TinnerFlow™4th YearPressure / LeakageYELLOW · Check First
If a large return duct pulls inward, the duct is not stiff enough for the negative pressure or operating condition. Stop, document, and add approved reinforcement or replace the section.
A return main can physically collapse inward when negative static exceeds what the duct panels and reinforcement can handle. This is not a cosmetic dent; it means the duct construction/reinforcement is wrong for the pressure condition or something is blocked.
Check whether the fan is running correctly, filters/dampers are open, openings are not blocked, and the duct matches the pressure/reinforcement detail. Recovery may include internal tie rods, external stiffeners, revised reinforcement, or replacement per the approved duct construction standard.
Use this as training guidance. The foreman, approved drawings, project specs, manufacturer installation instructions, employer safety policy, and AHJ/code requirements always control the final answer.