Originally Posted by
air1
I don't believe this statement is entirely accurate. Even tightly stretched flex duct is going to be more restrictive than sheet metal duct.
You are correct sir. A "properly" installed flex duct has the same friction loss as a round duct 2" smaller in diameter and I have rarely seen installers take the time to do it right. Also you should use mastic on the inside of the liner before sliding it over the metal collar so you will have a good seal. Panduit straps are worthless for the attaching the liner to the collar.
Should use a SS draw band. Use the panduit straps for attaching the insulation to the collar. Panduit will stretch with heat and soon become loose and your inner will either blow off of the collar or it will leak bad. SMACNA recommends using metal elbows by the way. I do have an old (can't get them anymore) flex ductulator which shows that a fully stretched flex has the same friction loss and velocity as a round duct 2" smaller and same for a 6" to 4" as a 14" to a 12" as a matter of fact. So do what you want but if you use flex up the sizes by 2" keep it tight, use metal elbows and hope it is installed right which probably won't happen. Thank you, thank you very much
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