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Help with vent floor size - Forno Bravo Forum: The Wood-Fired Oven Community

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  • Help with vent floor size

    I'm stumped with the amount of space I need to leave for the floor of the vent. I read (and reread) James' sticky posting about size of the hearth.

    I'm planning a 36 inch oven. The formula James posted translates to :
    36"(floor)+9 (walls)+6 (vent floor)+5 (rear insulation)+.5 (one wall)+12 (landing)= 68.5".

    After looking at all photos I could find of the vents and floors, it looks like the majority of the floors are 12" to 18" under their brick arches vs the 6 inches used in the formula. Am I misinterpreting the 12 inch oven landing portion?

    I'm getting ready to pour the vermiculite concrete portion of the hearth on a corner installation and want to make sure that I get enough coverage under this area., but not necessarily cover unnecessary areas as Vermiculte is hard to find around here.

    Can somebody please turn on the lights for me?

    Many thanks
    GJBingham
    -----------------------------------
    Everyone makes mistakes. The trick is to make mistakes when nobody is looking.

    -

  • #2
    Re: Help with vent floor size

    George,
    according to James plans the vent should be only 4" not 6"...here my plan for a corner oven with a dome of 37.5" internal...hope the picture would be visible...any input/suggestion from anybody? I am planning to do the foundation this week end if the weather will allow it...thanks!
    Carlo
    Attached Files
    Ciao Carlo

    Cost spreadsheet updated 4/22/08

    Pictures updated 5/28/08

    Comment


    • #3
      Re: Help with vent floor size

      Did you do a cross-section, which would be diagonal in this case? Remember that the dome is a hemisphere, and will be smaller at the level of the vent, giving you more room. A four inch vent slot seems really small to me.
      My geodesic oven project: part 1, part 2

      Comment


      • #4
        Re: Help with vent floor size

        As David notes, you have vertical space, where the vent can go up and a little back if you need the space. If you go 4"x8" with the vent opening, I think it will be OK. Double check my high school math, but if 4"X8" is 32 sq in, and 4"x6" is 24 sq in, and a 6" pipe is 3.14*9=28 sq inches, you should be OK.

        Also, do any of our engineers have a good description of how the physics of the vent work. My layman understanding is that the heat of the vent creates a low pressure area that effectively sucks air up. Does the idea of a more narrow vent opening in front of the chimnney stack create a more powerful draw?
        James
        Last edited by james; 10-25-2007, 01:11 PM.
        Pizza Ovens
        Outdoor Fireplaces

        Comment


        • #5
          Re: Help with vent floor size

          Originally posted by james View Post
          My layman understanding is that the heat of the vent creates a low pressure area that effectively sucks air up.
          James
          I wasn't sure how this worked ... so I looked it up.

          Here is a quote from this link:
          The Chimney


          "Chimneys operate on the principle that hot air rises above cold air thus, the hot gas in a chimney rises because it is less dense than the air outside the house. The rising gas creates a pressure difference called draft, which draws combustion air into the appliance and expels the exhaust gas outside through the chimney. The hotter the gas compared with the air outside, the stronger the draft."

          That helped me some.


          Dave
          My thread:
          http://www.fornobravo.com/forum/f8/d...ress-2476.html
          My costs:
          http://spreadsheets.google.com/pub?k...Xr0fvgxuh4s7Hw
          My pics:
          http://picasaweb.google.com/dawatsonator

          Comment


          • #6
            Re: Help with vent floor size

            OK. High school science. :-)

            The electrons get excited and spread out, making hot air lighter by volume than cold air, so it rises. It's the rising air that creates the pressure difference.

            Cool. Thanks Dave.

            James
            Pizza Ovens
            Outdoor Fireplaces

            Comment


            • #7
              Re: Help with vent floor size

              That's very interesting and useful, Dave.
              Since the draft of the chimney pulls in combustion air it would seem that the size of the arch and door relative to the chimney would also be important and again in relation to the oven size.

              would it be fair to say that the vent/chimney works as a pump?

              Comment


              • #8
                Re: Help with vent floor size

                I guess so.

                The relationship is very important.

                To big a chimney... to much draft.. hard-to-heat oven.

                To small.. I guess not enough draft... hard-to-heat oven.

                etc. etc. door size... 63%.etc.

                Thank goodness lots of others have already built and shed some light on the subject for the good of all of us following in their footsteps.
                My thread:
                http://www.fornobravo.com/forum/f8/d...ress-2476.html
                My costs:
                http://spreadsheets.google.com/pub?k...Xr0fvgxuh4s7Hw
                My pics:
                http://picasaweb.google.com/dawatsonator

                Comment


                • #9
                  Re: Help with vent floor size

                  Originally posted by james View Post
                  The electrons get excited
                  James
                  Electrons are cool..... or hot I guess in this case!
                  My thread:
                  http://www.fornobravo.com/forum/f8/d...ress-2476.html
                  My costs:
                  http://spreadsheets.google.com/pub?k...Xr0fvgxuh4s7Hw
                  My pics:
                  http://picasaweb.google.com/dawatsonator

                  Comment


                  • #10
                    Re: Help with vent floor size

                    Originally posted by james View Post
                    As David notes, you have vertical space, where the vent can go up and a little back if you need the space. If you go 4"x8" with the vent opening, I think it will be OK. Double check my high school math, but if 4"X8" is 32 sq in, and 4"x6" is 24 sq in, and a 6" pipe is 3.14*9=28 sq inches, you should be OK.

                    James

                    James's math is correct. If you are going with 6" pipe then you should go with the 4"x8". You want the chimney entrance to be larger than the area of the flue. This is to take advantage of Bernoulli's law, increasing speed of gas decreases pressure, this will increase the pressure drop of your vent creating a better draw.
                    Wade Lively

                    Comment


                    • #11
                      Re: Help with vent floor size

                      George,

                      I think we are mis-understanding your question. I interpret it how much space you need from the dome opening to your landing area. If that's the case, here are my numbers.

                      I have 12 inches of landing area. I am allowing 4 inches for decorative brick. My front arch brick is 5 3/4 inch, the vent opening is 6 inches and the back arch is 4 1/2 inches. All said; from the front of the landing to the inside of the dome will be 32 1/4 inches.

                      Les...
                      Check out my pictures here:
                      http://www.fornobravo.com/forum/f8/les-build-4207.html

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

                      Comment


                      • #12
                        Re: Help with vent floor size

                        I added Les measurement to the picture he has on the web so we can graphically confirm our understanding and use as guide. Hope this helps, check the link below.
                        Carlo
                        Attached Files
                        Ciao Carlo

                        Cost spreadsheet updated 4/22/08

                        Pictures updated 5/28/08

                        Comment


                        • #13
                          Re: Help with vent floor size

                          That's it.
                          James
                          Pizza Ovens
                          Outdoor Fireplaces

                          Comment


                          • #14
                            Re: Help with vent floor size

                            Originally posted by asudavew View Post
                            Here is a quote from this link:
                            The Chimney
                            Here is another quote from that link:

                            masonry chimneys need a foundation that extends below the frost line, which is a few metres below grade in most of Canada.
                            A few? Two? Three? Half a dozen? Hell, why not dig down to bedrock?
                            My geodesic oven project: part 1, part 2

                            Comment


                            • #15
                              Re: Help with vent floor size

                              I hear ya David - and THAT's funny!!! I think what they are implying is that two feet short of magma is best...

                              Les...
                              Check out my pictures here:
                              http://www.fornobravo.com/forum/f8/les-build-4207.html

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

                              Comment

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