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Hearth Design Philosophy - Forno Bravo Forum: The Wood-Fired Oven Community

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  • james
    started a topic Hearth Design Philosophy

    Hearth Design Philosophy

    This is a sticky posting that describes the logic behind the Forno Bravo hearth design.

    I think of the hearth and cooking floor assembly as serving three functions:

    1. Structural support -- holding the oven up and not sagging. Nothing thermal here, which is why you can use standard concrete (cheap) with rebar.

    2. Insulation -- keeping heat in the oven and floor. That's the insulating concrete layer, with vermiculite held together with portland cement.

    3. The cooking floor -- which is thermal. You want it to heat up, hold heat, and efficiently re-charge the heat in the floor from your live fire. The firebrick on its flat side gives you enough mass for typical backyard cooking and baking, and doesn't have too many seams. The Forno Bravo ovens have 2" floors that come in pie-shaped pieces. If you really want a little more mass under your floor (don't forget that commerical pizza ovens only have 3"-4" floors), you can add more mass under the floor, either with a second layer of bricks, or a poured disk of refractory mortar. That's the Island hearth (the extra mass under the cooking floor is surrounded by vermiculite concrete).

    That's why we have formally changed the recommended hearth design. For most installations it is good to put the cooking floor directly on the insulating layer, and if you want more mass you can add it, while taking advantage of a more efficient design.
    Last edited by james; 06-08-2006, 04:58 PM.

  • Aquaponicsguru
    replied
    Re: Hearth Design Philosophy

    I have seen glass bottles with perlite/vermiculite used for cob type pizza ovens. Anyone have any opinions on using glass bottles with a p/vcrete mix?

    Leave a comment:


  • Georgel
    replied
    Re: Hearth Design Philosophy

    Door shape: Is there and advantage of the rounded doors? My house has straight lines and I am thinking of a rectangle door. Is this going to be a problem? 47 inch oven with16 inch dome height and 10 inch high by 20 inch door.

    Leave a comment:


  • C5dad
    replied
    Re: Hearth Design Philosophy

    Originally posted by h12rpo View Post
    Someone has said have the floor 'floating' what exactly does that mean? I was intending to bed mine down using a quite wet mix of homebrew
    ( i intend to have my floor inside the soldier first chain).
    A floating floor is free to move with heating and cooling. Please do not bed your bricks and make sure there is a gap between the floor and dome (about the thickness of corrugated cardboard) to allow expansion of the floor outward without damaging the dome.

    Leave a comment:


  • C5dad
    replied
    Re: Hearth Design Philosophy

    Originally posted by Cappa View Post
    Hi,

    Looking to get some advice on the Hearth design in particular the insulation medium. I have purchased Vermiculite to blend at the suggested ratio of 5:1 with portland cement. Is the insulation cement strong enough to place the floor and dome directly on top of or is there an extra layer required to provide stiffness?
    Yes, it is more than strong enough to support your dome. You do not want your dome on up insulated material as the heat will get "sucked" out -that is a highly technical term ya know.

    Chris

    Leave a comment:


  • Cappa
    replied
    Re: Hearth Design Philosophy

    Hi,

    Looking to get some advice on the Hearth design in particular the insulation medium. I have purchased Vermiculite to blend at the suggested ratio of 5:1 with portland cement. Is the insulation cement strong enough to place the floor and dome directly on top of or is there an extra layer required to provide stiffness?

    Leave a comment:


  • adm
    replied
    Re: Hearth Design Philosophy

    Hi AT,

    Thanks for the info. Maybe I am using different terminology, being from the UK and all that.....I have attached a quick diagram of my proposed construction - please excuse lack of scale or detail as I am still earning SketchUp for this project....



    I hope this all makes sense. The "Thermalite" blocks are lightweight, insulating concrete blocks that are very common here. The Ceramic Fibre insulating board is what I think you all call "FB Board". Damp proofing membrane is a polythene sheet that will go under the base slab itself - I would also use another layer of DPM after the first ocurse of concrete blocks in the support walls.

    Cheers,

    Alasdair

    Leave a comment:


  • ATK406
    replied
    Re: Hearth Design Philosophy

    Originally posted by adm View Post
    Hi All,

    For the ceramic board - should that be cut to the same dimensions as the inside of the hearth itself with only the firebrick hearth on top of it, or should it cover the entire base of the oven with the dome itself also bearing on it? .....

    Anyway, as you can see I am still at early planning stages but would welcome any advice!
    Definitely insulate your dome walls from your support slab. It sounds like you are planning to use lots of insulation and that is a good thing. I must admit I'm a little confused by your description of the many layers of your slab/insulation/hearth floor. Have you sketched this out? Maybe a simple diagram of your plans and the associated materials would clear things up.

    AT

    Leave a comment:


  • adm
    replied
    Re: Hearth Design Philosophy

    Hi All,

    First post....so be gentle! I am in the UK....so metric dimensions I'm afraid!

    I am still at very early planning stage, currently deliberating between modular and brick built oven. I am leaning heavily toward full brick build though and am planning around that....

    I am currently trying to draw out my base and hearth, so could do with some advice....

    The base itself will be on a poured slab reinforced with rebar. I will put membrane down under the slab. Walls will be concrete block filled with concrete and rebar.

    On top of that, I will pour another slab for the base of the hearth. I am thinking of making this 100 or 150mm thick, then laying a course of thermalite blocks on their side on top of that. On top of those, I would put 50mm of the ceramic fire block board and then finally on top of that the firebrick for the hearth itself. I am leaning towards having the firebricks on their side as opposed to flat to get some extra thermal mass.

    So - does that sound about right? Or am I missing some vital ingredient? I see lots of vermicrete used in bases, but if I use thermalite blocks, the ceramic board and firebricks on edge, do I need it?

    I like the idea of having the firebrick hearth cut to fit inside the oven dome, with a soldier course of firebricks around the outside rather than have the dome bearing on the firebrick floor....which gives me another question:

    For the ceramic board - should that be cut to the same dimensions as the inside of the hearth itself with only the firebrick hearth on top of it, or should it cover the entire base of the oven with the dome itself also bearing on it? I am not sure how strong this stuff is in terms of compression and there would be a lot of loading on it from the dome.

    Anyway, as you can see I am still at early planning stages but would welcome any advice!

    Thanks,

    Alasdair

    Leave a comment:


  • h12rpo
    replied
    Re: Hearth Design Philosophy

    Someone has said have the floor 'floating' what exactly does that mean? I was intending to bed mine down using a quite wet mix of homebrew
    ( i intend to have my floor inside the soldier first chain).

    Leave a comment:


  • percy
    replied
    Re: Hearth Design Philosophy

    dino thank for the drawing. it gave me more ideas and you made my project easier. thank you. Percy

    Leave a comment:


  • Dagored154
    replied
    Re: Hearth Design Philosophy

    I would put more lintels in to reduce the gaps and it should work.

    Leave a comment:


  • Dagored154
    replied
    Re: Hearth Design Philosophy

    In speaking with a local hardscape builder who has built many ovens he has recommended that I put splits on the cooking surface. if the cooking surface gets damaged or worn out I can then easily replace them. it seems plausible, what do you all think? How do you actually replace bricks inside the oven ten years later? that would seem impossible to do.

    Leave a comment:


  • RobDodds
    replied
    Re: Hearth Design Philosophy

    Thanks - I think it would work - open span for my base is 1.1 metres so I reckon 4 lintels would only leave spans of 5-6" of Hardibacker (sorry to mix metric & imperial!). If it doesn't look to be enough I could always double up the Hardibacker and epoxy the boards together to make a constrained layer for strength. Planning to use a modular oven so probably less weight than a brick Pompeii.

    Leave a comment:


  • azatty
    replied
    Re: Hearth Design Philosophy

    It's all about the open span (distance between beams). You'd have to know the maximum load you can place on the backer board across the open span. My gut says you could probably get away with it in a modular design because you're working with larger pieces that will spread the load over larger areas. I wouldn't trust the design for a point load, though.

    Leave a comment:

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