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Thread: Splitting trunk duct into two thinner ducts?

  1. #1
    DIY Junior Member nezroy's Avatar
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    Default Splitting trunk duct into two thinner ducts?

    Due to the layout of our basement(*) we currently have our supply trunk ductwork running through the hallway, in front of the basement stairs, and through a doorway, all of which are extremely difficult to work with while we try to finish off the last bits of this space and make for horrible headroom. We are trying to get as much height as we can so we can put in a drop ceiling and regular size doors, and would like to shrink the height of the trunk duct as much as possible.

    Based on the existing dimensions of 14"x8" for the duct, I'm guessing the original numbers used were around 600cfm@771fpm (using an online ductulator). Every register is using a 5" round branch that I'm guessing was figured at 75cfm, based on the total of 8 branches off the trunk, and the size of the rooms involved (most rooms are ~80sqft, or around double that with two registers). There is a hard 90' bend after the (gas) furnace, then two 5" branches. A reduction to 12"x8" at that point (500cfm@750fpm?) and then the remaining six 5" branches.

    What we'd like to do is reduce the height of the trunk line from 8" to as low as possible, with plenty of width to work with (>40"). Looking at standard rectangular duct sizes, it looks like the best "regular" option would be a 4" height (100mm). However, the widest standard size for that is only 15.75" (400mm). That would work out to 300cfm@686fpm. If we split the original 14"x8" trunk from the furnace into two 15.75x4 lines (each at 300cfm@686fpm, each with four 5" branches), and reduce both of those to 11.75(300mm)x4 (each at 225cfm@689fpm) after the first 5" branch, are we going to get roughly the same performance? Also, what would be the best way to split this? I was thinking of doing a horizontal divider after the hard 90' bend, so that the top half (14"x4") goes into one, and the bottom half into another. Then they would just angle out to run side-by-side the rest of the way.

    (*) And yes, unfortunately the alternatives are even more work. The supply trunk currently runs along the furnace-side of the single central girder. To run it on the outer wall on the furnace side would require going under stairs (many bends) and also require ripping out and redoing the basement's bathroom shower, as well as altering the furnace plenum to change where the supply trunk attaches. To run it on the opposite side of the central girder (or on the far opposite wall) would require going over or under the girder, which would take 3 hard 90' bends and 4 easy 90' bends (or, alternately, redoing a bunch of electrical/plumbing lines in the furnace room), plus completely redoing all of the return ductwork as well.
    Last edited by nezroy; 05-10-2010 at 08:00 AM.

  2. #2
    Moderator & Master Plumber hj's Avatar
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    Stop trying to make a "do it yourself" installation. You need a sheet metal installer to construct 4"x30" ductwork and make proper transitions at both ends, along with any turns.There will also have to be custom takeoffs for the branches.
    Last edited by hj; 05-10-2010 at 06:22 PM.

  3. #3
    DIY Senior Member Hube's Avatar
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    I agree with Hj fully. Get a sheet metal worker or fully compentent furnace installer to fab you up with some fittings to acheive this depth reduction.
    If I was doing this job, having been a metal worker and in hvac for 50 years, I would make these 2 transitions from 14 x 8 to 4 x 20 and back to 14 x 8 at least 12" or longer in total length. Too short will add too much restriction to the air flow. Obviously, doing it this way will involve a sufficient length of 20 x 4 duct in between these transition pieces.But do you have the room (length) for this arrangement ?

  4. #4
    Plumber jimbo's Avatar
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    I think when you start splitting ducts you run into balance issues, so you really need to know what you are doing.

  5. #5
    Moderator & Master Plumber hj's Avatar
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    Which is why I said to have a single duct constructed.

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