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How do I calculate the height/width of the door/vent arch?

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  • How do I calculate the height/width of the door/vent arch?

    I'm planning the next steps in my build whilst my Vermiculite concrete insulating base cures. I've got the space for an external dome diameter of about 1.0m (40"). I'm planning a simple hemisphere with no soldiers/sailors in the dome wall construction. Proportionally, what should the height/width be for the door/vent arch compared to the external dome height/width?

    Thanks in anticipation!

  • #2


    • #3
      Petter, Thanks again for your post. I've checked out the link and that looks very useful. However, it always confuses me when folks talk about a 42" or 36" oven! Is this the internal or external measurement?


      • #4
        Woody, the most common reference dimensions for oven builders here are internal dimensions, so the 42 and 36 inch refer to inner diameter of the dome, and the "ideal" height for a door opening is 63% of the dome height (from floor to top of dome, typically half the diameter) Have you looked at the Forno Bravo plans yet? They are a good starting point and explain the basics pretty well. Starting with outside dimensions is unconventional and usually only done if someone has tight space constraints. If you are going to use the standard half brick approach that will make your ID 9" smaller than the OD, and if you use a minimum of 3" of insulation you are down another 6" making the ID of a 40" outside diameter oven a very small 25". You might want to step back and evaluate your plans. If you can't change anything now you might want to look at casting a thin oven..
        My build thread


        • #5
          Thanks, that helps clear things in my mind. I have had a look at the Forno Bravo plans and their simplicity has given me some good basic understanding.

          Where I am at the moment is....
          1. A standard concrete base has been cast and a 4" (100mm) Vermiculite insulation layer laid on top.
          2. My working area is constrained and it's basically a rectangle with the corners knocked off to accommodate paths, walls, etc. The basic dimensions are 40" (1.013m) X 50" (1.27m) which will allow me to have the dome and space for the door/vent arch.
          3. Yes, half brick construction so that's about 4.4" (111.5 mm).
          4. My ceramic blanket, now it's arrived, is about 1.2" (30 mm) thick and I have enough to do two layers.
          5. I'm thinking of a 2" (50 mm) Vermiculite 8:1 mix over this.
          6. Finishing with a cement screed of about 1" (25 mm).
          Your estimate of 9" wasn't far out! Are there any savings on the thickness of these layers that can be made? How would a thin cast oven compare and is it just the one layer?

          Any thoughts would be very helpful at this stage.


          • #6
            I am not sure you understood what I was trying to say. The estimate of 9" was only for the brick shell. Remember that you are working with a diameter, where every layer is on each side of the oven and you have to double the dimensions to see what your ID would be. If your bricks are 4.4" that takes 8.8" off of your total. Two layers of 1.2" thick insulation (2.4 each side) comes up to 4.8". The vermicrete @2" will be 4 and the final coat will be 2. Add these up and you have a reduction of 19.6" from your OD of 40" and you end up with 20.4" (smaller than what I originally estimated.
            What I highly recommend is doing a search for posts by DavidS and read all that you can find. David is the resident expert here in both cast ovens and smaller footprint ovens, and although the smaller ovens fall short of big thick ovens in terms of long term heat retention, they excel at fuel conservation, fast heating time, and maximizing internal diameter for a set outside diameter. Small thin(er) ovens are way out of my tiny area of expertise, but if I was going to build one I would follow the advise I gave you and study what David has had to say.
            My build thread


            • #7
              Many thanks for your ongoing reply! Apologies but I probably didn't make my response very clear. I see your calculations now and thanks for these. With my current plan, I'll have approximately 20" (0.5m) of dome thickness proposed leaving me with a similar internal diameter for the cooking area. So for the constricted size base area that I have, I'm not getting a decent-sized oven to cook in.

              I will certainly try and track down threads by DavidS for his advice.

              From my list of dome insulation and other coverings that I gave in my previous post, are there any that I could reasonably reduce and not lose a critical quantity of the insulation and protection from the elements?

              Thanks, as ever.


              • #8
                2" cast, 2" insulation blanket, 1" V-crete, 0.5" stucco = 100cm-11" = 72 cm inner diameter = decent cooking space.


                • #9
                  Thanks Petter. Looks like a plan.

                  What will this set up allow me to cook.....? Hopefully pizza! Will I be able to bake bread and will it hold enough heat to roast meat? I'm going to make an insulated door out of two sheets of galvanized 3mm steel with some ceramic blanket in between. Is there any science to the chimney cross-section area and length?

                  I've checked out some posts on a UK cast build which advises a two-part cast but is there anything you suggest I look out for?


                  • #10
                    This will make it possible to cook anything you want.

                    Don't use galvanized steel. The fumes are hazardous. Either plain hot rolled steel or stainless is good. If you put a ceramic fiber blanket between, make sure it is encapsulated since you should not eat fibers that falls out.

                    You can cast the dome and enterence arc separately. Just put a cardboard paper between. It will burn later, leaving an expansion gap.


                    • #11
                      Thanks for your reply Petter. Useful info on the galvanized sheet and door insulation. Thanks for that. I'll change my plans! I've just got to source needles and fibres now. I learn that the expansion gap between the two castings can be filled with Vermicrete, which is probably what I shall do.