Chimney Design

[stevenki]

My Mason decided ondoing a masonry chimney rather then the 8" pipe. Isthis ok. Height wise I have read if the ovenis 36"deep then the chimney should be at least that in height. Any concerns if itis 2 inches short not including the cap of course? Also If the chimney is not 8" square is that a problem? Again he made this thing taper in like a fireplace chimney. Just making sure it will still draw outside the dome.

Steve

[CanuckJim]

Chimneys

Steve,

Not sure exactly what type of oven you're building, but I think the following observations are more or less universal. There is all kinds of voodoo out there about chimney heights, most of which is unnecessary. The best source for the math I've found is still from the 18th century inventor Count Rumford of Revolutionary War fame. BUT, he was writing about inside fireplaces.

My oven, for example, has a 48" deep hearth. The actual cap height of the chimney is considerably less than that. For aesthetic reasons, I stopped at about 18 inches above the peak of the roof. For the interior, I corbelled in from the oven mouth until I could mortar in two pieces of half inch rebar on which to rest 7 inch round flue tile. I like the round tile because it has a male/female connection system that's quite stable, ending up with a very smooth transition, tile to tile. You'll be fine with somewhat less of a flue dimension than that provided by 8 inch square tile, but not a whole lot. The only problem with not using a liner is that your oven will reach very high temps (with corresponding temps in the gasses up the flue) and straight masonry will deteriorate over time. I'd recommend a liner of some sort that will prevent this.

It's worth saying that my oven draws like a steam locomotive on steroids.

The attached cold pic is very low res, but you'll get the idea. I will be taking better ones, in better weather, shortly. And I'll post them if needed.

Jim

[margepadgitt]

Chimney questions

Chimneys on outdoor bake ovens and fireplaces for that matter don't have the same rules as indoor chimneys. I suggest using a small flue liner (approx. 8" square or round) made of clay tile or stainless steel and I suggest a height of 3' above the dome. The chimney will draft when the temperature outside is considerably cooler than the flue gasses (which it will be any time of year with an oven). The thing to look out for is any nearby structures or trees that could inhibit draft - anything within 10' that might cause wind induced downdrafts might cause a problem, although this is rare. A flue liner is a good suggestion - I like the masonry idea. We have actually used heavy decorative clay chimney pots (not flue tiles) as chimneys without any brick masonry around them and they seem to work fine.

Marge Padgitt
Padgitt Chimney & Fireplace
Kansas City
www.chimkc.com