Originally Posted by joside
Right, I understand that but I also need to size the ducts to provide the full CFM for the room. So if I understand you correctly, you are saying that a room that I want to supply 400 CFM to can be supplied by two separate ducts that each are sized to supply 200 CFM?
Yes. Its done everyday, a few thousand times a day across the country. Often 2-7" are used to supply a room with CFM requirement of 200 CFM. Or 1-6" and 1-7" depending on the REL of the duct system.
I'm just wondering if it's ok to split a room in half or if I would be better off oversizing it because there is some loss at joints. ie. Are two ducts supplying 200 CFM each less efficient than a single duct supplying 400 CFM?
Depending on room. One supply is placed under/at each highest load point of the room. EG: Under or near windows, or door.
An 11" flex duct(it would be special order, its not a standard size, and not made as a stock item, probably cost you 500 to 800 bucks to get it made) moving 400 CFM, with a 10% leakage rate. Will leak 40 CFM.
2-9" flex ducts(yes, its made in 9"), moving 200 CFM each. With both having a 10% leakage rate, will each leak 20 CFM, for a total of 40 CFM.
All joints are to be sealed on all ducts by code(not enforced in many areas).