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Raised/insulated chimney - Forno Bravo Forum: The Wood-Fired Oven Community



I'm Peter Reinhart! Ask Me Anything! Monday, February 15, 2016 7:00-8:00 pm EST

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Raised/insulated chimney

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  • Raised/insulated chimney

    Hi All
    I reread the thread on things people would do differently and there are three elements would like to avoid with a plan I have. Now the challenge is to explain my plan in such a way that you will understand. :-) Let me try:
    The first two issues has to do with isolating the chimney and starting with a wider chimney. (As I have tried to explain with pics, the numbers speak to the attachment).
    1. This shows the footprint of the 42” oven as standard but with the footprint of two extra pillars of normal brick biult on either side.
    2. Shows how I plan to put lintels on top of the pillars which will thus carry the chimney.
    3. Shows the gap which will now be between the inner arch & oven and the pillars on the sides as well as at the top the gap between inner arch and the raised chimney.
    4. If I now box the area and “cast” normal 5:1 vermiculite in this gap as well as around the oven I believe my chimney will be insulated and I will have a wide (and flat) surface on the lintels to gradually taper the chimney entrance.
    5. As a consequence and as this cutaway down the centre of the oven (side view from the middle line) shows, there will be and area of open vermiculite mix visible at the deep end of the entrance “throat” before the inner arch
    Questions are if this will work and secondly what kind of temperatures could be expected at the outside of the vermiculite "strip", because if not too hot, I could clad the vermiculite strip with a thin stainless steel plate (if the bare vermiculite looks bad)?

    The third issue is the two doors thing. If the temp on the outside of this vermiculite strip (at the start of the outer throat) is low enough, I can build a sliding door (steel plate) frame (u channel) in-between the throat wall and the outer vermiculite into which a draft door can work as vertical sliding door. A stopper peg could ensure I leave draft space at the bottom.

    Thoughts? (...and thanks )

    Kind regards

  • #2
    Re: Raised/insulated chimney

    I think your isolation of the chimney structure will work fine, I like that the dome is independent of the chimney in both structure and heat. I’m concerned about vermicrete being the right material above the doorway. It’s my guess that the temps are going to be too high and you’ll find that it breaks down. Someone else might have some input here relating to where concrete starts to fall apart. If I remember right 1500F to 1800F is the last straw for cement and the cycles of extreme heat will erode things bit by bit.



    • #3
      Re: Raised/insulated chimney

      Just for curiosity's sake, I saved some of the leftover wet 8:1 vermicrete from my original hearth insulation and let it dry in a plastic tub. The resulting vermicrete disk, about 10" in diameter and 3" thick had excellent compressive strength when squeezed between my thumb and forefinger, but broke away easily when I pinched the edges.

      My first thought is to sandwich insulating firebrick in between your entryway and inner arch, or use a castable insulating refractory.


      • #4
        Re: Raised/insulated chimney

        John, Gianni, I wonder too about the castable. If you go with the castable, think about doing the door at the same time. I like the idea of casting it laying down and then rotating and mortaring it into place. The beauty is that your door jamb can be nice and flat and the door as well. With these two flat you should get a fit that's just right and be able to tune the pair even before installation.



        • #5
          Re: Raised/insulated chimney

          Excellent idea, casting matching pieces. Being the 'aerated' properties that make up an insulating material, I wonder how robust such an installation would be.


          • #6
            Re: Raised/insulated chimney

            The vermicrete is very porous. Unless coated you will have smoke leaking out everywhere. The insulated firebrick mentioned by GianniFocaccia may be your best bet. This stuff is light and very soft and can be easily "carved" into any shape you need.


            • #7
              Re: Raised/insulated chimney

              Thanks folks, seems like I will have to search for insulating bricks.
              But last try before I give up on the vermiculite: How about the stainless "edge" just to keep it all in?


              • #8
                Re: Raised/insulated chimney

                Dolf, I think you could do it using stainless, it really comes down to the door design slowing the heat flow. I wouldn't want to create a stainless steel pathway for the heat to transit out.

                I inadvertently did this with Soapstone running under my insulated door. I got smarter for it, now I understand that Soapstone conducts heat 6X faster than brick does.

                If you use Stainless, try to verify that it won’t corrode in the higher temps. My concern here relates to stainless on ocean going boats. The SSteel I see around here isn’t really corrosion free, it's just much better than other metals.

                If you insulate everywhere else well, you may find that for your trouble you don't really gain what you'd hope.

                When I first used my oven I got 3+ days of cooking temps then I retrofit my entry floor with soapstone. I now get 2+ days :-(.

                I'm having a piece of granite cut to fit that is an inch short of the dome structure. In this 1" space I'll be using some spare rigid insulation. My door is 2" thick rigid insulation and with this new break in place I should be better than before I retrofit with Soapstone :-).

                The granite will match the counter tops much better as well.

                Last edited by SCChris; 06-25-2010, 01:21 PM.


                • #9
                  Re: Raised/insulated chimney


                  Do you have an 'outside' door to your oven as well as your inside one? I've been thinking about the heat loss a soapstone entryway floor may cause and wondered if an outside door would help. I'm also thinking a soapstone entryway can be effectively used to let roasts, etc 'coast', warm buns, simmer stocks etc depending on the floor temp. Any ideas on this?


                  • #10
                    Re: Raised/insulated chimney

                    I do have a second exterior door, but the heat from the entry flows right up and out the chimney. The thermal break between the oven floor and the entry is my solution. The board will transfer heat at 2% of the SStone and about 10% of what the brick does.


                    PS As long as the inner door was open the entry will still work well for warming and holding food.
                    Last edited by SCChris; 06-25-2010, 02:29 PM.


                    • #11
                      Re: Raised/insulated chimney

                      Thanks Chris. Easier to go for insulating brick now than to retrofit, so I will let go of the vermiculite/SS plan & do it properly

                      Thanks again to all you guys for sharing your thoughts