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Tom B 08-12-2009 11:18 AM

How big a vent for a 42" oven.
Hello from Madison Wisconsin. My hopes for the Badgers football season are not very high so I've decided to focus on building an oven instead. I'm building a 42" Tuscan style dome oven (21" high), following the Forno Bravo plans (version 1.2) pretty close. The vermiculite insulating layer is poured and drying and now I need to lay out the oven, opening, etc. The insulating layer is 75 deep by 69 wide.

For the opening I was planning on following the plans: so about 20" wide by 12" tall, square because we will make it from angle iron and brick of course.

My question is about the vent size I should shoot for. I'm going to have a local shop fabricate a vent from SS and then use a commercial clay flue. The flue sizes are, according to my mason, 9 x 12, 12 x 12, 8 x 8. Those are OD measurements. I've got a call in to see about other sizes. Take away for the thickness of the clay and you have, I suppose a 7x10, 10x10, etc inside diameter for the flue.

If the opening is 20", I imagine the vent will be about that wide, or 20".

But what depth do I need to shoot for??? The pictures of steel vents in the plans are hard to follow, and I don't see any discussion of minimum sq inches of vent opening to get a good draw and keep the smoke from billowing out the opening arch. It seems logical that at some point it can get too big, but it also seems like more vent opening and a bigger flue size will lead to better draw and a better fire. My yard is sort of sheltered from the wind too, I might add.

Anybody have an opinion, I'd love to hear it. I'm getting ready to outline the footprint so I need to set that vent depth pretty soon.

P.S. Is it wise to "slope" the brick arch upward to the vent too??


splatgirl 08-13-2009 07:23 PM

Re: How big a vent for a 42" oven.
my understanding of vent design is basically the bigger the better, or the more volume at the start, the better, but I'm sure I'm oversimplifying. I my arch is 10-12"h x 18"w. My vent starts out just under 18 w. and tapers up to meet the 6"dia. anchor plate for my chimney over what amounts to about three courses of brick, or roughly 8". It draws beautifully with one 3' section of chimney.
If your arch width is 20"id, then yea, I guess I'd spec that assuming it's going to have a flange to attach it to the brick. Based on my experience, I think whatever taper you put on it will be just fine, but I'm certain I read something here about vent volume vs. chimney diameter. If you can't find it, google might work.

dmun 08-13-2009 08:48 PM

Re: How big a vent for a 42" oven.
There are rules about flue size in masonry fireplaces, where the flue area is a certain fraction of the fireplace opening. Since we have fairly small openings, we don't need huge flues. The 8 by 8 is what i used, and it draws like a champ, but my chimney is two stories tall which helps a lot. The rule of thumb about your transition is that the bottom should be twice the area of the top.

Neil2 08-14-2009 11:11 AM

Re: How big a vent for a 42" oven.
For a 42 inch oven, I suggest about 60 to 80 square inches of vent funneling up to about 30 to 40 square inches of chimney.

One other note. I see your dome is to be 42 inch. I suggest that the dome be elliptical in shape with a height about 85% to 90% of the radius which would be about 90% of 21 =18 3/4 inches in your case.

Another dimension, and this one is critical to a well functioning dome oven, your door height should be 62% of your inside oven height. Or, in your case, 62% of 18 3/4 = 11 5/8 inches.

sonomacast 02-10-2011 01:37 PM

Re: How big a vent for a 42" oven.
I found this on an italian Oven building web site
Diametro del forno diametro minimo canna fumaria in cm
da 60 a 80 ----cm 20
da 80 a 120--- cm 25
da 120 cm in--- su 30

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