I'm finishing my basement and I have a run of 18 inch by 8 inch rectangular cold air trunk that runs an additional 15 feet after the last feed into the wall cavity to the floor above. That additional 15 feet is used exclusively for a bedroom two floors above and has a six inch takeoff at the end from which a six in diameter duct runs another five or so feet and connects to the wall cavity to the bedroom 18 or so feet up (sorry if this is confusing).
Question: I am boxing in the ductwork and would really like to minimize the lost headroom area wise. Can I get rid of the 15 feet of
18 inch by 8 inch rectangular and simply run a 6 inch round in its place and connect it to the exisiting 6 inch takeoff at the end of the current run or will this totally screw up the balance of the system??Thanks - D. Cervoni