42" Oven in Raleigh, NC

[RandyJ]

Yes you need a 8" chimney with a 42" oven or a ton of smoke will poor out the front.

Randy

[UtahBeehiver]

Not entirely true, I have a 42" and used a 7" ID Selkirt double all SS vent and I do not have any smoking out the front issues beyond what we normally see. I would agree that 6" is probably a little small and 7" is an odd size, but I used it because in my scounging I got it for $25. So certainly greater than 6" ID.

[deejayoh]

Area comparison...
6" = 28 sq in
7" = 38.5 sq in
8" = 50 sq in

You definitely can't get a way with 6". It's just over half of the recommended size. Russell may be getting away with 7" but as he points out, it's an odd size and probably not really a cost savings if you have to source it.

Only caveat to all this is that you can probably use 6" if you have a super tall vent (think 8 feet or more). There's some formula for this somewhere - but at the end of the day that doesn't really save any money either...

[JRPizza]

Check out this thread. Tscarborough in post #2 recommends a flue size of 10-15% of door opening (not sure where those numbers came from). I used an 8" pipe for my 39" oven, where I have a hemispherical opening of around 198 in^2, which works out to be ~25%. I was not shooting for any ratio - I just saw that 36" was the recommended max oven for a 6" pipe and went larger. According to those calculations I could have gotten away with a 6" pipe (~14%) so I am not sure if the numbers are correct. A 10% pipe would be pretty small. Others in that thread recommend going with 8" for a 42" oven.
https://community.fornobravo.com/for...8-chimney-size

[deejayoh]

JR - I did the same as you - 8" flue on a 39" oven (and no problems ever) - but there is a trade off where they higher you have your flue, the greater the velocity of the exhaust.

As an example, this table is for heaters, not WFOs - but same idea. It looks like the flue height for a 6" would need to be roughly 5x the height of an 8" flue to give the same capacity

http://inspectapedia.com/chimneys/Ch...le_C9B_C9C.php

[boombaby31]

Thanks to all of you for the replies. I went back and checked our numbers and our oven opening is a little small since we went with a lower dome height. It comes out to 200 square inches, which would make the flue area 14% of the opening area. I'd be interested to know where those numbers are coming from because, from your comments and fornobravo's recommendation, it definitely sounds like we should get an 8" chimney. Deejay and JR, do you have any issues with smoke out the front?

[JRPizza]

I have a little staining of my outer arch, but I think that was from doing curing fires with yard scraps that were not well seasoned, and running without my chimney pipe installed on top of my arch. When I fire with a 3' pipe I don't get any smoke out the front.

[Gulf]

................... I'd be interested to know where those numbers are coming from...............

If you are talking about the "10 to 15%", it comes from fireplace opening/flue design. "10 to 1", cool air in, hot air out. 10% is the bare minimum for fireplaces in single story dwellings. A minimum of 12% is reccomended for taller residential chimneys. Since most say that "you can't over flue a WFO", I'm guessing the number range is stretched to up to 15%. I agree with that range. I don't necessarily agree with the statement "you can't over flue a WFO". I say that "you can't practically over flue a WFO" . That said, there are situations that any fireplace or WFO will allow some smoke to escape out the front on start up. No matter how perfect you size it. However, that should be only for about 30 seconds at the most while you are preheating the flue .

[deejayoh]

I only have ever gotten smoke out the front if I start a super smoky fire and the flue isn't hot. Not long enough to ever be an issue.

My opening is 22" across, and basically a half-circle set on top of one course of bricks - so (11^2*pi)/2 + 2.5 x 22 = 245 inches. Flue is 8" so that is a ~20% ratio.

I think given the amount of heat a WFO generates, you want to be well on the upper end of any ratio that is designed for a fireplace. There are a lot more BTUs moving out of a WFO than a fireplace.