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Vent dimensions. Please Help. - Forno Bravo Forum: The Wood-Fired Oven Community


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Vent dimensions. Please Help.

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  • Vent dimensions. Please Help.

    Ok, have finished my dome and am about to start the entry/vent arch but I just have a few questions first. Any help would be greatly appreciated.

    I gather from previous posts, that having a vent opening which narrows to half its original size is optimal. I am planning to have my arch opening 18" x 9" which will narrow to an approx. 9" x 9" flue. By my calculations, this gives me the correct ratio. (please, feel free to correct me). The vent dimensions are pretty much the entire opening x the length of one brick. I see most ovens only have a vent depth of about half a brick. Is a 9" vent depth too much and will it affect the ability to draw well, or is the reason most people only use half a brick depth purely for space considerations?

    My last question has to do with the height that this transition happens over. I am planning my vent height to be approx. 7" or 8", then chimney on top. Is this vent too short, or as long it narrows by half then the height isnt realy a question.

    Thanks for any help.

  • #2
    Re: Vent dimensions. Please Help.

    My oven opening is almost exactly the same size as yours (same width,slightly lower). I have used an opening with half-brick debth (4") spanning the whole width of the opening (18"). The flue channel is made by a thin (1") brick(one each side), fitting into the opening debth and inclined to leave a 13" wide(still 4" deep) opening at the top. From there on I have mounted a steel transition pipe going from the rectangular opening to a round flue pipe. The steel transition pice is formed with the same contraction as the side bricks mentioned. The opening works fine when hot. During the first 10-15 min. of firing I need to cover the upper part of the opening to "guide" the smoke. I start firing as close to the opening as possible to assure that the flue channel is quickly heated. I think the gradual contraction (with enough "space" inside) and the fast heating of the steel part (and in fact the complete steel chimney) helps a lot. From the smoke pattern I see it also seems important to catch smoke from the whole width of the oven opening.



    • #3
      Re: Vent dimensions. Please Help.

      The length of the transition piece is a factor in good exhaust flow. This change in area is increasing the velocity of the exhaust gas and in turn reducing the pressure of the upper vent.

      If my memory serves I believe the optimum angle is approx 30 degrees. Given our spaces to work with go with the longest transition you can if you can't get 30.
      Wade Lively


      • #4
        Re: Vent dimensions. Please Help.

        Thanks guys. My vent transitions at about 35 degrees so I think that thats ok. As for my vent depth, is a full brick length too much? my entire arch is going to be 1 1/2 brick lengths deep, so 1 brick length for the vent, and 1/2 in front for the entry. Is this too much vent area and too little entry area?


        • #5
          Re: Vent dimensions. Please Help.

          My intuition tells me that 1 brick debth is more than enough. But my experience tells me that I would spend some of that debth to make sure that I cover the whole width of the opening arch.

          regards from Karl


          • #6
            Re: Vent dimensions. Please Help.

            the way you have done the arch/vent with a 1/2 brick and one full brick seem to be fairly standard
            Real men cook with fire
            My Oven and Fireplace Build


            • #7
              Re: Vent dimensions. Please Help.

              The width of mine is 6-7 inches. I got there from the inside opening of the flue pipe. It draws very well. If I have any issues it's with the cover, until everything is pure gas it tends to throw the smoke down and out.

              Check out my pictures here:

              If at first you don't succeed... Skydiving isn't for you.


              • #8
                Re: Vent dimensions. Please Help.

                I just measured my opening and it is 9.5" x 15.15". I just formed it and have yet to complete the dome so I don't know how it will draw but this is just to let you know you have company in your vent opening size. I'll be putting on anchor plate for the 8" round flue pipe. I betting on good draw.
                Cheers, Dino
                "Life is a banquet and most poor sons-of-bitches are starving to death." -Auntie Mame

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                • #9
                  Re: Vent dimensions. Please Help.


                  the fact that I'm not the only one with such "massive" vent dimensions does help. I'm going to move forward, only maybe amending the depth by about 1/4", more for structural reasons, to help support the flue transition above it.
                  Thanks for letting me know.


                  • #10
                    Re: Vent dimensions. Please Help.

                    Has anybody established a sound ratio for flue opening/chimney size to the internal volume of the oven.
                    I know from the kiln fraternity this is a constant dilemma and debate


                    • #11
                      Re: Vent dimensions. Please Help.

                      if you want to make a correlation which make sense you need to include the chimney height. A reasonably good correlation should be obtained by including flue opening size (which is normally proportional to oven size), flue opening/chimney size ratio (area) and chimney height. This is based on the assumption that the flue opening to chimney is made as a gradual transition which is the recommended method. If there is a lot of variation in dimensions in this transition this parameter need to go in as well.
                      Maybe we should put a spreadsheet on the forum and let everyone plot in their characteristic design (the three parameters) and state their experience (good, fair, bad) and we may see a correlation as data is collected?



                      • #12
                        Re: Vent dimensions. Please Help.

                        I think Karl makes some good points.
                        With all the previous discussions regarding dimensions of flue outlets no one has mentioned oven volumes or chimney heights. Certainly, these are major considerations.
                        Surely someone, sinch the days of Pompeii, has come up with some formula for calculating the measurements related to venting a wood fired oven.


                        • #13
                          Re: Vent dimensions. Please Help.

                          The larger the chamber the more efficient it seems to be, so a small chamber will need to have a larger flue relative to its size. I once made a very small kiln for firing ceramic jewellery to high temps (1200C) I had to redesign the downdraft flue a couple of times and it was never particularly efficient. It had a large flue for its relative size. Another analogy is the fuel consumption of a small car relative to that of a truck. The car with a 1000 cc motor will use way more than a 1/5th of the fuel of a 5000 cc truck.
                          Kindled with zeal and fired with passion.


                          • #14
                            Re: Vent dimensions. Please Help.

                            My 21" oven works well with a 5" flue, but I don't think you would need to go as big as 10" for a 42"
                            Kindled with zeal and fired with passion.


                            • #15
                              Re: Vent dimensions. Please Help.

                              I agree with you David. My experience with kilns and ovens suggests the same.
                              However there is a maximum that is often quoted by kiln builders - "It's always easier to block a hole than open one up".
                              Isn't there some record of someone formulating the relative sizes of openings to volume out there. There is in the kiln 'Bible' and they vary greatly!