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Furnace floor insulation

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  • #31
    I have another question

    What do you think about building a furnace floor with a thermo bank according to the video below?
    Does anyone have experience with such a procedure?


    • #32
      Seems to me that he would of been much better off using another two or three layers of insulation board.


      • #33
        Yes me, I also tried to add an extra inch of heat bank under my floor... Maybe there's others that have better techniques, but mine cracked already during construction and introduced an element of instability. As a result, I now have an 8mm crack in my floor (luckily it has stabilised). It is one of the very few things I would do differently: I would go for a brick floor. And if I want extra heat capacity, then I'll put them on their side.

        PS: The oven of this Korean YouTuber (including the heat bank) is practically a carbon copy of the design (and build instructions) of an Australian kit supplier. I don't speak Korean (?) so I can't tell if they are giving due credit.


        • #34
          I would also add more insulation, but otherwise I like it.

          Kvanbael - how thick was your refractory concrete that it cracked.


          • #35
            It cracked during masonry or when firing the furnace.


            • #36
              It looks like he’s used one inch of calcium silicate. Builders here consider 2 inches the minimum amount of underfloor insulation. Much depends on your climate, but encapsulating insulation board with aluminium foil can seal in moisture, should it get in. Because aluminium is so conductive any heat loss by condtuction is increased if it sits against dense refractory or concrete. So it’s value as an insulator is only effective at reducing heat loss by radiation so long as it is surrounded by air or lightweight insulation.
              Do not dry cut firebricks with an angle grinder. It is dangerous for the operator and anyone near the site to breathe the dust, you must use a wet saw. Also never operate an angle grinder without the guard. The dust will also kill the angle grinder pretty quickly.
              Builders here generally recommend 2 inches of floor brick for all purpose cooking. Thicker floors require longer to saturate with heat, meaning more time and fuel consumption. Some builders who want to bake lots of bread in their ovens go for a thicker floor.
              If the floorbricks are mortared down they cannot expand freely which places them under additional stress. Also it makes their removal extremely difficult if required down the track. as does building the dome directly on top of them. To overcome this problem a thin layer of a dry mix of sand and powdered clay is an alternative method of levelling the floor bricks if their thickness is uneven.
              Refractory mortar is expensive and difficult to work with because of its extremely short working time, particularly in hot weather, as it is very temperature dependant. In addition all refractory mortars I’ve seen and used recommend that joints should not exceed 3/16”.Most builders have preferred to use the cheap and user friendly “homebrew” (3:1:1:1 sand, Portland cement, hydrated lime and clay).
              Last edited by david s; 08-12-2022, 09:55 PM.
              Kindled with zeal and fired with passion.


              • #37
                I am in the phase where I have poured heat-resistant concrete on the insulation board (the insulation is thicker than in the video - 7 cm vermiculite concrete + 5 cm insulation board) and now consider whether to glue the floor bricks or lay them loosely on fine sand with clay.
                You have pros and cons for both.
                I must mention that I decided on a furnace with a dome of a half cylinder with a vestibule and a chimney on it (instead of an igloo - a half cylinder), I am limited by space, otherwise I would make an igloo.

                3D model without dome (my furnace) - attached photo.
                what would you change or keep (depending on the availability of space)
                Click image for larger version

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                • #38
                  Originally posted by bencuch View Post
                  Kvanbael - how thick was your refractory concrete that it cracked.
                  It was about 1.5 inch on top of 2 inch calSil. With limited supplies in full lockdown I used regular (Portland) concrete. I think it cracked while hammering the floor tiles in place. But it did’t open up until heating to high temps.


                  • #39
                    If you had placed the bricks on the floor loosely on fine sand mixed with clay (place of gluing), you think you would have avoided a crack.


                    • #40
                      Bencuh, I am just wondering why you want to use this strange method?

                      I cant see any benefits of casting one large slab of high mass under the floor bricks, It will crack!
                      Perhaps a one piece 2-3” thick floor, cast from high grade refractory, vibrated on a special high frequency table, with added carbon fibers and nylon fibers might survive in a very small round oven but a rectangle or square floor would be more of a challenge due to spot heating and cooler edges.

                      The only way to avoid cracking would be to make it with mutable pieces to allow for spot heating and expansion.

                      If you dry fit fire bricks, they will last a long time and if one does crack (very rare if fitted loose) you can just lift it out and replace it.

                      I have fitted two Italian built kit ovens that come supplied with a three piece 2.5” cast refractory floor. The main floor is in two pieces and the gallery is a separate piece, in both cases the floors have cracked into four or five pieces, the ovens are still working fine but it does not look nice and the system is not a good one compared to individual bricks.
                      I have also worked on two commercial indoor ovens that have thin (1”) tiles cement fitted over a cast floor and in both cases the tiles are cracked and the base has forced the sides out causing large cracks in the dome!

                      Virtually every high mass material will expand and contract when heated and cooled, it is always a good idea to think about this when building a wood fired oven.

                      If you want a lot of floor mass then you can fit the bricks on their edges giving you a 4” thick floor, but… this will take and age to heat up and burn a lot of fuel in the process!

                      Having said all of that….there are no pizza oven police on this forum so you must do what appeals to you most.


                      • #41
                        I think you are right and I should not have poured the floor with refractory concrete. As far as I have to pour it, do you think it will be better if I lay the bricks loose on sand with clay?