TinnerFlow™1st YearFlex DuctGREEN · Standard Correction
Flex cannot make a sharp unsupported turn off a collar without choking. Use a smoother path, hard elbow/boot where needed, and support the flex so the core stays open.
A tight 90 right off a plenum box can crush the flex core even if the outside jacket looks connected. The room may be starved because the bend is acting like a closed damper.
The usual recovery is to give the turn a rigid elbow/boot or a wider supported bend, then reconnect the flex straight and smooth. Check strap placement, light fixtures, ceiling grid, and whether the run can be shortened or routed cleaner.
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.