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Chimney Primer

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  • Chimney Primer

    I need a real basic primer on chimney's and vents. I've read the threads about Bernouille's Law of physics already. Chimney needs to get more narrow as you go up. That makes sense.

    What about material? I haven't seen a discussion about the benefits/drawbacks of the different materials. I plan to completely enclose my chimney in a brick type enclosure. Does it matter if I use clay flue tiles vs. stainless steel (Duratech)? Is there a beneift of round over square?

    How about this shape?

    Does this have a better draw than the square? (hope you don't mind, Les, if I borrow your photo )
    Mike - Saginaw, MI

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    My oven build thread

  • #2
    Re: Chimney Primer

    If you are building a brick chimney, I like the ceramic flue tiles. They are cheap, and traditional. They go together with the same sort of refractory mortar you're already using. As you can see from Les' photo, or from this one:

    that you can use a piece of bigger flue tile and make your taper transition.

    The stainless units work well if you can afford them. As for shape, I don't think smoke cares what shape of pipe it goes through. Europeans traditionally use round flue pipe, where in the US we use square or rectangular shapes.
    My geodesic oven project: part 1, part 2

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    • #3
      Re: Chimney Primer

      Mike
      There is a difference between round and square. If you run the numbers through the flow rate calculator here

      http://www.fornobravo.com/forum/28/c...or-3905-2.html

      You will find that an 8 inch round pipe ( 2 meters tall) gives you a flow rate of 1005. An 8 inch square pipe has a larger cross section and, using the same height, has a flow rate of 1279. That's about a 27% increase in draw.
      Drop some numbers in the calculator while you are planning. I think the lowest number we have seen ( that appears to work ) is about 340 on the flow rate.
      Sharpei Diem.....Seize the wrinkle dog

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      • #4
        Re: Chimney Primer

        Thanks for the info. I'll play around with the calculator.

        I found this article on the web. Making Sense of Chimney Liners, by Michael Chotiner - Old House Journal

        It states
        "But square and rectangular flue tiles are not the most efficient shape for venting smoke. By nature, smoke spirals upward through a flue in a helical pattern, leaving incongruous air spaces at the margins. At best, these air spaces simply take up extra room within the chimney that may be needed for additional flues; at worst, they reduce draft. Round flues are much more efficient."

        Not sure if that is factual. Thoughts?
        Mike - Saginaw, MI

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        My oven build thread

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        • #5
          Re: Chimney Primer

          By nature, smoke spirals upward through a flue in a helical pattern, leaving incongruous air spaces at the margins.
          I think this is myth. Smoke, which is flavored hot air, is a gas. Gases are by nature fungible, meaning they will expand to fit the available space.

          In calm weather, it sure seems like the smoke rises from my chimney in a neat square before dispersing into the surrounding backyards.
          My geodesic oven project: part 1, part 2

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          • #6
            Re: Chimney Primer

            All phyics and laws of nature aside, either will perform to your satisfaction. I think the transition from the oven to the throat of the chimney is more important and even that is miniscule.

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            • #7
              Re: Chimney Primer

              Where do they sell these flue liners? It's not a typical Home Depot thing, is it?

              Also, are the dimensions (ex - 8x8 inches) typically inside dimensions or outside?
              Mike - Saginaw, MI

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              My oven build thread

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              • #8
                Re: Chimney Primer

                I've never seen them in any Lowes/Home Depot around here. I bought mine at a brickyard where they sold everything a contractor would buy in building a house...builder's supply stores. 8X8 is the outside dimension.

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                • #9
                  Re: Chimney Primer

                  Where do they sell these flue liners? It's not a typical Home Depot thing, is it?
                  This is a brickyard or mason's supply item. In a cold climate, they should be readily available.
                  Also, are the dimensions (ex - 8x8 inches) typically inside dimensions or outside?
                  This is a sore spot with me. They are just a hair under eight inches on each exterior side. As you may know, code requires that they be enclosed in a masonry chimney with one inch of space around them. Brick builds in eight inch increments, so building a proper brick enclosure means scores of little two inch slugs. Here's what I think. Build it with half bricks, leaving a twelve inch hole, or two inches on each side. If you aren't in an earthquake zone you should be fine (and if you are, you shouldn't be building out of brick anyway).
                  My geodesic oven project: part 1, part 2

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                  • #10
                    Re: Chimney Primer

                    Good thing there are no codes for outside barbecue structures where I live.

                    Comment


                    • #11
                      Re: Chimney Primer

                      Bruce. I played around with your calculator. If you intend on a tapered chimney, like those pictured above, what dimensions do you enter? Top dimension or bottom?
                      Mike - Saginaw, MI

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                      My oven build thread

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                      • #12
                        Re: Chimney Primer

                        Mike
                        I'm not sure. The dynamics of a tapered chimney are a lot different. Maybe one of the engineers on the forum can lend us a formula. If not, I would use the smallest measurement. That way you could be sure of the draw since a larger diameter would only draw better.
                        I'm of the over kill school of building. If it is worth building, it obviously needs crossbracing, large support beams, heavy brackets, deep footings etc. I just finished a septic system up grade that should work for 25 fiber eatin people for the next 100 years.
                        Sharpei Diem.....Seize the wrinkle dog

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                        • #13
                          Re: Chimney Primer

                          I'd luck to return to this discussion. Several questions.

                          First, regarding the above discussion on round vs. square, it appears that the square flue tiles are measured by the outside measurements, while the round are from internal dimensions. No logic, but I'm sure there is a reason. As it turns out, according to Superior Clay website Superior Clay Flue Liners
                          an 8.5x8.5 square flue tile has an effective area of 49 sq inch, while the 8inch round has an area of 50 sq inch. Essentially the same.

                          How important is the taper as you go up.

                          I plan to build a tapered vent entry into my arches. Mines not done, but similar to this


                          The bottom dimensions will be about 4.5x 21.5 inches, tapering up to a dimension of 4.5x 12.5 inches.

                          Locally, I can find 8x13 inch flue tiles that will span the opening. This would be in increase in the effective area as vent opening transitions into the flue tile. Will this cause a problem?

                          Is it important to alter the flue tile for the chimney to continue to taper as it goes up, like dmun and Les did -



                          Even more importantly, can my arch support this weight?

                          Thanks in advance. This chimney stuff is giving me a headache.
                          Mike - Saginaw, MI

                          Picasa Web Album
                          My oven build thread

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                          • #14
                            Re: Chimney Primer

                            [QUOTE=mfiore;43473


                            Even more importantly, can my arch support this weight?

                            Thanks in advance. This chimney stuff is giving me a headache.[/QUOTE]

                            I too, would like to use clay flue tiles on my oven, up to about 3' with the stainless insulated chimney inside of the tile extending 4' above that, both of which go through the top of a redwood pergola, but I don't know if the exhaust manifold/oven arch can support the weight. Any comments? Thanks

                            marco

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                            • #15
                              Re: Chimney Primer

                              would like to use clay flue tiles on my oven, up to about 3' with the stainless insulated chimney inside of the tile extending 4' above
                              I'm confused. Why would you use a stainless liner inside a clay liner? What goes on the outside?
                              My geodesic oven project: part 1, part 2

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