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I'm doing a basement development. Before I build bulkheads and close up the ceiling, I want to fix a few things about the ductwork.

As I understand it, you shouldn't have supply ducts coming right before the end of the main plenum. That is exactly what I have - two ducts coming off just before the end of the plenum, only a couple inches from the endcap. Both go underneath the last joist and supply air to our great room. This is going to require big bulkheads in two to-be basement rooms.

I would like to get up under that joist with just one supply duct, then y-split that to supply hot air to the two registers in the great room. These are located about 12 feet from each other and are 2 of 6 registers in that space. As a part of this change, my new supply duct will come from further away from the end of the plenum - about 18". Not sure if I can get a full 2' from the end and still have good spacing from the ducts that will supply hot air to the basement. That one side of the great room tends to be quite cold in the winter so I am hoping the new setup will improve heating air flow to that wall while increasing my finished ceiling height in the basement.

Hopefully this illustration makes it clear what I'm trying to do: paint sketch o ductwork

If I can do that, it'll give me a flat ceiling in one room and give me a nice, straight bulkhead in the other while fixing the design flaw with the existing ducts. To do the new runs, I'd have to use flex duct after the y-split but it'd be a perfectly straight, well supported run to the boots on each register.

Is y-splitting a supply duct effective, if I use the dampers to try and balance out the airflow? I don't think there is any space to do an alternative solution - I definitely don't want to add a second line coming off the right side of the plenum in my illustration, because that will eat up a ton of basement headroom.

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  • I'd be concerned that your proposed solution would, in effect, approximately half the amount of air volume being moved into your great room, which would cause more problems than you already have with a cold room during the wintertime. You could look at different profiles of ducting (low profile, which is very oval and not round) would help give you more basement headroom with a similar amount of airflow you have now. You'll probably also need to balance the rest of your living space with adjusting dampers to even our your heating throughout your living space.
    – Milwrdfan
    Jun 23, 2023 at 17:56
  • I thought that ducts at the end of a plenum get almost no supply? Is there a tool I could use to measure how much air I'm actually getting out of each vent? Oval ducting is unfortunately almost impossible to get up here... straight pipe is findable, barely, but elbows and the like would all be custom. Jun 23, 2023 at 18:13
  • Google says an anemometer. I will get that in, measure those two great room ducts and compare to ones further up the plenum to see what the difference may be. Jun 23, 2023 at 18:23
  • Where have you read/seen that ducts at the ends of a plenum get almost no supply? They'll get less than those significantly closer to the air handler, but whether they're at the end, or 2 feet from the end, anything past that is dead space with no movement so shouldn't make any measurable difference in flow.
    – Milwrdfan
    Jun 24, 2023 at 1:16
  • One of my top google results was a guy named John Puryear talking about the "two foot rule" applied to the the end of the duct run. Anybody else only uses it to space out takeoffs. So I guess it was bogus, bad advice. Measuring the takeoffs I get about 350cfm from each so there's no flow problem up there. Must just be usual high ceiling stuff keeping it cold in the winter. Jun 26, 2023 at 14:22

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You should only "fix" this to solve a current problem, like bad balance between other rooms. It's harder to balance a system with takeoffs at the end of the plenum, but it doesn't inherently hurt anything. Your proposed change will have much higher pressure drop and will pass less air. Dampers in the current takeoffs would be easier. A larger supply duct to the cold part of the room will probably be required to warm it.

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  • Would an alternative be to move the two takeoffs into the endcap itself? The plenum is large enough and the end is positioned just right, so that if I had two 90s coming out of it they would tuck up into the joist space perfectly. That would solve my ceiling height issue. Jun 23, 2023 at 18:25
  • that would give you even more flow, so if that's your goal, sure.
    – Tiger Guy
    Jun 24, 2023 at 18:47
  • Thank you, that is what I will likely end up doing then. Jun 26, 2023 at 16:35

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