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I’m in the midst of a project to make my basement space more usable, a big part of which will involve shuffling ductwork as it appears to have been installed (during the previous owner’s tenure) with reckless disregard for humans over 5’ 6”….

The section of supply I’m currently trying to address is all flex duct with two “hubs” (dunno the official word) from which there are runs to registers.

I’m thinking of switching to rigid round duct with takeoffs located close to the respective registers. I’m going with rigid since it’ll look better exposed in the basement (which I’ll be finishing), sit tighter to the joists for headroom, and hopefully will also perform better.

Refer to my sketch for the current setup and my best guess at what’ll work with rigid. The biggest design change is that I’d split the run to have rooms B and C start from the trunk rather than branch off room A (rooms B/C get poor airflow today). I have a ? for the size of the new duct supplying those rooms - thinking 6” might be sufficient?

duct redesign sketch

I’m exactly 0% an hvac pro, so by all means educate me on what I’m neglecting/alternative approaches/etc.

Obligatory: I’ve gotten quotes to redo all the ductwork and they were way out of budget, so I’m really hoping to make some improvements on my own. I’m a tenured construction DIYer and aim for code compliance + as-close-to-best-practice-as-I-can-get in my projects.

Thanks, people of the internet!

EDIT 1: as requested, I’m adding some photos of the branch of duct in question along with a sketch of the full system (supply only) for added context if helpful. The colors in the sketch correlate to the colored ducts in the photos. Fun puzzle.

The left side (branch supplied by a 10” flex - labeled “new work”) is the part of the system I’ve already converted to a (mostly) rigid duct setup. It’s not fully finished so I’m open to any suggestions on that side, too. The 12” line feeds two registers in an addition from the 90’s but that ductwork is buried so I’m not sure of sizing…but that room gets ample (probably too much) air, today.

Exhibit D: more shoddy sketches of full supply system

start of branch

first stop: a hub feeding bedroom 1

duct from first hub to second

next stop: a hub feeding bedroom 2 and bathroom

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    Where crossing under joists, a flattish rectangular duct can have more flow area than a round duct for equivalent headroom...or more headroom for equivalent flow.
    – Ecnerwal
    Commented Jan 7 at 22:02
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    The IRC specs a minimum ceiling height of 6'-4" at ducts (and beams and girders). It's 7'-0" everywhere else (actually not, but 7'-0" is safe).
    – popham
    Commented Jan 7 at 23:07
  • Your arrow pointing from the end of the trunk is alarming. There shouldn't be any duct coming off the trunk within a foot or two of its end.
    – popham
    Commented Jan 7 at 23:11
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    There's a duct systems IRC chapter, codes.iccsafe.org/content/IRC2021P2/chapter-16-duct-systems. THREE SCREWS!
    – popham
    Commented Jan 7 at 23:15
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    The “trunk” is far from to scale…just meant to be representative of where this branch begins. There’s a sizable and complex set of ductwork coming off the air handler I ignored in the sketch. It might technically not even be a “trunk” based design…more of a “spider”. This is just one of the four main branches…
    – JimboSlice
    Commented Jan 8 at 1:45

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While I don’t have the “right” answer (because HVAC ducting seems as much art as science), here’s the approach I settled on:

I’m thinking my attempt will be replacing the section in question with what I’ll call a “sub trunk” of 12x5 duct that’ll run the full length of the house (top to bottom on the diagram). From that duct I’ll pull off perpendicular runs of flex for each of the registers and tuck those between joists.

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    Yeah, going with a rigid sub-trunk instead of flex spaghetti is definitely a good move. Hopefully you'll be able to come back in and address the rest of the ductopus in the future :) Also, don't forget to make sure that the lining on the flex you are installing is fully stretched! Commented Jan 18 at 3:22
  • Appreciate the vote of confidence @ThreePhaseEel! The next round will be sorting through the returns. They’re arguably worse. Good news is I found a local sheet metal guy so the world of custom sizes and transitions is now wide open…which may be dangerous. We shall see!
    – JimboSlice
    Commented Jan 18 at 15:51

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