I suppose that is a theoretical question...but the practical part is here:
I want to open and use a previously hidden staircase; the only problem is that the return from a 3rd floor closet runs through the staircase. It comes through the 3rd floor to the floor of the 2nd floor, follows the stairs down to the 1st floor (ie at a 45 degree angle; it literally follows the line of the stairs) and then goes into a joist bay that is covered by sheet metal.
The return duct work measures 6" x 10" square. Could I replace it with a (flattened oval) 10" round duct that would have a volume of 78.5" square compared to the old volume of 60" square?