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I'm quite new to dealing with HVAC duct-work.

We would like to install a suspended ceiling in this room. Since the finished ceiling will be higher than this duct-work, we're going to build drywall bulkheads around them.

In order to reduce the wasted space (and having to build more bulkheads), we were thinking of running the two 7-inch ducts from the end of the main duct. This way we can build a single bulkhead around the main duct and continue it, containing the two smaller ducts, to the exterior wall where they turn apart and run upstairs. Both 7-inch lines lead to registers in the floor immediately above, approximately 10 feet apart, on the same wall.

Are there any good reasons not to do this (such as loss of pressure in one of the lines)? Would it be better to replace those two lines with a larger conduit and branch it when it reaches the wall?

Two 7-inch ducts branching from a main conduit.

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    Note, that's a damper control on the right duct in your picture (most likely in the closed position). You'll need to leave that accessible, which can be done with a small access panel in the drywall.
    – BMitch ♦
    Dec 6, 2011 at 0:11
  • I'm no expert, but it was my impression from my reading that taps shouldn't be in the end cap to allow the system to pressurize properly.
    – Josh Bush
    Dec 6, 2011 at 3:52

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I don't see a problem with that, other than the existing duct doesn't look wide enough to fit both. You'll have to get/build a reducer that goes from something slightly wider (that can support both circular ducts coming out of it) to the existing square duct size. Most metal shops can build this sort of thing for you - I had a couple 45-degree elbows built to re-route some ducts in my basement, it cost me around $50 I think. 

 | |   | |
 | |   | |    <--  existing two round ducts (one moved)
 | |   | |
+---------+
|        /
|       /    <-- new reducer
+------+
|      |
|      |     <-- existing square ducts
|      |

The main thing is to keep the number of elbows to a minimum. In fact, in your case it may work better as it looks like you'll reduce the number of elbows by one.

Another option would be to extend the main duct, and split the two round ducts off at the end. Not sure how long those are now, but the box stores carry a few regular square duct sizes you can buy and assemble/install yourself.

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  • Thanks @gregmac. Actually, we had considered extending the existing square duct since it only has to run about 10 feet before it reaches the wall.
    – msanford
    Dec 6, 2011 at 18:04

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