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  • Fan efficiency (CFM loss) due to ducting

    Created by Bill on


    I’m trying to size my ventilation system using inline fans such as the Vortex. Is there a table or rule of thumb for CFM loss through ducting? I would be interested in these values for 4″, 6″ and 8″ duct diameter. I may have longer runs, so this will be important.

    Thanks for your help.

  • Author
  • #207549

    Eric Vinje

    Hi Bill –

    Great question. When setting up a ventilation system we typically figure a 7% loss in CFM for every 25 ft. when using flexible ducting (a 3% loss if using metal duct pipe). You should also factor a 3% loss for every 90 degree bend in your ductwork. Keep in mind that these are ballpark numbers — each system is unique and actual numbers may vary.

    The following post should also help:


    Good luck!

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