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Smoke - Forno Bravo Forum: The Wood-Fired Oven Community

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Smoke

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  • Smoke

    Hi
    I have just completed my oven, which is loosely based on the Pompei.
    I am in the process of curing it and am not having a good time of it.
    I have used a 6" dia flue, which is surrounded by a 9" clay pipe. The space between the flue and pipe is filled with loose pumice to act as insulation for the flue. The flue is 24" above the arch, which is about 8" thick, giving total flue length of about 32".
    The problem is that smoke comes roiling out of the opening, covering me in smoke. The smoke is so bad that it fills up my house. I have to close the windows and doors but it still gets in.
    If all if the smoke went up the chimney I think the problem would be less noticeable.
    Can anyone offer advice? Will extending the flue make any difference? Note that I do not have a smoke chamber. The flue comes straight out of the arch.
    The way it is going I don't think I will use this oven much, if at all.
    It is really quite unpleasant.
    Max

  • #2
    Re: Smoke

    Originally posted by Massimilliano View Post
    Hi
    I have just completed my oven, which is loosely based on the Pompei.
    I am in the process of curing it and am not having a good time of it.
    I have used a 6" dia flue, which is surrounded by a 9" clay pipe. The space between the flue and pipe is filled with loose pumice to act as insulation for the flue. The flue is 24" above the arch, which is about 8" thick, giving total flue length of about 32".
    The problem is that smoke comes roiling out of the opening, covering me in smoke. The smoke is so bad that it fills up my house. I have to close the windows and doors but it still gets in.
    If all if the smoke went up the chimney I think the problem would be less noticeable.
    Can anyone offer advice? Will extending the flue make any difference? Note that I do not have a smoke chamber. The flue comes straight out of the arch.
    The way it is going I don't think I will use this oven much, if at all.
    It is really quite unpleasant.
    Max
    A 6" Flue is on the small side of things.

    Post some pics.....
    The English language was invented by people who couldnt spell.

    My Build.

    Books.

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    • #3
      Re: Smoke

      Max,
      What is the internal diameter of your oven? Your 6" flue may well be adequate depending on your oven size. If the diameter of your flue is ok then there are other causes of the oven smoking up.
      1. All new (wet) ovens have a tendency to be smokey until they're dried out.
      2. Damp wood is also a no no.
      3. Overloading the oven chamber with fuel.generally
      4. The entry not providing sufficient volume for the smoke to collect before exiting out the flue pipe. Think of an inverted funnel.
      A 6" diam flue is good for up to a 36" diam oven, any larger needs an 8".
      My oven is only 21" in diam and has a 5" flue that is more that adequate.
      Last edited by david s; 12-25-2013, 02:22 AM.
      Kindled with zeal and fired with passion.

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      • #4
        Re: Smoke

        Assuming you have a dry oven, and you are burning dry wood, there is another consideration.

        Inside ovens are subjected to negative pressure, unlike outside ovens. Does this problem always happen? If it does, you need to equalize the pressure in the room were the oven is.... Crack a window, install a fresh air intake, do not run exhaust fans, etc.
        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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        • #5
          Re: Smoke

          And especially in an indoor oven start with a small fire to heat the flue, like a couple pieces of paper held up to the flue to burn.

          Comment


          • #6
            Re: Smoke

            Pretty sure he is saying the oven is outdoors and the smoke is coming into his house. Just sounds like a new oven with a less then optimal vent to me.

            Comment


            • #7
              Re: Smoke

              Originally posted by shuboyje View Post
              Pretty sure he is saying the oven is outdoors and the smoke is coming into his house. Just sounds like a new oven with a less then optimal vent to me.
              If that is the case then agree, it's most likely inadequate venting and flue issues.
              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: Smoke

                I read the post as the oven is outside and the smoke makes its way into the house.

                I dont know who came up with the 6" flue size being adequate for any sized oven but it is obviously too small going by the number of smoke complaints that have appeared on the forum over the years.

                Using the 9" clay pipe for the flue would have been the better option.

                You cant over flue a wood oven, its not like a fireplace and chimney where the smoke is going vertically straight from the heat source.
                The English language was invented by people who couldnt spell.

                My Build.

                Books.

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                • #9
                  Re: Smoke

                  Originally posted by brickie in oz View Post
                  I read the post as the oven is outside and the smoke makes its way into the house.

                  I dont know who came up with the 6" flue size being adequate for any sized oven but it is obviously too small going by the number of smoke complaints that have appeared on the forum over the years.

                  Using the 9" clay pipe for the flue would have been the better option.

                  You cant over flue a wood oven, its not like a fireplace and chimney where the smoke is going vertically straight from the heat source.
                  I agree, but the required flue size depends on the size of the oven. Until th OP tells us the size of his oven discussion on the appropriate size of his flue is uninformed.
                  Kindled with zeal and fired with passion.

                  Comment


                  • #10
                    Re: Smoke

                    Flue size really has no direct relevance to oven size, it is related to door size. The flue has to be in proportion to the door, 10-15% is the normal range, depending upon the tallness of the stack.

                    A round 6" i.d. flue gives about 10 SqIn of cross section, so it should be fine for an opening of 100-150 SqIn, again, depending upon the height of the stack.

                    Comment


                    • #11
                      Re: Smoke

                      Originally posted by Tscarborough View Post
                      Flue size really has no direct relevance to oven size, it is related to door size. The flue has to be in proportion to the door, 10-15% is the normal range, depending upon the tallness of the stack.

                      A round 6" i.d. flue gives about 10 SqIn of cross section, so it should be fine for an opening of 100-150 SqIn, again, depending upon the height of the stack.
                      But bigger would be better in any situation, no?
                      The English language was invented by people who couldnt spell.

                      My Build.

                      Books.

                      Comment


                      • #12
                        Re: Smoke

                        Max,
                        I have been waiting for you to fill in some more details of your oven which is "loosely based on the Pompei"i. You might want to reply with some pics and the actual design that you have built. There may be other factors at play, here. I looked at your profile. I see that you intended to build an 850 mm/33.5" oven. I am very sorry to see that you asked about the "flu size" question in the designing stages twice with no responses. I will take my share of the responsibilty for that . But, for the size oven that you were intending to build, you may be in specs.

                        Can anyone offer advice? Will extending the flue make any difference? Note that I do not have a smoke chamber. The flue comes straight out of the arch.
                        A smoke chamber, that transitions from the narrow depth rectangle to the round (the "upside down funnel" that David S. mentioned) is the key to this. Going sharply from gathering the smoke from your door width to a 6" opening is a stretch IMO.

                        For the mean time though, experiment with a cheap section of 6" stove pipe (or two) added to the top of your flu. That will help with the draw, and may help the smoke to clear your house. (note: smoke will rise faster in cold weather and when the barometer is reading higher. It will lay close to the ground when the barometer is lower) When we see the smoke laying close too the ground, we think that rain is in the forecast.

                        My oven is not “under flued”. But, we do have weather extremes. Windy, is a given. We also have temperature swings. It can be in the 20's F. at daylight and in the 60's F. by around noon. That results in a reverse draw due to the chimney cooling the ambient air. Cool air will fall in warmer ambient air.

                        Try some of the cheaper solutons (preheat and or extend the flu) before you do something drastic.............. like you mentioned here.

                        The way it is going I don't think I will use this oven much, if at all
                        .

                        That is not an option .
                        Last edited by Gulf; 12-25-2013, 07:09 PM.
                        I don't care what folks say behind my back........They are either braggin' or.......lyin'

                        joe watson

                        My Build
                        My Picasa Web Album

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                        • #13
                          Re: Smoke

                          Bigger is almost always better, Brickie, for certain.

                          Comment


                          • #14
                            Re: Smoke

                            I consider my 6 inch flue "adequate" for my 750mm/29.5 in oven. I do have a nice fabricated s/steel hood funnelling the smoke from a rectangular opening in the arch to the round section of the flue though.
                            I would go 8 inches for anything bigger than 30 inches though.
                            If the oven is pumping that much smoke it's getting into the house, that might be a combination of things. e.g. damp firewood, damp oven, short chimney, etc.
                            Last edited by wotavidone; 12-25-2013, 07:48 PM.

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                            • #15
                              Re: Smoke

                              Originally posted by Tscarborough View Post
                              Flue size really has no direct relevance to oven size, it is related to door size. The flue has to be in proportion to the door, 10-15% is the normal range, depending upon the tallness of the stack.

                              A round 6" i.d. flue gives about 10 SqIn of cross section, so it should be fine for an opening of 100-150 SqIn, again, depending upon the height of the stack.
                              A flue is the engine that creates the draft. The larger the flue the bigger the draft. You can increase the diameter or the height to do this, but increasing the diameter is more powerful in obtaining more draft. A large oven needs a bigger draft than a small one, because the volume and fire is larger.

                              A round 6" flue has a cross section of 28 sq in not 10

                              The 10-15% rule may be ok for a 42" oven, but for a small oven you need to go more like 20%

                              Restricting the air intake from say 1500% down to around 25% of the flue area actually creates an extremely efficient combustion atmosphere, as some members have found by using a blast door.

                              My own oven has a 5" flue, a 21" internal diameter and an opening of 100 sq in which makes the flue cross section 20% of the opening. No smoke staining issues after four or five years.
                              Last edited by david s; 12-26-2013, 12:12 AM.
                              Kindled with zeal and fired with passion.

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