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In the last picture I am moving a register (1) to (2). The left side of the picture shows where the 4" duct is sitting at the top of the wall (running across the joists). This is a bathroom. I need the duct to land at the base of that wall.

I have two options:

  • use a 4 to 3 reducer and an 3 to 3 elbow and continue to use 3" all the way down to the register boot, practically reducing the size of the duct (this increases the static pressure in the AC system and will send more air to other parts of the system)
  • use an elbow transition from 4" to a boot that is 10x3-1/4 and use a range duct, shown in the last picture (if this is OK)

The right side of the picture show the framing details. As far as I can see from what was done on the opposite wall (where the duct is landing now) they cut the horizontal stud and created two segments on that wall and added studs to close the two cuts (practically creating two frames joined by the bottom horizontal stud
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The room size is 102x78x96=763776 cubinc in or 442 cubic feet

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    Note that 'Stack Duct' is a standard term for these kind of rectangular ducts, and the 'for Range Hood' part probably just means it's sized to use in that application. There are also tons of similar stack ducts available that don't say that, e.g. here: homedepot.com/b/…
    – Nate S.
    Commented Aug 28, 2020 at 18:07

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A 3" pipe has roughly half the cross-sectional area of a 4" pipe (28.3 in2 vs. 50.3). You need to maintain that size as a 4" duct is already quite small. Even if you had a larger duct to begin with that's too much of a reduction.

Use the rectangular parts and/or oval ducts that meet your current flow requirements.

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  • Yeah that makes sense but I am not sure if I need that much cold air there ..heat is never enough in the winter in a basement :-) if you ask my wife How about the framing and fittings, are they OK ? The Home Depot guy (was a girl :-) ) told me to use duct tape and self taping screws to put these together. I still need to take some youtube lessons to put the two halves of the rectangular 'range' duct together...why is it even called range duct ? Is it recommended just for range ?
    – MiniMe
    Commented Aug 28, 2020 at 17:59
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    Doesn't really matter what parts you use as long as they're large enough. They make oval ducts for 2x4 walls. Use what's available. Duct tape hasn't been an approved technique for decades. Use foil tape for sealing and screws for fastening.
    – isherwood
    Commented Aug 28, 2020 at 18:05
  • Damn I mean foil tape (aluminium tape use for duct work) Oval ducts are not available in the big stores here in Canada
    – MiniMe
    Commented Aug 28, 2020 at 18:20
  • It seems that I am going to have to cum my own fittings as the off the shelf ones do not allow me to transition properly from round to retacangular provided that the round one is close to a joist which makes it impossible to install a 4x3x10 boot
    – MiniMe
    Commented Aug 28, 2020 at 23:12
  • Wouldn't be the first time. it's not difficult if you learn how to cut tabs on the end of your pipe and alternate over-under when you attach to a box.
    – isherwood
    Commented Aug 29, 2020 at 22:47

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