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Thread: DC - Rectangular/Oval Ducts?

  1. #1
    Join Date
    Dec 2006

    DC - Rectangular/Oval Ducts?

    I've been thinking about dust collection for my new shop. For those of you watching, you may recall that I am planning to put in a wood raised floor over the concrete slab. I would like to run dust collection under this raised floor. If I used 2x6s on edge, I should be able to get a 5" pipe under it which would be sufficient for my shop. I'd like to explore the possibility of using 2x4s on edge to gain a few inches and cut down on cost a bit. If I do that, I obviously would not be able to use a 5" pipe. My question is, would it be proper/efficient to have rectangular ducts (maybe with rounded corners, or, even an oval shape) that carry the same amount of volume as a 5", but are shorter? I realize I may need to have these fabricated. The cost involved in doing that may kill the idea but I was wondering if you guys think that would work. Am I being clear here with my question?

  2. #2
    Join Date
    Nov 2006
    Austin, Texas
    I have seen dust collectors collapse ordinary round metal heating ducts, and flat ducts would have even less strength against collapse.

    You might consider boxing the beams closed, if you can get it fairly air tight, and using the space without a duct. I used the "thick wall" trick to get a cold air return in a very old house that didn't have room for a regular furnace return duct.

    I would be worried about the friction of the rough "non-duct" and how you would clear a clog if something got caught and started to dam up the flow.
    Charlie Plesums, Austin Texas
    (Retired early to become a custom furnituremaker)
    Lots of my free advice at

  3. #3
    Join Date
    Nov 2006
    Constantine, MI
    Hi Rob,

    If you're looking to run your DC under the floor with only 2x6's how will you go perpendicular to the joists without having to cut the entire joist through? Are they actually supporting your wood floor above the concrete, or are they on the concrete as 'sleepers'? If they are being used as sleepers then you would have no problem running duct work perpendicular. Otherwise, I'd look into 9.5" I Joists. You can have a round hole as large as 6.5" as close as 5' from the bearing, or a 5" hole as close as 2'6" from the bearing. (Providing ceiling height is not an issue)

    The cost is a bit more, but might be comparable to the custom made ducts and would not be as much of a hassle.

    just my $.02
    Host of the 2017 Family Woodworking Gathering - Sunken Wood

    “We all die. The goal isn't to live forever; the goal is to create something that will.” - Chuck Palahniuk

  4. #4
    Join Date
    Nov 2006
    Delton, Michigan

    in agreement

    with charlie on this one, you will get cloggs if you use it. unless you are very particular.. if its in the open then you will stand a better chance to not have them, but as soon as you put yourself at murhys law mercy, it WILL get you also i would be intersted in your costing on puting down that wood floor,, i have thught about it and it appears to be prety steep choice rob.
    If in Doubt, Build it Stout!
    One hand washes the other!
    Don't put off today till tomorrow!

  5. #5
    Join Date
    Dec 2006
    If you're going to all the trouble of raising the floor, you might as well do a stemwall with joist floor. The drastic reduction in concrete and remesh costs would likely offset any added cost due to larger joists, and you could have plenty of room for ducting underneath. Then you just vent the space and don't have to worry about moisture. There must be something I'm missing, question is, what?


  6. #6
    Join Date
    Nov 2006
    Odessa, Tx
    I remember seeing an article that included all the math for round, square and rectangular ducts, and there was a tremendous effeciency loss when using anything but round ducts. Seems like it was about a 40% loss (IIRC) when using the same square inch cross section. You would also have to be making pure 90* turn to come up to the machines unless you plug in a "Y".

    I ran a channel in my concrete to carry my main trunk duct and it's deep enough that I can come off it with a 40* "Y" and a short but slow curve hookup to the machines.

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