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I too am interseted in info regarding:
1. the worth of a chimney on a dome oven
2. whether there is a minumum height before it will effectively draw, and any other tips on the best chimney dimensions.
Hi Sammy and Steve.
I am yet to finish the chimney on my 40" (1000mm diameter) Pompeii. See the link: http://www.fornobravo.com/forum/21/n...-a-2045-2.html
to see the stainless vent with an 8" (this one is a temporary galvanised section) chimney which I will extend with 3 stainless flue sections, raiing it around 3 metre high. This will take the smoke high so that the neighbours are not affected as my oven is well below the fence line in an excavated area. It draws exceptionally well at present.
I would go for the 8" myself and have that extra capacity in reserve, what harm can a larger one do anyway?
Prevention is better than cure, - do it right the first time!
The more I learn, the more I realise how little I know
I know from old steamship design that taller is better for draw. But have not seen any formulas or calculations.
My oven is a 42" pompeii. I have an approx 12" transition plus a 24" run of 8" duravent and you can easliy hear the "rush" of exhaust air go up the chimney from the arch opeing. I sized mine based on the roof pitch and looks. But hey, it works.
I believe I've read that a 36" oven works best with an 8" (or equivelent) and a 10" is recommended for a 42" oven. Opening width I believe should be 50% of dome diameter and the height of the opening around 60% of the dome height. So you are spot on.....actually your dimensions are identical to mine.
Keep in mind, many variations and personal adaptations have been made by all of us.
Based on how I wanted my oven to look and the size of the overall footprint I wanted to maintain, I cut down the overall slab dimensions; this lead to giving up a couple of inches in the depth of my entry arch/chimney area. I ended up using the 6" pipe - 48" length.
I too had heard a taller flue pipe draws better; since I gave up 2" in the diameter I went with the 48" length hoping for better draw. Mine works and draws well, with a little smoke coming out of the entry on start up. I've found building your fire correctly greatly reduces the smoke.
If I could change one thing about my oven it would be to take back a few of the inches I cut things down and make my landing and entry a little deeper so I could have used the 8" pipe.
Hope this helps
My opening is very close to 18"x 11" opening (arch not square shape)
I have a 7inch flue which is about 2300mm (7.5 feet) tall - on my oven. It draws very well (no/minimal smoke out the front). I'm guessing the following help:
- the flue top sits about the roof line where its gets the breeze.
- bottom of flue is immediately above the top of oven opening - most direct path for heat/smoke to exit
-medium sized landing in front of oven
Would too big a flue....mean less draw (suction) at a certain point...
Well I did some searching and found this general rule.
Masonry fireplace flue sizing is determined by the size of the fireplace opening below. Masons traditionally calculate the size of fireplace flues using a "rule of thumb" that the CSA (Cross-Sectional Area) of the masonry flue be at least 1/10 the CSA of the fireplace opening. For example, an 8x12 (id) flue liner is used in a chimney venting a fireplace with an opening measuring up to 40" wide x 24" tall
And these two which are much more technical and do give equations. 19b
So for Sammy's oven opening, it looks like a flue of 5" (min) would be suitable, based on this 10% formula. My opening is a similar size, and I was thinking of 8" diameter, but if 6" would be more efficient, I might need to reconsider.
We recommend 6" ID double wall (DuraVent) for the 31" and 35" precast FB ovens, and 8" for 39" and 43" ovens. Higher chimney systems draw better, but a shorter run that only gets you to 24" higher than your enclosure works fine.
A wonder of nature, brick ovens naturally draw very well.