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Flexible Duct Design: Maximum Length, Sag and Pressure-Loss Effects

Flexible ductwork is widely used in HVAC systems because it is lightweight, quick to install and capable of connecting rigid duct branches to ceiling diffusers without complex fabricated fittings. However, its apparent simplicity often leads to poor design and installation practices.


A flexible duct may have the correct nominal diameter and still fail to deliver the required airflow because it is too long, insufficiently stretched, sharply bent, compressed, pinched or inadequately supported. These defects increase pressure loss, reduce terminal airflow, raise fan energy consumption and make air balancing difficult.

Effective flexible-duct design therefore requires more than selecting a diameter. The designer must control: (Flexible Duct Design: Maximum Length Sag and Pressure-Loss Effects)


  • Installed length

  • Straight-line compression

  • Vertical and horizontal sag

  • Bend geometry

HVAC system layout and airflow analysis

Download the full design guide PDF with detailed calculations, fan sizing, code compliance, and real project insights. Built for HVAC engineers and MEP consultants. (Flexible Duct Design: Maximum Length, Sag and Pressure-Loss Effects)



For detailed calculations, project-specific design, and authority-compliant solutions, contact our engineering team.

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