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Old 05-13-2010, 12:12 PM   #1
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Multiple Vents, Same Room


I have a question about how to calculate duct size when there are multiple supply or return vents in the same room.

Say I have a room that requires 400 cfm. Using a ductulator for flex ducting tells me I need 11" diameter. Now if I have two vents at opposite sides of this room, how do I calculate the duct size for these vents.

Do I still need them to both be 11" or would I redo the calculation using half the room size for each vent?

Is there a difference in this calculation for supply vs. return?

Thanks in advance.

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Old 05-13-2010, 08:22 PM   #2
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Multiple Vents, Same Room


You would size the duct for the CFM that duct is to carry. So how many CFM is each one going to carry, individually. 200? Or will one be 250, and the other 150?

You size for the ducts CFM.

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Old 05-14-2010, 10:40 AM   #3
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Multiple Vents, Same Room


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?

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?
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Old 05-14-2010, 12:31 PM   #4
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Multiple Vents, Same Room


Quote:
Originally Posted by joside View Post
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).
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Old 05-14-2010, 12:49 PM   #5
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Multiple Vents, Same Room


Thank you beenthere, you have answered my question perfectly.
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