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)