Air Flow is slow: Duct sizing

[ontargett]

Help, I have high air flow everywhere in my house but one room (it is the farthest from the furnance). The duct is 10 inch that splits into two 10 inch ducts that go to the floor registers. The flow is slow. Should the duct size change at the Y to something smaller? One other note. Off the plenum, this 10 inch is the second largest besides the one that goes to the upstairs. Some of the smaller ones 6 and 8inch flow like a jet engine. Thanks for your suggestions.
Cold in Ca

[Jay11J]

The 10" spliting off to two 10" is the problem.. I would split down to 6" or 5" pipe to feed the vent.

[ontargett]

Jay,
Thanks for the advice. To clarify, you suggest that I take the 10" feed from the plenum, run that to the room and then split it down to two 5 or 6 inch feeders to the two vents in a Y fashion? 10 inch from plenum to room, then down to the two 5 or 6 inchers with a Y junction to the vents in the room?

[Ed Imeduc]

lets rethink what you have there. You dont say how far the runs are. But a 10" pipe should Y to two 8"and the 8" pipe should Y to two 6" pipe.
So you can take that 10" pipe and run 2-- 8"Y off of it to that room. But how big is that room??????? It sounds like your whole duct work should be reworked. If a 6" and a 8"pipe sound like a jet things are not right there.
Cold in Ca you have to get more air off the plenum there.

ED ;)

[ontargett]

Thanks for helping me with this unsolved mystery, the 10 inch run off the plenum is approx. 20 feet to where it Y's off. From the Y, each run to the vent is about 16. The room is approx. 350 sq ft.

[Dennis B.]

There are a few things you could do that would help. One is you could damper down the runs that are blowing real hard either at the floor register or you could put a round damper in the pipes that are coming off the plenuim. You can get some silver tape and seal all the joints on the pipe. It's not a bad idea to do them all. If this does not help enough than you can run another run off the plunium and put another register in the room that is cold.
Hope this helps.