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secondary chimney vent? - Forno Bravo Forum: The Wood-Fired Oven Community

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Dome Installation Video - Casa / Premio / Modena

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secondary chimney vent?

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  • secondary chimney vent?

    I have noticed the black discoloring left form the smoke on the decorative part of almost every oven.
    Has anyone designed a chimney that eliminates this problem?
    I was thinking of placing a thinner secondary transition into the chimney closer to my decorative arch.
    Maybe just use smaller slices of brick for the top inner vent arch right before it reaches the decorative arch or entry way.
    This should give the black smoke another chance to move up into the chimney instead of exiting through the front.
    Or at least cut down on the amount that comes out the front.
    Gino

  • #2
    Re: secondary chimney vent?

    The smoke could be avoided by using a baffle outside of the chimney to lower the entry height a couple of inches and still allow wood to be added. This sort of accessory might be just a little stainless sheet metal that extends up into the smoke collection area, down a couple of inches lower than the oven entry height and spanning the full width of the entry in this area, this all supported by some sort of side legs. So for my oven that has a 13.5 inch exit height for the actual oven and a width in the area described of about 24 inches, I’d start with a removable baffle that allows 12 inches of vertical for putting wood into the oven.

    Doing this could also help to create a faster flow of incoming, fresh air. Once the fire is rolling and the chimney is warm very little smoke comes out of the entry anyway, but until then smoke can sneak out the front.

    Can you provide an illustration of your idea.


    Thanks

    Chris

    PS Like a fireplace, if you start the oven slow, and allow for a bit of warming, very little smoke will make its way forward. Eliminating smoke out the front can be done, but it might be more trouble than it's worth..
    Last edited by SCChris; 10-17-2011, 11:21 AM.

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    • #3
      Re: secondary chimney vent?

      I like the stainless steel baffle idea until the fire gets going hotter.
      I thought about a permanet baffle by dropping the outter arch a little but then I would not be able to get my door in and out.
      Hope this link works.

      Snapfish: Share Photo:Registration

      the black represents the chimney inside, the outter arch prior to my outside finish look is where I wanted to cut the arch brick thinner to create more area for the smoke to go up instead of out.
      Maybe I can incorporate a stainless baffle that slides up into the arch area that I can pull down when needed.
      Gino

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      • #4
        Re: secondary chimney vent?

        The only reason the fronts get sooty is because the flue isnt big enough, or the flue gather isnt built correctly, smoke always finds the easiest way out.
        The English language was invented by people who couldnt spell.

        My Build.

        Books.

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        • #5
          Re: secondary chimney vent?

          Brickie,

          I agree with you, smoke always finds the easiest way out, but it needs to be said that as the chimney heats, the ability to pull the smoke out of the entry increases.

          An analogy would be sink with and without an attached drain pipe.. The added mass of the water flowing out the drain pipe helps to pull the water out of the sink. A siphon is another example, water won’t flow above the water supply, but it can be pulled higher, with a siphon, as long as the end of the hose is lower than the source supply.

          Once the chimney is hot, the higher the chimney, the better the draw all things being equal.. Many of the problems with smoke escaping the front of the entry relate directly to not having enough area to service the quantity of smoke for the given chimney height. I’m not advocating a 30 foot chimney on a 42 inch oven, I’m just saying, your right “Smoke always finds the easiest way out”

          gtofani,
          Time to time I have to remind myself.
          "Keep it simple stupid"
          "Don't be penny wise and pound foolish"
          "Do it right" and
          "If it's important don't compromise"

          Chris

          PS.
          One more thing; "She is almost always right and she knows it and will remind me of it"
          Best to all..
          Last edited by SCChris; 10-17-2011, 01:02 PM.

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          • #6
            Re: secondary chimney vent?

            It is the volume of the flue that creates the draw. You can increase the height or the cross sectional area, or both. Using the top down method also eliminates smoke at start up. I have a mosaic decorative arch (glossy tiles) occasionally I give it a rub with damp newspaper- no more smoke staining.
            Kindled with zeal and fired with passion.

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            • #7
              Re: secondary chimney vent?

              My chimney works great for the most part. Sometimes as the fire just starts to get big and just the right gust of wind feeds the oven I get smoke out the door. I think if you don't like that look you can always tile the opening so that if it does get dirty it will easily clean(like what David S said). personally I don't mind the markings I think it adds charm to my oven

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              • #8
                Re: secondary chimney vent?

                Originally posted by Faith In Virginia View Post
                My chimney works great for the most part. Sometimes as the fire just starts to get big and just the right gust of wind feeds the oven I get smoke out the door.
                Have you tried a blast door?
                The English language was invented by people who couldnt spell.

                My Build.

                Books.

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                • #9
                  Re: secondary chimney vent?

                  You can increase the height or the cross sectional area, or both
                  I am currently designing my flue and in particular, the size and shape of the flue gather, or vent. I understand that the longer a given flue is the higher the pressure and draw it will provide. Is it incorrect to assume a shorter but larger diameter flue would not provide the same amout of pressure that the longer/narrower flue could?

                  Comment


                  • #10
                    Re: secondary chimney vent?

                    No Al, I have not tried a blast door what are the benefits?

                    Comment


                    • #11
                      Re: secondary chimney vent?

                      Originally posted by Faith In Virginia View Post
                      No Al, I have not tried a blast door what are the benefits?
                      It increases the draw dramatically at start up, a piece of cardboard with a cut out at the bottom across the opening will give you an idea of the effect.
                      The English language was invented by people who couldnt spell.

                      My Build.

                      Books.

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                      • #12
                        Re: secondary chimney vent?

                        Being the impatient soul that I am, I usually start a pretty decent sized fire straight away, and I was definitely having issues with smoke giving my decorative arch the 'rustic look' that I wasn't really going for!

                        My neighbor Dave has a very simple solution for this problem; Cut a piece of solid timber as though you were making a door for your oven - mine looks like a semi-circle (see attached sketch). Then, to allow fresh air to be drawn in, cut decent sized gaps in the base of this semi-circle. If you sit this in the doorway just inside your decorative arch, it will ensure the smoke goes straight up the flue without staining the front of the oven. This door can then be removed once your flue has heated to the point that it is drawing good and hard. Simple and effective.
                        Attached Files
                        The Melbourne Fire Brick Company

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                        • #13
                          Re: secondary chimney vent?

                          The second calculator on this page is perfect for your question. Simply plug in some numbers and find out.

                          Air Flow and Velocities due to Natural Draft

                          Originally posted by GianniFocaccia View Post
                          I am currently designing my flue and in particular, the size and shape of the flue gather, or vent. I understand that the longer a given flue is the higher the pressure and draw it will provide. Is it incorrect to assume a shorter but larger diameter flue would not provide the same amout of pressure that the longer/narrower flue could?

                          Comment


                          • #14
                            Re: secondary chimney vent?

                            Originally posted by benguilford View Post
                            Simple and effective.
                            Its a blast door, there are lots of oven users using them..
                            The English language was invented by people who couldnt spell.

                            My Build.

                            Books.

                            Comment


                            • #15
                              Re: secondary chimney vent?

                              Originally posted by GianniFocaccia View Post
                              I am currently designing my flue and in particular, the size and shape of the flue gather, or vent. I understand that the longer a given flue is the higher the pressure and draw it will provide. Is it incorrect to assume a shorter but larger diameter flue would not provide the same amout of pressure that the longer/narrower flue could?
                              Not sure if thats correct, if you read up on fluid dynamics you will see that the longer the pipe the more friction and resistance you will get and thats with pressurized fluids.
                              Smoke from your oven isnt pressurized so has to coaxed to find the flue.

                              My last oven had a very short flue of about 400mm and worked fine, my current flue is over 2M tall and works fine.

                              It comes down to internal sizing over door opening size plus any restrictions like incorrect flue gather.
                              The English language was invented by people who couldnt spell.

                              My Build.

                              Books.

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