I'm looking for a source of info on sizing plenums.... We have a situation in a job where because of the location of the furnace we may have to limit the length of the supply plenum. It will end up being 10 or 12 inches long (from a horizontally set Carrier unit) with a single 14" duct coming directly out of the end. Ends up being an adapter more or less to attach duct system to furnace. Any thoughts on problems this might cause?
I've done similar things in the past. Either because of tight quarters like you probably have, but I've also had a few really long runs of duct before the 1st register, so then I just used a transition and elbow to tie into the duct.
Seems to me this would be more efficient, but it wouldn't "mix" the air up quite as well, not sure if that's a concern or not though.
I'm not a sheet metal guy but you didn't mention the size of the furnace and the size of its plenum edges I think those will tell the story, if the plenum opening of the furnace is much larger than the 14" duct I think it will be a problem creating a high external static pressure that the blower probably won't be able to overcome. The blower just loads up and won't be able to move the correct volume.
The size of the opening is 16" x 19" or about 300 Sq in. A 14"Duct will give me about half of that in size. The unit we are planning to use is a Carrier 060-08 and we need about 900 cfm which would be low speed at a static preasure of 0.1 or high speed at a staatic preasure of 0.5. The return is a short run of square duct 20 x 20.
Thanks for your responses!
You should not have any problems
with that setup.I have had to a
lot worse thing because of space
restraints.A fourteen inch should
be just peachy for 900cfm.If you
have a real concern and have no
space for a 16"x19" to 14" round
then Use that 10" of length you
said you have and make a 16"x19"
to a 16'x16".This would leave you
a 1" to tap the 14"round into.This
would help steer the air flow
directly into the 14"
round elbow.Not perfect but its
not a perfect world in a attic
Best of luck.