Why is a short expanding transition noisy or choking airflow?

Plain-English answer

Fast, abrupt expansion is a performance problem, not just a sheet-metal shape problem. The duct may physically connect, but the air can separate, tumble, whistle, or starve the downstream equipment/branch.

Check the available run length, system type, downstream device, and approved detail. Likely recovery options are a longer transition, revised fitting, offset/route change, or engineered vane/splitter detail if the project allows it. Do not invent a transition angle from memory; use the approved standard or detail.

Field checklist

• Measure the inlet size, outlet size, and actual transition length available in the ceiling or shaft.
• Check what is downstream: VAV, fan inlet/outlet, elbow, louver, diffuser branch, or balancing point.
• Look for noise, vibration, oil-canning, or signs the fitting is acting like a restriction.
• Check whether there is room to lengthen the transition or move the next fitting.
• Document the field space and ask before changing the fitting geometry.

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