabilize the casing with approved stiffeners or tie-rod reinforcement before the metal fatigues or tears.

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Plain-English answer

A return plenum can look fine until the fan pulls against it. If the side walls boom inward, the casing is not stiff enough for the negative pressure and panel size.

Do not ignore the oil-canning. Verify the fan/return connection, panel condition, hanger support, and whether the plenum needs cross-breaking, external stiffeners, or internal tie-rod reinforcement per the approved detail. If the panel has creased or torn, stop and get direction before startup continues.

Ask Foreman

The return plenum at [unit/location] is buckling inward on startup. I checked the panel, seams, and AHU connection. Do you want stiffeners/tie-rods added per detail or should we hold for detailer direction?

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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