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Heat retention versus oven size

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  • mongota
    replied
    Originally posted by paulkjrobbins View Post
    Mongota, that is some awesome stone and concrete work on the outside. Did you do that yourself?
    Did you have to use any special concrete mix when capping it? By that I mean, was heat an issue, or was it negligible since it was on the outside of the flue?
    Thanks.

    Yes, I built it myself.

    The concrete corner pillars, slabs, arches entry shelf, and chimney cap are all plain ole Quikrete with carbon black/charcoal colorant added. Heat is not an issue with any of those castings. I had thought I might need a heat shield of some sort for the underside of the arch casting that surrounds the opening on the front landing, as I though it might see too much heat when starting fires right at the opening. It has not been a problem.

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  • paulkjrobbins
    replied
    Mongota, that is some awesome stone and concrete work on the outside. Did you do that yourself?
    Did you have to use any special concrete mix when capping it? By that I mean, was heat an issue, or was it negligible since it was on the outside of the flue?

    Leave a comment:


  • mongota
    replied
    Originally posted by paulkjrobbins View Post
    TI have read a lot of posts that comment on how attaching the anchor plate to the firebrick with expansion bolts will crack over time.
    I used four anchor bolts to secure my chimney plate. No cracks in the bricks. The drilled holes are larger than the actual bolt diameter, so very little of the anchor contacts the brick. I have a ~2-1/2" thick cast concrete cap that covers the anchor plate. It's going no where. Zero movement/sway in the chimney.

    Attached Files

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  • paulkjrobbins
    replied
    Thanks David, that picture explains it reall well.

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  • david s
    replied
    Originally posted by paulkjrobbins View Post
    The weather has been wet here so I am still waiting to start mortaring in my first course. One more question I am wondering about is in relation to the chimney pipe. I am doing an 8" double-walled SS pipe. My oven is being built just outside of a pavilion so the chimney will extend 2' past the ridge of the pavilion giving it plenty of draw, it should be pretty stable as it will have a support rod from the pavilion roof. I have read a lot of posts that comment on how attaching the anchor plate to the firebrick with expansion bolts will crack over time. What have people done instead of anchor bolts? Is an anchor plate needed necessarily? Can I just flange the bottom section of the chimney pipe and encase it in homebrew or vermicrete? Perhaps mortar another layer of firebrick on top to encase the seam? I am doing a gabled enclosure with hardiboard so it won't be visible and doesn't need to be super pretty. Any thoughts would be appreciated.
    This is my solution, it works pretty well. If mortar is sitting around the flue pipe the hot expanding pie can crack it. I use vermicrete to support the base of the pipe which has some give to it. If you are using a double flue this shouldn't be a problem as the outer flue will be considerably cooler.

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  • MarkJerling
    replied
    My oven's flue pipe is simply bricked and mortared in. It's been 100% stable.

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  • paulkjrobbins
    replied
    The weather has been wet here so I am still waiting to start mortaring in my first course. One more question I am wondering about is in relation to the chimney pipe. I am doing an 8" double-walled SS pipe. My oven is being built just outside of a pavilion so the chimney will extend 2' past the ridge of the pavilion giving it plenty of draw, it should be pretty stable as it will have a support rod from the pavilion roof. I have read a lot of posts that comment on how attaching the anchor plate to the firebrick with expansion bolts will crack over time. What have people done instead of anchor bolts? Is an anchor plate needed necessarily? Can I just flange the bottom section of the chimney pipe and encase it in homebrew or vermicrete? Perhaps mortar another layer of firebrick on top to encase the seam? I am doing a gabled enclosure with hardiboard so it won't be visible and doesn't need to be super pretty. Any thoughts would be appreciated.

    Leave a comment:


  • UtahBeehiver
    replied
    It 63-65% of dome height is a rule of thumb and subject to much discussion. An old white paper called Bread Ovens of Quebec showed that the majority of the early pioneer ovens had opening is the 63% range. The FB is for the 42" is 59.5 range. Will it make a difference, probably not..More recently builders have been using the 63-65% ratio, builders choice. On the IT, as long as you are aware of how the vertical and horizontal offsets effect the dome diameter and make adjustment you are fine. The offset does effect how the dome and arch tie together if you do a tapered inner arch (recommended).

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  • paulkjrobbins
    replied
    Thanks for the advice Russell. I will lower mine in. I know there will still be a little variance since the fulcrum from the caster wheel pivot is not centered over the center of the dome arch but I think I am just getting picky if I get too crazy with things like that. Is the 65% calculation preferred over the recommendation in the general forno bravo plans (attached- pg.13)?
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  • UtahBeehiver
    replied
    I can't see the detail from the pics you attached. Now that you are doing a 42", this changes your inner arch height, 65% of 21" is 13.65" instead of the 12.5" in the previous post. I see you are also aware of how the vertical offset at the pivot point changes the dome height. You could do like Gulf did and make a wood block the size of the fire brick and low the pivot point down to the floor elevation. Several builders have use Gulf's idea with much success.

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  • paulkjrobbins
    replied
    Click image for larger version  Name:	IT2.PNG Views:	0 Size:	28.4 KB ID:	440418Here are pictures of my IT tool. I just recently realized that I will need to cut off the bottom support so it is the same width as the top so that I can get a good profile to trace when doing the inside parts of my inner arch bricks, but I will trim those down when I get to that point. I was trying to get something that would work with what I had and in the end just had to buy a threaded rod for it. The bottom pivot is a 1.5" caster. I am actually going to do the full 42" oven, my wife wants to have the option of roasting a pig in it in the foreseeable future. I plan on adjusting the IT for each level taking into account the distance from the brick to the IT (1/2" plywood plus 3/4" to pivot of caster) to keep the radius close to 42" for the whole dome. Any advice on this? Would it just be better to dial the IT in and then just keep it at that length except when adjusting to use with a pencil? Thanks for your help.Click image for larger version  Name:	IT1.PNG Views:	0 Size:	27.8 KB ID:	440417
    Last edited by paulkjrobbins; 07-30-2021, 01:51 PM.

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  • paulkjrobbins
    replied
    I can never get myself to use software for math calculations. I use to teach Calculus & Physics & I guess I got used to writing everything out by hand. I use my TI-89 for just about everything.

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  • JRPizza
    replied
    I built my 39" oven with a 12.25 inch high opening - a number I arrived at by interpolating the FB plans between their 36 and 42 inch oven recommendations. A 12.25 opening in a 39'er works out to be 62.8% which was close enough to 63 for me Also if you build a hemispherical arch (not what you are showing) the 9.75" radius that gives you a 19.5" opening sits perfectly on a 2.5 inch brick at the bottom of the arch.

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  • UtahBeehiver
    replied
    12.5" is right in the range for a 39" ID dome. Rule of thumb is 63-65% of dome height so you are there. Depending on the type of chimney material, ie brick, double wall SS, etc. buttressing of the side walls may be required (tall brick chimney). SS should be fine sans buttressing. To help with the arch bricks on a axed arch I did a quick design using some software I have called the Anglelizer. It is no longer available from General but I think a forum member posted a link to a public domain software archive.

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  • paulkjrobbins
    replied
    Russell, I'll try to remember to take a picture of my IT after work today. Here are some calculations I did for determining my raised arch curve. Hope to put together a template tonight.
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