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Chimney Size and Design - Forno Bravo Forum: The Wood-Fired Oven Community

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Chimney Size and Design

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  • Chimney Size and Design

    This my first post at this Forum although I used it a lot during my oven building adventure. I want to thank you all for the plans, the great information and help that this forum gave me. I used the Pompeii Plans and built a 36" oven. I followed the plans carefully with the exception of the flue size. I live in the tropical highlands of Chiriqui, Panama and do not have easy access to many of things that you can easily buy at a HomeDepot. I cannot buy stove pipe here. I had some extra 5" Simpson Dura that I brought down in 2006 for another project so I decided to use the bird in hand. I only had enough for about 30 inches of chimney. I figured this would be too short. The rectangular flu opening from the entry is 2.5" x 22". I cast a concrete transition to the pipe. The oven works great but sends a lot of smoke out the entry especially with heavy firing. I positioned the stove on our patio so I can run the stove pipe up through the ceiling and out the roof. I have ordered 8 more feet of pipe and will try this extension as it will be the easiest fix. What do you think? Will this work? The worst case scenario is that it doesn't and I will have to rebuild the chimney section. You highly experienced guys, please take a look at the photos at this link and let me know what you think and recommend: Wood Fired Oven Chimney
    Best, Lloyd
    Last edited by lcripe; 03-10-2010, 12:01 PM.

  • #2
    Re: Chimney Size and Design

    I would guess that for sure additional rise on the chimney would increase its draw once it heats up. I had to add an additional length of chimney tile to my oven and the draft up the chimney increased beautifully. No more smoke in the face or out the opening. Up and out would do it. Good luck.
    Bill
    Last edited by BurntFingers; 03-10-2010, 12:26 PM. Reason: oops, typo

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    • #3
      Re: Chimney Size and Design

      Thanks for your reply. Do you think I can get by with the 5" pipe? Lloyd

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      • #4
        Re: Chimney Size and Design

        I believe that there is a formula for the door opening height ratio to the inside height of the dome to allow for the correct draw. I remember something around 60% of the height of the dome would allow for the best proportion. The diameter of the flue pipe is important too. I am not sure what would work best. My terra cotta chimney tile is 8"X8" and it is two lengths high, ending close to 5 feet above the top of the door. Once it gets warm (just from the kindling) it draws well in all types of weather. There are some photos at: Bill P.'s Page - My Wood Oven
        Bill

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        • #5
          Re: Chimney Size and Design

          First of all, I like your flue transition. That's a nice design.



          A five inch flue pipe is marginal. FB recommends a six inch round flue unit for the 36 inch oven. A five inch circle has an area of 19.63 sq in, and a six inch 28.27 sq in. You can see that it's like one third bigger. That said, more length will absolutely help. It will get that smoke out of your patio area, too. Why not do a test with cheap galvanized stove pipe to see if it has the draw you want before you spring for the expensive duravent unit?
          My geodesic oven project: part 1, part 2

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          • #6
            Re: Chimney Size and Design

            Thanks for appreciating the work I put into the transition casting.

            I already ordered the dura vent and it is on the way. Should be here in a couple of weeks and I can't stop the process now. I will give it a try and am sure the draw will improve, but whether it will completely solve the problem is unknown. Unfortunately there is no simple galvanized pipe here in Panama.

            I did the same calculations on the area differences between 5" and 6" pipe ans saw that the difference in area is considerable. I took a risk with the 5" piece I had in hand. This is why I am concerned.

            I grew up around wood burning stoves and know that well cured wood is essential. I have seen stoves with 5" and 4" pipes function very well, but they didn't have such large intake areas. If the experiment doesn't work, I will simply have to rebuild the flue.

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            • #7
              Re: Chimney Size and Design

              I spent a week last spring in David, Boquete seemed like a nice place for an oven

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              • #8
                Re: Chimney Size and Design

                Yes, a wood-fired oven is perfect in Paradise! Checkout my weather web site: Boquete Weather Home to learn more about the weather and climate here.

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                • #9
                  Re: Chimney Size and Design

                  Does any have thoughts on placing the chimney on the BACK BOTTOM of the oven?
                  Wouldn't this vent the cooler air to maintain more heat in the oven??
                  Eli Rogosa

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                  • #10
                    Re: Chimney Size and Design

                    The rule of thumb when building an open fireplace is, flue size should be 10% or more than opening size
                    The English language was invented by people who couldnt spell.

                    My Build.

                    Books.

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                    • #11
                      Re: Chimney Size and Design

                      Placing the flue at the back and the bottom sounds rather like a downdraft kiln set up. I think that this would work extremely well once you got the thing up to about 350 C, but below that I think you would have trouble with smoke deciding that it would prefer to exit out the front door. I am certain someone would have tried this and found it didn't work well. You have to get a really god draft happening to draw a flame downwards. Wood as a fuel also easily provides an excess of fuel leading to unburnt fuel (smoke) The idea of having a flue somewhere other than at the front is attractive because of the flue creating xtra problems of reaching into the oven.
                      Kindled with zeal and fired with passion.

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                      • #12
                        Re: Chimney Size and Design

                        Thanks. This is the kind of information I needed. Will crunch a calculation today and see how it looks. May have to rebuild the chimney exit and use larger pipe. Will let you know. Best, Lloyd

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                        • #13
                          Re: Chimney Size and Design

                          I checked the numbers. The opening is 204.25 sq inches which means I need 20.43 sq inches of flu (10%). The 5 inch pipe is 19.63 sq inches. It will be close, but should work. The pipe is enroute in the Atlantic on a ship and should be here within a couple of weeks. I will install and give you a report. Thanks again to all that responded to my questions. Lloyd

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                          • #14
                            Re: Chimney Size and Design

                            Wanted to let you know that I finished repairing the chimney. It now goes through the patio ceiling and roof and is about 12 feet long. It draws well and smoke goes where it should! It was quite a job cleaning off all the soot and smoke from the patio walls and ceiling and then repainting it. The 5 inch pipe is working so will not have to change to 6 inch. Thanks for all your input. Here is link to photos of the repair:

                            Wood Fired Oven Project - Chimney Repair

                            Lloyd

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                            • #15
                              Re: Chimney Size and Design

                              Beautiful pictures, love the dome and arch combination.
                              Our Facebook Page:http://www.facebook.com/pages/Stoneh...60738907277443

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