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chimeny diameter - Forno Bravo Forum: The Wood-Fired Oven Community

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chimeny diameter

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  • chimeny diameter

    I've completed the insulation of my oven used it 4 times now and it's worked great!
    1.5 hours to 400c, uses about 12 - 15kgs of wood to get to that temp and stay there for 4 hours.
    now i've just got to put in the chimeny and cement render the outside, but i've hit a bit of a snag with the chimeny - what diameter should it be and does it need to be straight or can it have a 45 degree bend in it?

    my oven is under my patio area so putting a chimeny straight up would involve cutting into the colour bond roof which i'm not real keen on. the other issue is the price of materials - 75mm galvanized pipe used for drain pipes is cheap ($10/m) but is 75mm diameter going to cut it? most oven's i have seen have a chimeny 2x this size.
    the smoke goes straight out of the gap i left for a chimney in the top of my front arch really well so i'm thinking the hot air rising will push it up a 75mm opening pretty well?

  • #2
    Re: chimeny diameter

    timmarhy,
    you are going to need much larger than one , two and maybe three 75mm chimneys if your oven is anywhere around a 36" diameter or larger. A 3" diameter pipe has an area of 9 1/2 sq inches, 2 will give you 19 sq inches and 3 flues 28 1/2 sq inches. I have a 1 metre diameter Pompei with an 8" (an area of 25 sq inches) chimney and no smoke comes out the front entrance arch.
    If you are planning a bend or 2, then they should be 30˚ rather than 45˚. Remember that these will also reduce or hinder the flow of the smoke and hot gasses from your oven, ie. it reduces the draw efficiency.
    What's wrong with passing your chimney up through the colorbond roof, Mone works a treat and is easily sealed.

    Rastys
    If you don't succeed the first time, try again and again until you get it right!

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    • #3
      Re: chimeny diameter

      i'm more worried about stuffing up and not getting the chimney straight cutting through the colour bond. any tips if i decide to go that route?
      i'll hunt around and look for a more economical larger pipe - the only 150mm galv chimney pipe i've seen is $30 a meter and i need 3 meters of it, it makes my "i'm being ripped off" spidey senses tingle.

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      • #4
        Re: chimeny diameter

        Less than six inches? Even small ovens are best with an eight inch chimney. And single wall galvanized stovepipe? Through a roof, or immediately adjacent to it? This sets off all sorts of fire safety alarms in my head. Also, building code calls for no more that 30 degree bends in chimneys if you decide to go that route.
        My geodesic oven project: part 1, part 2

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        • #5
          Re: chimeny diameter

          it's a steel patio roof with steel trusses and it's 2m above the oven -it's not going to catch on fire if that's what your concerned about. i'll also slip a heat resistant silicon sleeve over it the same as they use for indoor wood heaters.
          my real concern with going through the roof is marking out the hole and cutting it right because replacing the sheet on the roof would be a major pain in the ass.

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          • #6
            Re: chimeny diameter

            timmarhy
            go to yourwood stove or heater suppliers and get 3 lengths of 8" flue and one length of 9".
            This is used to provide an outer sleeve that I filled with a 1/2' layer of superwool insulation blanket and fits snugly in the colorbond patio roof.
            To get your exact location over your oven chimney outlet, you can locate the exact position in one of 2 ways:
            1. get a spirit level in your oven chimney outlet with a longer dowell or wood extension to reach your roof, get the level vertical and mark the roof on all 4 sides of the outlet. Measure the centre, drill a small in the centre and using a compass mark the circle and cut the hole out with an angle grinder using a 1mm cutting disc. Yes, you can cut circle with these very easily, OR
            2. Using a plumb bob (a weight on a string ) and holding the string up against the roof with the bob suspended exactly over your oven chimney centre, mark the roof to get your centre and follow the instructions as above.
            To seal the outer flue, go to your plumbing supplier and get the largest neoprene vent sealer (a 9" flue can seal beautifully).
            The only other thing you need to do is to carefully hammer the 9" flue ends over so that the 8" inner pipe will fit snugly through it. I then sealed the joint with a little high temp silicon sealer.

            Rastys
            Attached Files
            If you don't succeed the first time, try again and again until you get it right!

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            • #7
              Re: chimeny diameter

              Originally posted by timmarhy View Post
              it's a steel patio roof with steel trusses and it's 2m above the oven -it's not going to catch on fire if that's what your concerned about. i'll also slip a heat resistant silicon sleeve over it the same as they use for indoor wood heaters.
              my real concern with going through the roof is marking out the hole and cutting it right because replacing the sheet on the roof would be a major pain in the ass.
              Try centering a plumb line from the roof to your oven opening to mark your chimney center.

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              • #8
                Re: chimeny diameter

                Sorry,Didn't read rasty's last post.

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