Should I do this (ductwork)?

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  • billfrommich
    Forum Newbie
    • Jan 2007
    • 74

    Should I do this (ductwork)?

    The shop I am building in a portion of my basement will have a 13 ft. x 29 ft. rectangular floorplan. The ceiling consists of exposed 2x10 joists on 16" centers that run parallel to the long dimension of the shop. Much of the room between the joists is allready filled up with the home's forced air HVAC ductwork, electrical wiring, etc. I'll probably use some additional room between the joists when I wire the shop and install 6" diameter sheet steel ductwork for a dust collection system. Even so, I hope to have at least one pair of adjacent joists with nothing between them that extends the entire 29 ft. length of the shop. I'm considering closing up the bottom of the joists (with galvanized sheet, drywall, or plywood) which would give me an "air cleaner" duct with a rectangular cross section of roughly 8" x 14" running the full length of the shop. I would like to use one or two furnace type blowers to suck in air (through filters) at the ends of the shop and return the filtered air through this ductwork to the center of the shop (or possibly intake air at one end and return it at the other end). Any reason not to do this? I'm concerned with safety and code issues, primarily. Incidently, in my 1960's built home, a portion of the forced air HVAC cold air return ductwork is constructed this way - galvanized sheet metal nailed to the bottom of the joists. Do they still do it that way today?
    Last edited by billfrommich; 07-16-2008, 11:07 PM. Reason: grammer correction
  • capncarl
    Veteran Member
    • Jan 2007
    • 3564
    • Leesburg Georgia USA
    • SawStop CTS

    #2
    The duct that you are creating will be too large and the velocity of the air flow will drop, causing all of your dust and chips to "fall out" and remain in the duct. You create a situation like this when you want to remove dust and chips from the air stream, ie a cyclone. On long runs you sometimes have to reduce the duct size to carry the chips along.

    capncarl

    Comment

    • Uncle Cracker
      The Full Monte
      • May 2007
      • 7091
      • Sunshine State
      • BT3000

      #3
      The above advice is correct. You will need to size your ducting to coincide with the CFM moved by your air handler(s), in order to keep air velocity at a predetermined level. This would normally not be so critical for AC return air ducting, but unlike AC, you are wanting to move air that is (or may be) heavily particle-laden. The result of too-slow air movement could be a heavy buildup of dust in the ducting, and this would be a huge fire hazard. I am not sure the of the velocity constant you should shoot for, but someone here (Loring?) may know what it is...

      Comment

      • BobSch
        • Aug 2004
        • 4385
        • Minneapolis, MN, USA.
        • BT3100

        #4
        What kind of DC do you have? If it's anything like the HF unit a 6" duct is too big. 4 or 5" would work better. Got to keep the velocity up to carry the solids.
        Bob

        Bad decisions make good stories.

        Comment

        • LarryG
          The Full Monte
          • May 2004
          • 6693
          • Off The Back
          • Powermatic PM2000, BT3100-1

          #5
          My understanding is that Bill wants to use this to duct air from an air cleaner, not to a dust collector. Also, note that the air in this duct would be coming from the cleaner, not going to it. So, at least in theory, the air it ducts is "clean."

          I've seen the described type of "duct" in quite a few older homes. Haven't seen it lately -- in large part because I now live in an area where basements are rare. I don't know what the code ramifications are, nowadays. Modern concerns about mold may enter the picture for ducting for a home's HVAC system. But for what you propose, I don't see why it wouldn't work (at least in theory) as long as it's well-sealed, so that the "duct" doesn't become a conduit by which the air in the shop escapes into other parts of the house.

          But as others have noted, build-up of dust could be a concern if your intake filters are not highly efficient. You'd want to build in some access panels so you could monitor the situation and periodically clean it out, if necessary.
          Last edited by LarryG; 07-17-2008, 08:02 AM.
          Larry

          Comment

          • Uncle Cracker
            The Full Monte
            • May 2007
            • 7091
            • Sunshine State
            • BT3000

            #6
            Originally posted by LarryG
            My understanding is that Bill wants to use this to duct air to an air cleaner, not to a dust collector.
            Understood, but there will be dust in the air, regardless of the DC in use (otherwise, there would be no need for this setup). One major component of the dust will actually be that which gets through the fabric bag on the DC.

            Comment

            • billfrommich
              Forum Newbie
              • Jan 2007
              • 74

              #7
              LarryG is correct, I want to use the "between the joists ducting" as part of an air cleaning system, not as part of my dust collection system. Furthermore, the air would be filtered down to micron size level or so before entering the ductwork, so if the filtering is working and intact, the only dust in the ductwork would be micron size level or so, which should easily move through the ductwork even with the low velocities that accompany the large area ductwork. The purpose of the ductwork is to set up a continuous cross flow of filtered air in the shop. In a sense I am engineering my own forced air recirculation system, but I'm not an expert in the field - is there something I might be missing? Again, I'm pretty sure the system would work very nicely as an air cleaner - but what about possible safety issues, code violations, etc.? I realize I could totally line the area between the joists with rectangular sheet metal ducting, at a cost. If I did it as described above I would definitely follow LarryG's idea of having some hinged, removable , and or transparent access panels that would let me monitor the situation in the ductwork.
              Last edited by billfrommich; 07-17-2008, 08:37 AM. Reason: additional content

              Comment

              • cgallery
                Veteran Member
                • Sep 2004
                • 4503
                • Milwaukee, WI
                • BT3K

                #8
                Provided you can adequately seal it all up, it will work, and is very ingenious.

                Comment

                • pelligrini
                  Veteran Member
                  • Apr 2007
                  • 4217
                  • Fort Worth, TX
                  • Craftsman 21829

                  #9
                  I don't know of any code violations, but I have never totally read the Mechanical Code or the corresponding section in the IRC (International Residential Code). Safety wise - I would think that if you monitor any dust buildup it would be ok.

                  You might caulk any joints in the exposed subfloor if it is just a single layer of decking.
                  Erik

                  Comment

                  • billfrommich
                    Forum Newbie
                    • Jan 2007
                    • 74

                    #10
                    Originally posted by pelligrini
                    You might caulk any joints in the exposed subfloor if it is just a single layer of decking.
                    The decking is actually 1" x 6" planks laid diagonally across the joists, with ocassional 1/8" gaps between the planks, and the ocassional knot hole in the planking that has fallen out!. In some places there are nails penetrating through to the basement. The planking appears to be covered with some type of black "tar paper". In the living area above my shop, there is a mixture of hardwood flooring and linoleum/vinyl flooring. I'm not sure if there is anything else (sheetgood type subflooring) between the "tar paper " and the flooring. Unless I install a drywall ceiling, this will be the situation over the entire shop. I'll have to get one of those inexpensive dust monitors and see if there is a spike in readings taken in the house while using the shop, with or without the air cleaner.

                    Comment

                    • LCHIEN
                      Internet Fact Checker
                      • Dec 2002
                      • 20913
                      • Katy, TX, USA.
                      • BT3000 vintage 1999

                      #11
                      my concern would be for what gets sucked in from upstairs in the house, the floorboards over the joist will not be sealed tight unless you do something like line the "duct" with plastic.

                      If I read your post, you plan to "pull" a suction on this duct rather than pressurize it. That'll cause household air to enter and you may get dust buildup upstairs around any cracks leading to the joists in use. Might even get dust lines on the carpet with the carpet acting like a filter if you have carpet over the flooring.
                      Loring in Katy, TX USA
                      If your only tool is a hammer, you tend to treat all problems as if they were nails.
                      BT3 FAQ - https://www.sawdustzone.org/forum/di...sked-questions

                      Comment

                      • dbhost
                        Slow and steady
                        • Apr 2008
                        • 9209
                        • League City, Texas
                        • Ryobi BT3100

                        #12
                        I think you would be better off hanging duct, than trying to enclose the rafters for making duct... You would have FAR less leakage and better air control that way...

                        As far as the 6" DC duct work goes, unless you are using a DC that is WAY too big for that space, you are going to lose velocity on your airflow, and drop solids from the airstream in the duct, not a good thing...

                        Depending on your budget, and your machines, you will most likely want to duct this space with either 4" or even 2.5".

                        I have my shop run with 2.5" going into a Ridgid 12 gallon wet / dry vac, and going full bore with my planer causes no problems. More than a few folks tried to tell me this setup wouldn't work well... They were more than a little wrong. Maybe a big jointer might clog it up. But I don't own one...
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                        Comment

                        • pelligrini
                          Veteran Member
                          • Apr 2007
                          • 4217
                          • Fort Worth, TX
                          • Craftsman 21829

                          #13
                          After consideration, it's probably not that big of a deal. I was thinking that there might be some minor dust infiltration blowing up into the house through butt-joints, like maybe through carpet. I didn't consider a carpet pad or anything else on top of the subfloor either. I would think that cailking all those joints wouldn't be worth it at all.
                          Erik

                          Comment

                          • LarryG
                            The Full Monte
                            • May 2004
                            • 6693
                            • Off The Back
                            • Powermatic PM2000, BT3100-1

                            #14
                            As far as the 6" DC duct work goes, unless you are using a DC that is WAY too big for that space
                            I don't understand this comment. The size (CFM capacity) of a dust collector has no direct relationship to the size of the shop space it's serving. What do you mean?

                            I have my shop run with 2.5" going into a Ridgid 12 gallon wet / dry vac, and going full bore with my planer causes no problems. More than a few folks tried to tell me this setup wouldn't work well... They were more than a little wrong. Maybe a big jointer might clog it up. But I don't own one...
                            I think what people were probably trying to say is that while a shop vac-powered DC system can help you keep a cleaner shop, it's not going to do anything much to protect your health. The reason is that no shop vac can move a sufficiently large quantity of air to collect all the fine particles right at the source, where the dust is being created. Unless that can be done -- and my HF "2HP" DC with 4" hard piping can't do it, either -- the fines will escape into the shop air and eventually find your way into your lungs. A shop vac like yours and smaller dust collectors like mine can indeed do very well at chip collection, but they're lousy for bona-fide dust collection.
                            Last edited by LarryG; 07-17-2008, 12:07 PM.
                            Larry

                            Comment

                            • billfrommich
                              Forum Newbie
                              • Jan 2007
                              • 74

                              #15
                              Originally posted by LCHIEN
                              my concern would be for what gets sucked in from upstairs in the house, the floorboards over the joist will not be sealed tight unless you do something like line the "duct" with plastic.

                              If I read your post, you plan to "pull" a suction on this duct rather than pressurize it. That'll cause household air to enter and you may get dust buildup upstairs around any cracks leading to the joists in use. Might even get dust lines on the carpet with the carpet acting like a filter if you have carpet over the flooring.
                              Actually, I suppose I could put the filters at one end, the blower at the other end, and pull filtered air through the duct (creating a negative gauge pressure situation in the duct) or put the filter and blower at the same end and pull air through the filter and push it into the ductwork (creating a positive gauge pressure in the duct). So if there were any leaks in the duct, I'd either be pulling dust from upstairs into the shop, or pushing shop dust (ostensibly filtered) into the upstairs. As has been said before, to perform the air filtering I desire requires no leaks in the duct. At this point, I guess I probably should look into the cost of rectangular ductwork.

                              Comment

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