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From my A/C supply plenum, I will have a trunk (approx 20ft straight run) with 3 branches coming off (each about 2 ft runs). This 20ft straight run would be perfect for a large flexible duct, and ductwork wyes are available to create the smaller branches.

Thing is, I've always seen trunks being made from steel rigid-duct, or in very few instances fiberglass board. Is there a reason I don't see trunks made of flex, and would this be a viable application of flex?

(note: Calculations have already been made for CFM, pressures, and output velocities. This question pertains specifically to the usage of flex-duct as an appropriate trunk in an hvac system)

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There may be some opinion on top of what I believe to be true.
There are some large flex duct sizes but the ribs are horribly inefficient. Metal trunk is also better suited when going vertical it is easier to secure. Frame work is square to box-in a round trunk the box would be much larger. I have seen DIY main trunk in a home we were replacing the “mess” that really did not work well , added adjustable baffle dampers and the system worked where prior the furthest rooms had no flow graduated size ducting fixed the mess. So sure it can be done but the trunks will take up more room and not work as well.

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    Well said! Flex duct can be done well and can work well.. but often it isn't done well and doesn't work well. It's important that the liner be pulled taut to minimize the trouble from the ribbing. Duct Disaster Science of Sag
    – Greg Hill
    Jun 4, 2020 at 20:36

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