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Thermal break between dome and chimney box

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  • Thermal break between dome and chimney box

    I've poked around the site and did not find much on this subject. Or I looked in all the wrong places....

    It seems like a pretty big heat sink is attached at the front (or back) of many ovens in the form of an un-insulated chimney.

    Has any one used extra insulfrax or other insulation to thermally isolate the chimney similar to the floating hearth concept? Does the general consensus think it will make a reasonable difference in heat retention?

    Thanks

    Christo
    My oven progress -
    http://www.fornobravo.com/forum/f8/c...cina-1227.html
    sigpic

  • #2
    chimney

    Most people use triple wall engineered flues for their ovens, which have a LOT of insulation between the inside and the outside. They are designed to withstand a chimney fire of highly flamable creosote without setting the house on fire (you see a lot of them installed in a wood frame riser).

    Even masonry chimneys are supposed to have a half inch of free air space for insulation between the refractory flue tile and the four inches of masonry surrounding it.

    So yes, the chimneys are supposed to be insulated.
    My geodesic oven project: part 1, part 2

    Comment


    • #3
      Thermal Break

      I'm not doing very well communicating my question.

      I'm asking if it makes sense to break the oven in half and provide separation between the oven half and the pizza entry tunnel (piehole) that supports the chimneys on most ovens.

      Sort of an addendum to the island hearth discussion - breaking more of the oven off and isolating it.

      In most ovens I've seen here, the pie hole and chimney support extend about 16 inches from the opening in the dome. That seems like a lot of masonry (even if you exclude a triple wall chimney duct) that could suck heat from the dome. There is also extra flooring bricks that could pull heat from the floor.

      I was considering leaving a 1 inch gap at top, bottom and sides around the dome opening and filling it with castable refractory insulation after I build the chimney section.

      I guess the real question here - do oven entrys feel warm to the touch, does the floor of the oven entry pull significant heat from the oven floor?

      As always, Thanks!!!

      Christo
      My oven progress -
      http://www.fornobravo.com/forum/f8/c...cina-1227.html
      sigpic

      Comment


      • #4
        The oven entry feels warm, but per our fireplace discussion, remember that there is radiant heat from the oven glowing through the entry. Perhaps an engineer could answer the question whether the heat conducting from the oven into the masonry of the landing area and chimney vent areas is significant compared with radiant heat loss and convection (cold air entering through the opening). From the practical standpoint, I doubt you have much to gain here from the pizza making standpoint. The real question would be whether this improves retained heat baking.

        Comment


        • #5
          Re: Thermal break between dome and chimney box

          I am designing a thermal break between my dome and my chimney/entrance.

          I have attached a few photos of where I am at this time but looking for advice and input before I start cutting brick.

          The first photo shows a 45 degree chamfer below the floor leaving a 1 inch vertical surface. I will be filling the void with FB Board in the chamfered area and the entrance brick will have a similar but opposite angle cutout. I will be leaving about a 1/8 inch gap in the floor to fill with ash.

          My real question comes regarding how to handle the thermal break on the arch. I believe I have three options.

          1) Looking at the remaining photos you can see that I have drawn 2 arcs on the face of the inner arch. The lighter inner arc represents the location of the interior the brick for the outer arc; this leaves a 1 inch landing for the sealing face of the inner insulating door.

          The darker outer line represents where I would cut a relief chamfer into the inner arch similar to what I have done to the floor in photo one.

          The brick at the base of this arch is a minimum of 5 inches deep going into the oven on the inside of the arch and gets up to 6 inches at the outside edge. At the top of the oven the bricks are 7 inches minimum deep and become 8 inches deep at the thickest point. So I believe that the internal arch is strong enough to be able to deal with the chamfer.

          you'll notice in the photos that I have a tape measure pointing to the lower part of the arch and considering not chamfering this area because I have gone to the trouble of buttressing this area with additional bricks. The thermal loss in this area albeit potentially significant may be offset by the weakening of the inner arch. I would still maintain the 1/8 inch gap in this area between the inner arch and outer arch.

          This chamfered area would then be filled with FB insulation.

          2) The second option would be to not modify the inner arch. And have a similar design associated with the outer arch (chimney area) putting the chamfer on the outer arch brick.

          The problem I perceive by putting the chamfer on the outer arch is that it may weaken the area underneath the chimney stack. I will be using a double wall (no internal insulation) stainless steel stack so the weight is not significant.

          3) the third option would be to put a small chamfer going back about a half-inch into both the inner arch and the outer arch transitioning to a flat parallel to the existing face.

          This third option would be a little more difficult to create but it is an option.

          One other thought, I'm not sure if FB insulation is the best choice for this break, other options are insulating firebrick which would be very difficult to cut, FB board which may not be so difficult to cut but still difficult, or Perlcreate, or vermicreate. clearly the FB insulation or the vermicreate Perlcreate options would be the easiest to install.

          In any case I will be putting a sheet of cardboard as a spacer between the bricks that will burn out after a few firings.

          Thanks for your thoughts and advice in advance.

          Chip
          Attached Files
          Chip

          Comment


          • #6
            Re: Thermal break between dome and chimney box

            With regard to what material to use for the insulation in your arch thermal break, why not cut a strip of ceramic fiber blanket and stuff it in the gap, then seal it in with mortar?

            Love the addition, but will it be worth it in terms of heat retention? What do you plan on baking with all that insulation?

            Comment


            • #7
              Re: Thermal break between dome and chimney box

              Originally posted by calipizzanapoletana View Post
              With regard to what material to use for the insulation in your arch thermal break, why not cut a strip of ceramic fiber blanket and stuff it in the gap, then seal it in with mortar?

              Love the addition, but will it be worth it in terms of heat retention? What do you plan on baking with all that insulation?
              I would like to use my oven all year round and with temps in the -20F range in the winter I will be trying to retain all the heat I can the wintertime.

              I feel that optimizing loss will be a significant issue here in winter.

              Chip
              Chip

              Comment


              • #8
                Re: Thermal break between dome and chimney box

                Folk are always banging on about insulation being so important, yet attach a heavy uninsulated entry to the front of the oven and another heavy uninsulated decorative arch to the front of that. I believe it is a great idea to isolate these from the main oven to reduce heat conduction as well as allowing the oven to expand reducing pressure and resulting cracking problems around the entry and decorative arch. I have been advocating this for years. My approach was to make the entry as light as possible (8 Kgs cast refractory) and isolate the decorative arch with an 8mm gap filled with vermicrete.
                My outer arch, after two hours firing, is still only warm to the touch so something is working ok.There is some discussion about all this on a thread somewhere.Keep searching.
                Kindled with zeal and fired with passion.

                Comment


                • #9
                  Re: Thermal break between dome and chimney box

                  Chip,
                  Kudos for taking a proactive approach to what David accurately describes as a great idea to isolate a heavy uninsulated entry (giant, exposed heat sink) from the main oven.

                  I have two thoughts on this:

                  First, a thermal, or heat break (not brake) is a good idea, IMO. Not for baking pies, but for extended (multiple-day) cooking using residual heat. I will be going with a heat break closer to your second option, which I think is easiest and best since it doesn't compromise your inner-arch and a hemispherical entryway is inherently stronger than a straight-sided entryway. My original plan was a 1" gap filled with Insblock 19 between inner arch/entryway, but tscarborough's recommendation of a simple 1/4" air gap has merit. I have also considered filling the gap with ceramic blanket or ceramic rope since there won't be any exposure to the inside of the entryway and food in any way. At this point I am still undecided on which direction to go.

                  Second, since heat transfer rate is partially a function of the difference of the two entities swapping heat, over time, a hotter entryway will wick less heat from the oven than a colder entryway will. Safety issues aside, I believe a hot, well-insulated entryway heated primarily by exhaust gasses will benefit from an outer door and more thermal mass provided a heat break exists.

                  There are a few builders here who have reported (no cook/insulated door) residual heat losses of -100F per day which I think is fantastic. This is the primary reason (along with the $350/cord cost of oak locally) I'm going with a thermal break. I am interested to see which direction you choose.

                  John

                  Comment


                  • #10
                    Re: Thermal break between dome and chimney box

                    Originally posted by GianniFocaccia View Post

                    Second, since heat transfer rate is partially a function of the difference of the two entities swapping heat, over time, a hotter entryway will wick less heat from the oven than a colder entryway will. Safety issues aside, I believe a hot, well-insulated entryway heated primarily by exhaust gasses will benefit from an outer door and more thermal mass provided a heat break exists.

                    John
                    John,

                    In order to reduce the thermal loss via the flue I am considering using a chimney cap damper - one of the products is called a Chim-A-Lator it is a fairly simple device that caps the chimney and seals it to reduce down drafts and flue core cooling. This would add to the thermal retention of the entry.

                    I am still thinking over my options on the thermal break. The 1/4 inch gap seems quite large at the floor but I think the sides could tolerate this type of air gap. As long as they were backed up with tightly packed insulation, insulation brick or board. I would not want any flames to get out via this large a gap.

                    I still think that I will not compromise the integrity of my inner arch by doing the changes in options 1 or 3 because the bricks are so integrated into the structure of the dome and are significantly oversize compared to the thickness of the dome.... but I am still thinking this over.

                    Thanks for your input.

                    Chip.
                    Chip

                    Comment


                    • #11
                      Re: Thermal break between dome and chimney box

                      I didn't find much on this subject either. But did manage to put heat brakes in three different places on our oven entry. You can see the brakes and read my comments citing actual temperatures here.

                      If I could go back, I'd make the air space between the dome and the inner arch a bit wider.
                      Last edited by Lburou; 08-19-2011, 07:12 PM.
                      Lee B.
                      DFW area, Texas, USA

                      If you are thinking about building a brick oven, my advice is Here.
                      Our One Meter Pompeii Oven album is here.
                      An album showing our Thermal Breaks is Here.

                      I try to learn from my mistakes, and from yours when you give me a heads up.

                      Comment


                      • #12
                        Re: Thermal break between dome and chimney box

                        Originally posted by Lburou View Post
                        I didn't find much on this subject either. But did manage to put heat brakes in three different places on our oven entry. You can see the brakes and read my comments citing actual temperatures here.

                        If I could go back, I'd make the air space between the dome and the inner arch a bit wider.
                        You said you would have a 1/2 inch air gap between the inner arch and the flue but when I look at your photos it looks like it might be about 1/2 inches already, what type of gap do you have now?


                        Chip
                        Chip

                        Comment


                        • #13
                          Re: Thermal break between dome and chimney box

                          The 1/4 inch gap seems quite large at the floor but I think the sides could tolerate this type of air gap
                          True to form, I didn't fully explain the configuration of my gap. I plan to maintain a continuous brick surface between the arch and entryway via the entryway brick that butts up against the inner arch (see illustration) The contact area in this graphic has been set at 1" and the gap here is 1/2". This is my idea of a thermal break. I could make the contact area less than an inch but am concerned that over time thermal cycling may make the portion of contacting brick brittle and subsequently break. My other thought was to leave a true (1/2" gap) and fill it with say, a 1/2-1" strip of 1/8" -thick stainless steel on the inside and insulation (or not) behind the stainless.

                          My plan right now is to incorporate a stainless steel entryway floor so there will be no gap between the entryway and oven floor.

                          Any thoughts?
                          Attached Files

                          Comment


                          • #14
                            Re: Thermal break between dome and chimney box

                            Originally posted by GianniFocaccia View Post
                            . My other thought was to leave a true (1/2" gap) and fill it with say, a 1/2-1" strip of 1/8" -thick stainless steel on the inside and insulation (or not) behind the stainless.

                            My plan right now is to incorporate a stainless steel entryway floor so there will be no gap between the entryway and oven floor.

                            Any thoughts?
                            The idea of stainless at the entry is a great idea from more than one perspective. The entry floor can be easily cleaned and the underlying area could be entirely insulation no need for conducting brick mass there.

                            The stainless could be bent down at the edges and provide a nice clean edge that would not catch tools and it would provide a turned down wrap over the insulation below.

                            I have been toying with the idea of a metal bumper strip at the edge of the inner arch to prevent what others have experienced and that is excessive chipping of the edges of the inner arch.

                            If a piece of stainless angle was placed at the corner of the inner arch and wrapped around the inner arch it would create a nice strong corner. It could be built to fit loose enough to not cause any thermal expansion issues.

                            The problem would be fabrication of this very unique shape. I am not sure if you could heat angle with a torch and get it to bend into such a radical curve.

                            See drawing for below

                            The other possibility is not to use angle but a larger sheet or a number of smaller sheets that just sit between the inner arch and the entry arch. The material does not really need to wrap around the corner into the entry of the oven it just needs to protect the corner. Cutting the stainless to match the arch would be quite easy. And multiple pieces could be cut to lock together to prevent shifting. Additionally attaching a piece of insulation gasket rope to the stainless should be a piece-o-cake and further secure the stainless should they be multiple pieces.


                            Chip
                            Attached Files
                            Last edited by mrchipster; 08-19-2011, 08:28 PM.
                            Chip

                            Comment


                            • #15
                              Re: Thermal break between dome and chimney box

                              Originally posted by mrchipster View Post
                              You said you would have a 1/2 inch air gap between the inner arch and the flue but when I look at your photos it looks like it might be about 1/2 inches already, what type of gap do you have now?


                              Chip
                              Its about 3/16ths of an inch. What I want to communicate is that I wanted more of a temperature difference between the dome and the inner arch. I don't think my design hurt the performance of the oven....But, a slightly larger gap could make a larger temperature difference.

                              The sixteenth inch gap between the granite and firebrick on the entry floor gives about half as much temperature difference as the difference between the inner arch and the dome, ergo, the larger the gap, the slower the heat loss. The question is, how big a gap is too much? And, where does the thermal brake do the most good?
                              Lee B.
                              DFW area, Texas, USA

                              If you are thinking about building a brick oven, my advice is Here.
                              Our One Meter Pompeii Oven album is here.
                              An album showing our Thermal Breaks is Here.

                              I try to learn from my mistakes, and from yours when you give me a heads up.

                              Comment

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