I have a two story home, with 6 x 18 steel ducts used as a main supply
trunk. The outlet to each room is off of this main trunk. I have really poor
air flow in all the rooms, even with all vents open, or balanced, or all
registers removed, anything!! the problem as far as I can tell is that my 22
foot long, as the crow flies, 6x18 inch ducts have four 90 degree mitered
corner bends each (upper and lower floors) . I am no engineer, but it seems
to me that square comers are bad news when trying to move air!!!!
as to the particulars.
5 ton hp (this should be 2200 cfm, as per mfg)
1200 sq inches of return grill, so I don't think I am starving the return
two 18 inch flex ducts for the return, total flex run, less then 10 feet.
2500 sq foot home
14 supply registers, 3 6 x18, 6 6x18 and 5 6x8
all metal duct, including the feed to the 6x18 ducts.
6x18 does NOT step down from outlet to outlet, thus the end of the line has
very little throw!!
all duct is buried in walls, zig zaging through out the load bearing areas
etc. NO way to change the 6x18, unless I take down miles of drywall and
so my question is, how can I or what can I do to optimize the mitered
corners?? Some internet investigation found "turning corner vanes" but are
there other ideas???
on a side note, originally the 6x18 duct was fed with about 23 feet of 16
inch flex, total air flow out of the supply vents was about 700 cfm. I took
out the flex and replaced it with round metal and the cfm went to about
900!!! I also sealed all the ducts I could access with mastic or tape.
additionally, I have fooled around with stepping down the 6x18 duct as it
nears the ends and have gotten better throw.
thanks in advance, for any ideas!!!