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Wood Burner Conversion II - Forno Bravo Forum: The Wood-Fired Oven Community



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Wood Burner Conversion II

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  • Wood Burner Conversion II

    Greetings everyone-

    About a year ago, I asked for and received some fantastic advice about converting a steel wood burner I found to a WFO. The thread is here: http://www.fornobravo.com/forum/f43/...ion-18785.html

    Unfortunately, other things arose and I never got to it. But now I'm going to start the project. Before going to too much trouble, I'm going to give it a test run just to get a sense of whether it will be worth the effort. Here is the oven:



    To close up the opening of the floor, I bought two steel plates:


    The fire brick goes on top of them. If this goes well, I'll eventually insulate underneath the oven and all of that.


    Then, I'm lining the walls with more fire brick.


    Oh, and of course, I'll close up the chimney on top.


    So I'm wondering about what the experts think so far? One little concern is that I'm cutting off the depth of the oven a little with the bricks on the wall in back. I could cut more fire bricks so as not to lose that much of the hearth, but I'd only be gaining another inch and a half. Does that matter?

    Thanks in advance!


  • #2
    Re: Wood Burner Conversion II


    Loren from Utah has a friend that had a steel body oven lined with firebrick. They fired it up the other day so contact Loren on the attached thread and maybe he can give you some feedback.

    Build Link............... Picassa Photo Album Link


    • #3
      Re: Wood Burner Conversion II

      Thank you!


      • #4
        Re: Wood Burner Conversion II

        Last night, I did a test run with the oven, just to see what would happen. I built a large fire and let it burn for a couple of hours, then moved it to the side to keep it going. I was able to cook a few pizzas, but I clearly lost heat fast. The first one is pictured below. It took about 6 minutes. According to my oven thermometer, I never got above a lowly 350 degrees. I don't know what the hearth temp was, but the bottom of the pizza was reasonably crispy and browned.

        So I now have several questions:

        1. I am clearly losing tons of heat. I assume I'm losing most of it from around the top, sides, and back of the oven since there is not insulation at all? Should I be concerned about losing heat out the door?

        2. I am thinking of using ceramic blanket to insulate around the oven. Is that the best choice-- as opposed to perlcrete or something?

        3. For insulating the bottom, I am considering FB Board. Will that work?

        4. I am considering creating (probably by welding) an arched entry, which would also include a chimney. Would that help?

        Thanks in advance for any advice.



        • #5
          Re: Wood Burner Conversion II

          Some thoughts and suggestions:

          Thought: I think it is good to keep in mind that a WFO is in essence a pocket/hole in a non-combustable medium in which we build a fire; the purpose of which is to store heat from the fire in this medium, the hearth and walls and ceiling. It is this stored heat (plus some small amount which is direct from the fire) which we use to bake pizzas. When we place a pizza into our ovens we are diminishing that stored heat. The number of pizzas one can bake before recharging the WFO is directly dependent upon the amount of heat stored. This is the major hurtle IMHO for any of the small lightweight WFOs suggested such as the pedicab/cycle powered WFO. Limited mass therefore limited heat storage resulting most likely in slow pizza production. One might have to pause between each pizza to reheat the WFO. This lost time may or may not be a problem economically for them. But I digress, back to your WFO and a suggestion:

          So my first suggestion: Place the firebrick outside the steel liner on the sides and top. The brick will need to be in intimate contact with the shell and this may pose some construction issues. Castable works easier in this case than rectangular bricks. However, one of the great advantages of a steel lined WFO is the fact that steel more rapidly conducts heat than does firebrick both to the refractory directly behind the steel but thru the steel sideways to other refractory. Therefore a steel liner more effectively transmits heat from the fire to the surrounding firebricks. This makes for a faster heat up time; and a shorter time between match and first pizza.

          However, if you do not wish to move the firebricks to the outside of the shell then cut firebricks to fill in the spaces presently void of bricks inside the WFO. The bricks are stacked on the back wall but there are significant areas where there is no firebrick. Filling the void areas will increase the mass as well as provide some uniformity within the WFO. Not wishing to get into a detailed discussion but basically heat radiated from a heated brick inside a WFO (the interior of which is completely firebrick) goes into baking a pizza or into another firebrick, where it is either absorbed or reflected. The only other place for it to go is to be lost out the WFO's opening (yes, some may argue correctly that there is loss thru the insulation as well). As presently constructed, in the areas without brick the steel actually acts a place where the heat can escape the WFO. I found firebricks shape quite easily using a hand held grinder and diamond disk (both available inexpensively thru Harbor Freight).
          If you elect to keep the bricks inside the steel shell your WFO will behave basically like a brick WFO. Although as the bricks are not mortared together and acting as a single mass, the brick to brick transmission of heat by conduction is less than a traditional brick WFO and so will have a longer heat up time. But the WFO is small and this may not be significant.

          Second suggestion: If it is economically viable choice, then I would suggest going with ceramic fiber insulation. If only because it will allow further modification without such a mess of removing vermicrete or perlcrete. Just remember to wear a mask when working with the stuff.

          Hope this helps,


          • #6
            Re: Wood Burner Conversion II


            Thanks so much. I hope you don't mind a couple follow ups:

            So, basically you think I'm better off without bricks lining the inside. That's good news, actually. I think I was working on a suggestion from someone a long time ago by putting them there. But what will that mean in terms of heat retention? Let's say I insulate all around with fiber blanket insulation. Am I going to maintain heat inside the oven?

            Secondly, do you think it's a good idea to build an entry, an extension to the door, that would also include the chimney? I'm thinking a little of yours here.



            • #7
              Re: Wood Burner Conversion II

              Regardless of where they are placed (inside or outside) you are going to need some sort of heat reservoir. IMHO if you can place the bricks on the outside (bedded in mortar so there is intimate contact with the outside of the steel shell) that would be best solution. The heat reservoir would then need to be insulated. The mantra of this forum (if it had one) would be "Insulate Insulate Insulate". Placing the bricks outside would give you the most interior depth, which if memory serves with your WFO is the smaller dimension of the rectangle of the hearth.

              And of course you have the option of having some sort of door by which you can, at least part of the time, alter the heat loss thru the large entrance. A door might change the heat dynamics so that between pizzas the heat loss would be less and you could still have the open entrance watching the pizza bake part of the experience. The door does not have to cut off the whole entrance. Furnaces for glass blowers often have doors that open and allow a large piece to enter the furnace and close leaving a smaller opening for the blowpipe. The doors are to conserve heat.... mostly to keep the high temperatures and heat costs money and the economics of glass blowing are crazy IMHO. (I have a couple friends who blow glass and their propane bill is absurd).

              I know with my WFO if I only have another couple and my wife and I then I'll bake one or two pizzas one right after the other. Then I'll rake the coals over the hearth and throw a few sticks of wood on the coals and go sit with the group. I'll have plenty of time to enjoy the company, the pizzas and a glass of wine or a salad and then when it looks like time for more pizza, it's an easy matter to go push the coals off the hearth and build more pizzas. By the time the pizzas are assembled the hearth temp has dropped to a useable range. I blow off the ash and have at. I have found this method/process to be very sustainable with a small group wherein I want to be part of the sit down eating conviviality.

              As for the entrance area: I have not regretted having the large outside the entrance work area on my WFO. I think most on the forum use Ove Gloves for handling hot pans and roasters etc.. They work well but even with the gloves one cannot hold a hot handle a long time especially if the pan is cast iron and the whole is heavy (think a chick-can with the pan a good way filled with rendered chicken grease) It's really great to be able to slide a screaming hot cast iron pan onto the side area outside the WFO. I usually slide the pan onto a sacrificial piece of plywood to lessen any thermal shock to the granite. I have burn rings deep into the plywood, and better it than any table. But this is a personal choice of function and aesthetics.

              As for a chimney again it's a matter of choice. Personally, when I first enjoyed pizza from a neighbors cob WFO I walked right up to the entrance and caught myself just in time from being burned from the transparent exhaust gasses. His WFO is significantly lower to the ground and without any entrance ledge/shelf. Right then I decided that I wanted something very different for my WFO. A chimney will duct the extremely hot gasses safely away from the entrance and curious guests who want a "look see".

              Hope this helps,


              • #8
                Re: Wood Burner Conversion II

                A little update. I insulated the oven with 2 inches of blanket and insulated the underside with 2-inch insulating board. I lit a nice test fire and pretty quickly got the oven to over 600 degrees (as high as the thermometer would go).

                Even better, it seemed to hold heat pretty well. 5 hours later and it was still around 300 degrees. A couple more hours with the door off, it was still around 200.

                I think it's going to be a keeper!