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Amount of space to allow for vent - Forno Bravo Forum: The Wood-Fired Oven Community

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Amount of space to allow for vent

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  • Amount of space to allow for vent

    We are ready to place our 36" pompeii on our hearth. We have a 62 x 70 inch block stand.

    We are planning to use a 8.5" clay flue. How many inches should I allow from the outside of the inner arch to the outside of the outer arch?

    Pam

  • #2
    Re: Amount of space to allow for vent

    Obviously it depends upon how you build it. Normally it will be about 6" wide from outside of inner arch to inside of outer arch to accommodate the inside diameter of the flue.

    That gives from face of inner arch: 6" flue span+outer arch material.

    Comment


    • #3
      Re: Amount of space to allow for vent

      One brick length is plenty. Depending on the brick size and if you add a thermal break, that's about 8.5"-9.5". Unless you want a deep vent for dome reason....
      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

      Comment


      • #4
        Re: Amount of space to allow for vent

        This shows what I am talking about. It is about the simplest method to do it.

        Smoke Chamber

        Comment


        • #5
          Re: Amount of space to allow for vent

          I searched the forum a while ago about this when I was in the process of laying out my oven. I recall reading some posts about problems with smoke coming out the front with short vent landings. From that perspective, the minimum space required to build it might not be the best choice. Since mine is under a roof and will be finished in white stucco, I opted to make my vent landing deeper in hopes that I can reduce smoke problems. We'll see how it turns out. Everything is a compromise, I guess.

          Comment


          • #6
            Re: Amount of space to allow for vent

            Deeper will help, but lighting the fire correctly will help more.

            Comment


            • #7
              Re: Amount of space to allow for vent

              The depth of the vent is not that important.... it's the size of the throat opening and the flue that effects draw the most.
              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

              Comment


              • #8
                Re: Amount of space to allow for vent

                Originally posted by stonecutter View Post
                The depth of the vent is not that important.... it's the size of the throat opening and the flue that effects draw the most.
                And, how it is transitioned between the two .
                I don't care what folks say behind my back........They are either braggin' or.......lyin'

                joe watson

                My Build
                My Picasa Web Album

                Comment


                • #9
                  Re: Amount of space to allow for vent

                  Can you guys elaborate a bit?

                  I have read recommendations to preheat the chimney to avoid smoke problems, but took it as a work around for a problem with the vent design or chimney size/height, not as a general requirement. What is the correct way to light the fire to keep smoke to a minimum?

                  What exactly is the throat opening and what is a good size for it? How should it be transitioned?

                  Comment


                  • #10
                    Re: Amount of space to allow for vent

                    Originally posted by hubert_s View Post
                    Can you guys elaborate a bit?

                    I have read recommendations to preheat the chimney to avoid smoke problems, but took it as a work around for a problem with the vent design or chimney size/height, not as a general requirement. What is the correct way to light the fire to keep smoke to a minimum?

                    What exactly is the throat opening and what is a good size for it? How should it be transitioned?
                    Not a "work around", though preheating can be used to lessen the problem. Any oven can smoke, no matter the how the flu is designed, if the wrong weather conditions line up against you.

                    I'm sure you will get a lot of anwers on the "correct way to light the fire" But, holding the lit starter, (wether it be) a match, paper or torch high in the flu before lighting the bundle in the oven is the first step.

                    As for as transitioning, I like the funnel design.
                    I don't care what folks say behind my back........They are either braggin' or.......lyin'

                    joe watson

                    My Build
                    My Picasa Web Album

                    Comment


                    • #11
                      Re: Amount of space to allow for vent

                      As for as transitioning, I like the funnel design.
                      Where would I find some good examples/pictures of the funnel design?
                      Pam

                      Comment


                      • #12
                        Re: Amount of space to allow for vent

                        Originally posted by hubert_s View Post
                        Can you guys elaborate a bit?

                        I have read recommendations to preheat the chimney to avoid smoke problems, but took it as a work around for a problem with the vent design or chimney size/height, not as a general requirement. What is the correct way to light the fire to keep smoke to a minimum?

                        What exactly is the throat opening and what is a good size for it? How should it be transitioned?
                        The throat opening is simply the opening at the apex of the vent arch, were the exhaust from the oven SHOULD exit...if a vent has been built. I have found that if the throat opening is 65-75% (more won't hurt either) the width of the oven opening, you do not need anything to help start a draw.

                        That dimension will transition up to the flue in two-three courses. That creates a funnel..what Gulf is talking about. It behaves similarly to a smoke shelf, an area for gas and smoke to accumulate when the volume of exhaust increases, preventing it from bellowing out the front. Sometimes no matter what, smoke will make it past the outer arch, because of high winds. But a proper oven throat and flue will draw cold, and won't smoke at start up.
                        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

                        Comment


                        • #13
                          Re: Amount of space to allow for vent

                          Originally posted by stonecutter View Post
                          The throat opening is simply the opening at the apex of the vent arch, were the exhaust from the oven SHOULD exit...if a vent has been built. I have found that if the throat opening is 65-75% (more won't hurt either) the width of the oven opening, you do not need anything to help start a draw.

                          That dimension will transition up to the flue in two-three courses. That creates a funnel..what Gulf is talking about. It behaves similarly to a smoke shelf, an area for gas and smoke to accumulate when the volume of exhaust increases, preventing it from bellowing out the front. Sometimes no matter what, smoke will make it past the outer arch, because of high winds. But a proper oven throat and flue will draw cold, and won't smoke at start up.
                          Thank you for that explanation, stonecutter. I recall reading a post here that said the volume of the vent under the flue should be big enough to avoid excessive smoke problems and suggested a deeper vent to increase that volume. I guess deeper, wider and higher is better by that reasoning. It is good to see some actual numbers, 65-75% of the oven opening.

                          Earlier in this thread, you mentioned one brick depth should be enough. Does this require using the inner arch as a support, or can you fit the vent support in one brick length even if you build it completely separate from the dome? If it is built separately, a 6" opening would only leave 1.5" on either side of the hole for a 9" brick. Is this enough?

                          Comment


                          • #14
                            Re: Amount of space to allow for vent

                            Originally posted by hubert_s View Post
                            Thank you for that explanation, stonecutter. I recall reading a post here that said the volume of the vent under the flue should be big enough to avoid excessive smoke problems and suggested a deeper vent to increase that volume. I guess deeper, wider and higher is better by that reasoning. It is good to see some actual numbers, 65-75% of the oven opening.

                            Earlier in this thread, you mentioned one brick depth should be enough. Does this require using the inner arch as a support, or can you fit the vent support in one brick length even if you build it completely separate from the dome? If it is built separately, a 6" opening would only leave 1.5" on either side of the hole for a 9" brick. Is this enough?
                            I had bricks that were 8"...that's what they had. But when I build vents, I don't want a deep, almost vault like structure. So all my arches are one brick length and once I hit the mark for my throat opening on each side of the intrados, the rest of the arch brick were cut to 3". That gave me a 5"+ ( because of the 3/4" thermal gap) wide, 10.75" long throat opening for a 33" oven, with a 17.25" opening.

                            You have 9" brick so you can do a wider throat. And yes, 1.5 would be fine, but 2" would be better.
                            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

                            Comment


                            • #15
                              Re: Amount of space to allow for vent

                              This is helping me picture this vent/chimney area.

                              If I have a 36" oven, 19" wide x 12" tall opening, 4.5" inner arch, would you recommend a 8.5" x 8.5" OD or a 8.5 x 13" OD clay flue?

                              So the back side of the clay flue would set on the inner arch?

                              I'm using 9" brick.

                              Pam

                              Comment

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