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Flue sizing - Forno Bravo Forum: The Wood-Fired Oven Community

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Flue sizing

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  • Flue sizing

    Hi all, I am building a 36 inch pizza oven and trying to decide what is the best flue size to use. No doubt it has been asked before, but I was wondering if anyone can offer any advice from experience as to whether a six inch or an eight inch flue works best for a 36 inch oven. Also, is there a maximum or minimum height to work efficiently? Any help would be appreciated. Regards Dave.

  • #2
    Re: Flue sizing

    Venturi effect

    4 inch , any obstacles nearby may cause low pressure areas

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    • #3
      Re: Flue sizing

      Originally posted by sandgroper View Post
      Venturi effect

      4 inch , any obstacles nearby may cause low pressure areas
      The smoke will find the easiest way out which is the door.

      A venturi only really works on a closed system without the larger escape hole for the smoke.
      The English language was invented by people who couldnt spell.

      My Build.

      Books.

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      • #4
        Re: Flue sizing

        In that case any Rumford fire would smoke like a dragon

        Principles of venturi according a fire size the flue should draw especially if tall enough.

        Try the 8 then

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        • #5
          Re: Flue sizing

          Originally posted by sandgroper View Post
          In that case any Rumford fire would smoke like a dragon
          Fair enough, point taken.
          But in building a fireplace and chimney there is a slow gather of the brickwork above the fire box to coax the smoke to go straight up, which it wants to do anyway due to heat and the expanding volume of the gases.

          In an wood fired oven situation the smoke is making a rapid exit for the door, you have to then persuade the smoke to do a right angled turn and go up the flue, making the flue smaller will inhibit this.
          Last edited by brickie in oz; 05-05-2013, 11:55 PM.
          The English language was invented by people who couldnt spell.

          My Build.

          Books.

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          • #6
            Re: Flue sizing

            6 inch is perfect for my 36 inch oven.
            What makes the smoke move up the flue is the draw. Heat rises so it you can create a draw. The longer the chimney, the stronger the draw.

            A draw can be strong enough to move smoke sideways and even downward.
            Rocket stoves have flames and smoke move that move sideways and down.

            I use stackable 6 inch hvac for my chimney. The longer I make that chimney, the stronger the draw.

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            • #7
              Re: Flue sizing

              Originally posted by sandgroper View Post
              In that case any Rumford fire would smoke like a dragon

              Principles of venturi according a fire size the flue should draw especially if tall enough.

              Try the 8 then
              Conventional draw characteristics change dramatically with outside fire chambers....which is why you can't over flue a wood oven or outdoor fireplace.
              Old World Stone & Garden

              Current WFO build - Dry Stone Base & Gothic Vault

              When we build, let us think that we build for ever.
              John Ruskin

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              • #8
                Re: Flue sizing

                I have a 6 inch flue on my 32 inch oven and it works fantastically. After the oven (and flue) heat up I get no visible smoke out of the front. I suspect the 6 inch flue would work just as well for your slightly larger 36 inch oven. Make sure you do not constrict the air flow prior to it entering the flue.

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                • #9
                  Re: Flue sizing

                  So if I am using a 20cm or 8" flue on a 1000mm oven I should be okay? Does it have to be double walled it is outside completely?
                  Cheers,
                  Rosemary

                  My build so far
                  http://www.fornobravo.com/forum/f51/...ish-19084.html

                  Starting out collecting info and materials
                  http://www.fornobravo.com/forum/f3/h...lia-18943.html

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                  • #10
                    Re: Flue sizing

                    I think the double wall is necessary just for safety incase the flue passes through a building structure. If the oven is located outside and the flue does not pass through a building structure, then a double flue is unnecessary.

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