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Heat Drops fast! - Forno Bravo Forum: The Wood-Fired Oven Community

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Heat Drops fast!

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  • Heat Drops fast!

    Hi everyone,
    I think maybe this is a curing issue but this is how it goes.
    I have one layer of 25mm on the side of the oven and two on the top an just a layer of perlite for leveling of steping gap between the two layers.
    After the second fire the temp's reach 300c and the dome is white, two hours late with the door closed and empers inside the dome the temp is 150c!!!!
    The external of the dome is just hot to the touch, i thing about 40c.
    Just for the info, the dome is ready for the past month now so it's prety dry...i think.
    Hope i didn't make something wrong.

    Thank you all in advanced.

  • #2
    Re: Heat Drops fast!

    Hot on the outside and temp dropping fast sound to me like the oven is still wet. The remedy is to keep firing. My bet is that if you do this a few more times you will find that the outside is barely warm, the temp rise much faster and heat retention much better. But be careful and take it slow. Forget pizza for a while and just roast and bake until it's really dry.
    Kindled with zeal and fired with passion.

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    • #3
      Re: Heat Drops fast!

      What insulation have you got underneath?
      The English language was invented by people who couldnt spell.

      My Build.

      Books.

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      • #4
        Re: Heat Drops fast!

        Its 2 layers of 20cm and 10cm of 10:1 perlite mix.

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        • #5
          Re: Heat Drops fast!

          30 cm! That sounds like a lot. If it is all a perlite/cement mix then you have added a ton of water. Do a calculation of how much perlite you used in total and divide that by 3. That will be about how much water you have added. It takes ages for this mix to dry. See attached doc. Slow fires are the only remedy and as you have already rendered over it, the moisture will not be easily driven off. Remember that one litre of water will make 1600 litres of steam. That's the big advantage of using blanket, rather than perlcrete or vermicrete, it doesn't contain water that has to be removed.
          Good Luck.
          Attached Files
          Kindled with zeal and fired with passion.

          Comment


          • #6
            Re: Heat Drops fast!

            Originally posted by david s View Post
            30 cm! That sounds like a lot. If it is all a perlite/cement mix then you have added a ton of water. Do a calculation of how much perlite you used in total and divide that by 3. That will be about how much water you have added. It takes ages for this mix to dry. See attached doc. Slow fires are the only remedy and as you have already rendered over it, the moisture will not be easily driven off. Remember that one litre of water will make 1600 litres of steam. That's the big advantage of using blanket, rather than perlcrete or vermicrete, it doesn't contain water that has to be removed.
            Good Luck.
            The first layer of perlite (15cm) has been set from the an old cob oven, over that layer has been poured 15cm additional so the moister i think is from there.
            I haven't render the final layer of the dome yet because if there is a insulation failure i want to have the choice to remove the Ceramic Blanker an fix the problem.
            Last edited by dimitrisbizakis; 04-03-2013, 06:12 AM.

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            • #7
              Re: Heat Drops fast!

              Originally posted by david s View Post
              Hot on the outside and temp dropping fast sound to me like the oven is still wet. The remedy is to keep firing. My bet is that if you do this a few more times you will find that the outside is barely warm, the temp rise much faster and heat retention much better. But be careful and take it slow. Forget pizza for a while and just roast and bake until it's really dry.
              I will take your advice and start with baking small batches of bread!
              I just need to make a good oven door now...

              Comment


              • #8
                Re: Heat Drops fast!

                my dome insulation is perlite and mortar mix, about 4 inches thick. There is another 2 inches of plain mortar over that for weather proofing. When the oven is at operating temp you can hardly feel any warmth on the dome. I have a thermal break between dome and outer arch, this makes quite a difference. My door is 3/8 steel plate, first one was wood - lined with sheetmetal, it only lasted two firings! The oven hardly needed any drying out

                My oven floor sits on hollow blocks, not mortared nor backfilled with concrete. It was super easy to build like that.

                The oven bricks were perlite as were the floor tiles, very light and easy to cut with small angle grinder. At first I agonized over setting out the bricks but soon realized that it doesn't matter too much if things are not perfect. I made sure I did not use any mortar to fill any gaps inside the dome as I was afraid of falling cement getting into my pizzas at a later date. The fit between the bricks was reasonable and the whole setup works very well.
                Last edited by tonybaker; 04-16-2013, 02:16 AM. Reason: missed info

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                • #9
                  Re: Heat Drops fast!

                  Originally posted by tonybaker View Post

                  The oven bricks were perlite as were the floor tiles, very light and easy to cut with small angle grinder. .
                  Theres your problem, perlite is an insulator not thermal mass.
                  You need to build an oven with thermal mass and save the perlite for the insulation.
                  The English language was invented by people who couldnt spell.

                  My Build.

                  Books.

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                  • #10
                    Re: Heat Drops fast!

                    Originally posted by brickie in oz View Post
                    Theres your problem, perlite is an insulator not thermal mass.
                    You need to build an oven with thermal mass and save the perlite for the insulation.
                    Oops, sorry m8 brickie in oz is right!
                    Exept if we misudenrstude something.
                    You could build an inner layer to your walls and floor with firebricks, its a hard way and you will reduce the size of your oven but the other way is to start all over.

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                    • #11
                      Re: Heat Drops fast!

                      Oops, sorry m8 brickie in oz is right!
                      Exept if we misudenrstude something.
                      You could build an inner layer to your walls and floor with firebricks, its a hard way and you will reduce the size of your oven but the other way is to start all over.[/QUOTE]

                      sorry, I forgot to say that on the floor of the oven in the middle are some very special high density bricks that act as a heat sink. \

                      I agree entirely with your thoughts and they make sense, however the oven works fine and the perlite bricks were so easy to cut.

                      I could always throw some railway irons into the base if it became a problem with heat storage?

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                      • #12
                        Re: Heat Drops fast!

                        Originally posted by tonybaker View Post
                        sorry, I forgot to say that on the floor of the oven in the middle are some very special high density bricks that act as a heat sink.
                        I agree entirely with your thoughts and they make sense, however the oven works fine and the perlite bricks were so easy to cut.
                        I could always throw some railway irons into the base if it became a problem with heat storage?
                        If this works for you is ok m8!

                        Comment


                        • #13
                          Re: Heat Drops fast!

                          Originally posted by tonybaker View Post
                          sorry, I forgot to say that on the floor of the oven in the middle are some very special high density bricks that act as a heat sink.
                          Not that special if the oven doesnt retain heat for any time.
                          An oven should retain heat for a week.

                          Originally posted by tonybaker View Post
                          I agree entirely with your thoughts and they make sense, however the oven works fine and the perlite bricks were so easy to cut.
                          Good functional ovens are hard work to build, theres a trade off here.

                          Originally posted by tonybaker View Post
                          I could always throw some railway irons into the base if it became a problem with heat storage?
                          Scrap iron pizza?
                          The English language was invented by people who couldnt spell.

                          My Build.

                          Books.

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                          • #14
                            Re: Heat Drops fast!

                            Ok, new data about the temp drop!
                            Today i did the Second burning with the insulation and render layer on, I previously burned the oven one more time without the insulation and the render.
                            After the soot have gone I've added some wood, total burning time 2 hours approximately.
                            After removing the coals i placed my door,the door temp stopped rising at 226 C, the outside of the dome wasn't even warm at touch.
                            After an hour passed with door closed the temp dropped dramatically to 165 c, and the oven was slightly warm at some spots on the top of the dome.
                            I have an internal oven thermometer that showed 260 c but the Mississippi count go beyond 10 so i guess the door thermometer was right.
                            Please give me your thoughs!

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                            • #15
                              Re: Heat Drops fast!

                              If you had used dense firebrick for your floor and dome, after a two hour burn the oven temperature would be in the region of 400 C
                              Kindled with zeal and fired with passion.

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