Chimney Flue Sizing

[jbarnes]

Does anyone have a formula/info that would provide optimum size and shape for a chimney flue relative to the oven size?

[brickie in oz]

No less than 10% of oven opening size, round is best but if built in brick rectangle or square work ok too.

[david s]

Divide the oven diameter by 5 to get to get an adequate flue diam.(if using a round flue pipe)
eg. a 42" oven should have at least an 8" flue. A 21" should have at least 4"
You can't really over flue the oven so go bigger if in doubt. The draw can also be increased by making the stack higher as the power of the flue is governed by its volume. My 21" oven has a 5" flue which is more than adequate but works really well. This is a bit of a rough rule as the smaller you go the more the need to increase the size, relative to the diam.

[sonomacast]

Flue size. The US underwriters Laboratories state that it is best to use a flue as large as you need and no larger;
So if you were burning 12 lbs/hour of wood you would be generating 97,000 btu's and therefore need a 6" Flue (My 41" oven would burn less than this)
16.5 lbs/hr=7" according to The US underwriters Laboratories.
21.6 lbs/hr=8"
27.3 lbs/hr=9" and so on
Also 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

Costruzione e uso del forno a legna

Costruzione e uso del forno a legna

[brickie in oz]

6" is a tad small.

[david s]

"Flue size. The US underwriters Laboratories state that it is best to use a flue as large as you need and no larger;"

They are presumably referring to fireplaces in this context and would be attempting to minimize heat loss up the flue. Most fireplaces are a simple updraft configuration. A pizza oven is a bit different especially as most of the designs use a cross draft system which requires extra flue pull, particularly at start up to prevent smoking out the front .It is therefore desirable to have more rather than less draw. This equates to a larger, rather than a smaller flue.

[david s]

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

Costruzione e uso del forno a legna

Does this mean they are suggesting a 100cm (36") oven should have a 25 cm (10") flue?
I'm all in favour of a bigger flue, but I think that is way bigger than necessary. Maybe they use this formula for indoor ovens, where smoke is more of an issue.

[jbarnes]

Thanks to all for your inputs. You have confirmed my belief that for my 42" oven with an arched opening of roughly 21" x 12.5, "a 8" round clay flue 48" tall will be adequate for my oven. I intend to wrap it with 1" insulation, wire and finish with stucco. Any comments?