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double venturi/chimney redo

updated wed 30 apr 97 on fri 11 apr 97

I am rebuilding the chimney on my Bourry box wood kiln. It does not fire
properly and though I've made several changes that have improved things, I
think it's time to get to the heart of the problem which is insufficient

I've read and re-read THE ART OF FIRING and am trying to rebuild the
chimney using the double venturi concept.I bricked up all the exit flues,
except for one, which I partially bricked up to make it a 4.5" by 7"
opening with the 7" on the vertical. And there's the problem. Because the
7" is on the vertical instead of the horizontal, I can't follow the plan in
the book.Near as I can figure I need to maintain that 4.5" X 7" cross
section at the damper. To do that, I have to "turn" the opening. If I make
a flue box measuring 7" X 9" (cross section) by 7" tall, then I can create
the 4.5" X 7" opening at the top of the flue box.
The 9" X 9" above the damper and construction of the 2nd venturi is not a

Does all this make sense? Or is the larger volume of the flue box (over
what's in the book) going to create problems. I've built models of both and
it LOOKS like it might work. Then again, I wonder if this doesn't create 3
venturis. If anyone has any input on this problem, I'd appreciate it,
before I rebuild the whole chimney. Nils?


Ruth Ballou

John Weber on sat 12 apr 97

While I don't have the answer for you, I just rebuilt my 40 cu ft downdraft
kiln which uses natural draft burners positioned so they enter from the base
of the kiln (like a Geil). The kiln is a Minn. Flat-top similar to Nils
Lou's shown in the Art of Firing. I could not get draft when the doulbe
venturi box was in place so I had to open the initial exit flue to 9 x 4 1/2.
I also increased the size of the second venturi (the point where the flue
box opens into the pipe flue (same a Art of Firing except only 10' stack
sitting on top of 3' brick flue box.). The kiln now drafts well in
oxidation. I recognize that Nils Lou suggests using a 16' stack, so that may
of been my problem, but these changes made my kiln work. I think our kilns
are similar in that there is no assisted air, like Nils Lou who uses blowers
with his burners. I hope this helps.