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

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.

['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.']

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