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Keep the old duct layout or switch to two branches?

agurkas | Posted in Mechanicals on

Trunk A has rotted out and is filled with nasty old fiberglass lining, so I need to replace it. While I am at it, I wonder if I would get better performance from registers, if I also replaced the 8X8 existing branch with two 8″ branches straight off the trunk.

HVAC unit is only serving the large living room with cathedral ceilings and two vents on each side (so 4 total). Vents are all floor 14″X4″ In the original layout my concern is air distribution between the two registers, since you have this fat supply duct connecting via a 90 into 8X8 and closest floor register is about 2′ away from the connection

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  1. GBA Editor
    Martin Holladay | | #1

    You can probably use an 8-inch round duct to serve each of the two registers. But you definitely don't want the branches to come "straight off the duct."

    When an 8-inch round duct branches off a rectangular trunk, you use something called a take-off fitting. Your sketch doesn't show these take-off fittings. The take-off fittings make a more gradual transition, reducing turbulence.

    The shorter of your two 8-inch round ducts should be connected to the trunk at the end cap, not at the side of the trunk.

  2. agurkas | | #2


    If I were to go with plan B, I was planning using square to round take-off, which has the lowest impact on total equivalent length.

    One point I recall reading somewhere is that you not supposed to do take-off for register from the end of the supply trunk. I think you supposed to leave 2 feet. Something to do with more even air pressure to other ducts.

  3. GBA Editor
    Martin Holladay | | #3

    If you read that in a reputable document -- and I don't doubt you did -- then I stand corrected. Sorry to give bad advice.

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