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Chimney issues - Forno Bravo Forum: The Wood-Fired Oven Community



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Chimney issues

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  • Chimney issues

    Hey all,

    So a couple of questions around the chimney, currently mine is removable from my oven.... I'm gonna wager this is not a good thing...

    What affect does the height of the chimney have on the oven?? Mines currently maybe 24" on a good day, if I was to make it 48" what would that change??
    Im using 8" single wall.

    Would adding a damper to the chimney help keep in heat? When should I open it and when should I have it closed??

    Has anyone tried the fire place rope between chimney and brick?? Or is the high heat silicone a Better shot?? I swear I tried that at one point and it came away from the metal chimney after not so long..

    Here's my build


  • #2
    Re: Chimney issues

    The draw of the chimney is roughly proportional to its volume . A damper in the chimney for a Pompeii style oven is unnecessary.
    Whatever you use to seal the flue from the oven keep doing it as it is an area of water entry if it gets weather on it.
    Last edited by david s; 06-27-2012, 11:59 PM.
    Kindled with zeal and fired with passion.


    • #3
      Re: Chimney issues

      The effect of a change in height is essentially the square root of the new height. The effect of a change in width is the new width. That's a not entirely correct simplification if the formula for draw, but it illustrates the point: changing width creates more draw for the given increase. In other words, if I double the width, I double the draw. If I double the height, I only get an increase equal to a root of the height, which is smaller.


      • #4
        Re: Chimney issues

        Thanks for that clarification azatty. So to increase draw it would be better to increase the flue cross sectional area. rather than increase height ? Presumably there is also an ideal relationship between height and cross sectional area ?
        Kindled with zeal and fired with passion.


        • #5
          Re: Chimney issues

          Yes, increasing cross section increases draw "faster" than height. The formula is CA[sqrt(2gH(Ti-Te/Te))], where C is the discharge coefficient (0.65 to 0.70), A is the cross sectional area of the chimney, g is the gravitational constant (9.8 m/s2), H is the height, Te is the temperature outside the chimney, and Ti is the temperature inside the chimney (in degrees Kelvin--so at absolute zero the chimney won't draw. What a surprise!).

          I think the "ideal" height and width can only be determined in the context of a particular target draw. If we know that we need to draw 5 cubic meters per second to keep smoke put if our eyes, we could figure the necessary height and width.

          I remember seeing a spreadsheet of experimental data from WFOs that attempted to find the ideal draw. The ideal draw was the figure at which the oven didn't smoke out the front. I thought some people on this forum put it together. So I messed with the height on my chimney until I hit the required draw figure. I'll see if I bookmarked it.

          Edit: Found it, I think. A post from four years ago has a calculator and smoke/no smoke data in a zip file. Might be something worth stickying. Xene has been looking for the same thing.

          Last edited by azatty; 06-28-2012, 06:03 AM.


          • #6
            Re: Chimney issues

            This is one of those areas where I am not confident in calculations. There are too many variables in the throat area to accurately compute it. Good old rule of thumbs work here:

            Flue area at least 10% of the inner arch area.

            You should generally use the largest flue that will work with the throat area available.

            Round is better than square or rectangular (calculate area of square flues by drawing a circle to fit, for rectangular ones draw 2 circles).

            Make the throat as smooth and free-flowing as possible.

            Make the outer opening arch area as close as feasible to the inner arch area (but big enough to allow the door to be fitted).


            • #7
              Re: Chimney issues

              Thanks for the link azatty!
              My oven album is here



              • #8
                Re: Chimney issues

                Any suggestions for reattaching or creating a better seal between the chimney and the brick??
                Here's my build



                • #9
                  Re: Chimney issues

                  Originally posted by wotavidone
                  I find the discussion on chimneys quite fascinating. In particular, I got interested in the oft repeated assertion that increasing the diameter of your flue increases the draw. I guess that's true, but surely there must be a top limit to this? Draw is caused by the temperature differential, right?
                  Draw is actually caused by a pressure differential. Heating a gas causes its pressure to increase, and it will flow from an area of higher pressure to lower pressure. However, since Gay-Lussac's law tells us that pressure and temperature are proportionate, most of us tend to think of draw in terms of temperature differentials.

                  As stated the ideal draw is when the oven doesn't smoke out the front. This is when you are capturing all available hot gas. After that, any extra draw is pulling in cold air from the front, diluting your hot gas, and reducing the temperature differential. Ultimately, if you keep increasing the diameter, would you get to where you have the equivalent of no flue, as if your hot gases were coming out of the oven into an open space?
                  In the limit (that is, as the variables approach infinity) the functions become nonsensical for all practical purposes.

                  I think we have to do some thinking about some of these numbers. For example, at work we use a 1300 horsepower motor (yes that's 1300 horsepower, not a typo) to move 13 cubic meters of gas per second. Therefore, I find it hard to visualise a pizza oven flue moving 5 cubic meters per second. Did you mean 5 m/sec velocity, maybe?
                  Nature moves tremendous volumes of air--more than human machines can move--with pressure differentials. A wind storm is nothing more than a relatuvely small pressure differential.

                  That said, I pulled 5m3/s out if the air just as a number, not an actual representation of draw. WFO draw is measured in cubic centimeters per second.

                  That's the other thing about increasing the size of your flue - you might move more gas, but you will probably reduce the overall velocity, and exit velocity is very important, the faster the gas leaves your chimney the higfher it will go in the open air.
                  Gas velocity results from the pressure differential. Change it, and the gas will stop flowing no matter how fast it's moving. That's why a cold chimney will initially smoke you out even if you have a roaring fire. The gas may have a high initial velocity, but when it hits the cold air, the pressure differential plummets and the gas stops moving.
                  Last edited by azatty; 06-30-2012, 08:12 PM.


                  • #10
                    Re: Chimney issues

                    My original investications into chimneys found all these calculations and suggestions.....and stacks of ovens with smoke stains...seems they all don't act the way they should till properly hot! I also considered the amount of kids that have climbed over the oven during construction and a steel pipe didn't seem to be kid proof.
                    So finally construction was a low brick chimney wide as the oven door 1/2 brick deap and 4 bricks high...no smoke stains ....draws well from cold. Oven is surounded by trees and bushes so no problems with smoke being blown down such a short chimney.

                    Regards Dave
                    Last edited by cobblerdave; 06-30-2012, 11:08 PM. Reason: spelling
                    Measure twice
                    Cut once
                    Fit in position with largest hammer

                    My Build
                    My Door


                    • #11
                      Re: Chimney issues

                      Originally posted by Kiwi_in_canada View Post
                      Any suggestions for reattaching or creating a better seal between the chimney and the brick??
                      I used a masonry flue. So, to connect to the oven I just made a bed of refractory mortar and lowered the flue on to it, making sure it was nice and level. I believe you want to avoid connecting the flue to the chimney because as it expands and contracts with the heat,things will break.So what you do is put some newspaper between the mortar and flue on the way up. This breaks adhesion


                      • #12
                        Re: Chimney issues

                        keep it simple! Don't bother with a chimney...


                        • #13
                          Re: Chimney issues

                          Originally posted by tonybaker View Post
                          keep it simple! Don't bother with a chimney...
                          Thats fine if you like smoke and heat blowing in your face as you cook.
                          The English language was invented by people who couldnt spell.

                          My Build.



                          • #14
                            Re: Chimney issues

                            I did not want any soot on my beautiful stone work or a steel chimney climbing up high, so I went for volume over velocity...... 13ga. steel 23in x 9inch and 24in high from the arch .. bedded into vermicrete which seems to expand with the steel . It draws fantastic , 3 years later still no stains !