HVAC installers added two 10in. Return Air Ducts (Insulated Flex Duct with metal connectors) from a ceiling box back to the Plenum above my downflow furnace. This is a very short run but since the Flex Duct came packaged in 25ft.lengths, it was snaked in, on and around the roof trusses and joists with significant bends in places. Does this not impair air flow significantly? Or does the 10in. diameter compensate? Can flex duct be cut?
Of course bends (and the rough interior surface of flex itself) impedes air flow when compared to smooth straight ducting. You blower motor will have to work a bit harder to compensate while delivering less static pressure. A pair of 10 inch round ducts is only about 150 square inches. Compare that to the area of your plenum. Flex can be cut.
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