Flue dimensions for 37" Pompeii Oven

[Whackers]

I wanted to know before I finalise the design of the oven opening arch, whether a rectangular flue is an issue, it seems that a circular 6" flue pipe is suitable up to 36" and that would have a surface area of 182 cm squared. The alternative would be the rectangular flue which I think works out at 301cm squared so unless I've forgotten my geometry calculations from school the flow of air is at least 50% better if I go down this route - assuming that the height of the flue is the same for a fair comparison of course. Secondly, any strong feelings about suitable chimney height in a relatively sheltered corner of the garden, I was thinking just 400mm, but could afford and build up to 600mm if it would make a huge difference?

Here was my original post which got lost on my main build page as a topic:

1. For the size of oven (37inch diameter) is there any problem with selecting the following middle sized option of these cavity liners (by my calculations it has a larger surface area than a 6 inch circular flue) and would work well. *(see photo) they are 215mm x 140mm https://www.selcobw.com/products/bui...er-215-x-140mm and would be easier to connect to my intended rectangular exit through the oven entrance archway than a 6 inch diameter circular alternative. The 215mm x 215mm flue seems far to big for the purpose.

2. What height should I go to for the chimney, I've read a few thoughts and was planning on having three of these, connected with home brew mortar which would take the height up to 600mm from the point of connection with the arch, could/should I just opt for two and 400mm of height in the chimney?

[hughjamton]

I can't answer your query with absolute certainty, but logically there should be no problem with the area of the flue.
Two things I do know, with certainty, the taller the flue the more the draw, although realistically you only the the smoke to go above peoples heads, and more importantly, you will have to insulate those flue liners or they will crack.

[JRPizza]

I don't know where you are building your oven but if it is close to a structure or if you are going to build a roof there are most likely local codes you should follow. I did not need a permit to build my oven or roof but followed local code for height above structure to cover us in case there was ever a fire.

[Whackers]

I can't answer your query with absolute certainty, but logically there should be no problem with the area of the flue.
Two things I do know, with certainty, the taller the flue the more the draw, although realistically you only the the smoke to go above peoples heads, and more importantly, you will have to insulate those flue liners or they will crack.

Guessing I can use either insulation blanket (used for the dome insulation too) or vermiculite fed in to the gap between the bricks I intend to run up alongside the flue with a very small gap between brickwork and the flue? Would I need the insulation if I have the brickwork running up alongside the flue?

[SableSprings]

You might enjoy (or be terrified...) watching this video of thermal shock on a chimney tile. Insulating your rectangular chimney tile is a good idea as is keeping your flue clean. The insulation simply helps moderate the thermal shock in the flue tile material when you start up your oven. Generally, with a good smoke collection chamber (think of an upside down funnel that leads into your chimney) and normal firing of the oven, you won't see the extreme thermal shock generated in the video. Your flue will expand and contract slightly with firing/cool down, so leaving a little cushion between it and your chimney bricks is a good idea IMHO...

https://www.youtube.com/watch?v=bxOkwQyY-2w

[Whackers]

You might enjoy (or be terrified...) watching this video of thermal shock on a chimney tile. Insulating your rectangular chimney tile is a good idea as is keeping your flue clean. The insulation simply helps moderate the thermal shock in the flue tile material when you start up your oven. Generally, with a good smoke collection chamber (think of an upside down funnel that leads into your chimney) and normal firing of the oven, you won't see the extreme thermal shock generated in the video. Your flue will expand and contract slightly with firing/cool down, so leaving a little cushion between it and your chimney bricks is a good idea IMHO...

https://www.youtube.com/watch?v=bxOkwQyY-2w

Thanks - and wow, that's very stark warning about getting this right!

[Ope-dog]

Hi Whackers, I am actually considering the same material for the chimney in my build. I found 3 pieces of clay flue tile, approx 8x17 ( or 203mmx431mm in your terms.) While this was a larger opening than I was intending, the pieces came from my father and I thought it might be fun to incorporate some of his old yard flare into the build.

Does anyone know if an 7" x 16" (ID) chimney opening would be "too" big for a 36" oven? There's lots of mind boggling info on chimneys, but I'm having trouble nailing down the best size window to be in. I suspect the main disadvantage would be thermal loss through a larger chimney?

[SableSprings]

I don't have the extensive dynamics of flue flow () in my background, but thermal loss would not be the problem with a larger chimney diameter. Since chimneys work best as they heat up a bit (increasing the draw), the problem I see with a large diameter flue is that it may not pull all the smoke up...you'd probably have a significant amount of smoke coming out the front of the oven as is going up the chimney. I suspect that if you used a piece of that larger flue to funnel smoke into a 6" to 8" pipe (normal size for a 36" to 39" oven) it would work really well. The smaller pipe producing the suction required to pull that smoke up into the intended path. Your transition to the smaller pipe would present some interesting problems, but I suspect it would be doable. I agree that it would be great to incorporate something of your Dad's in the build. You could also just stack them to the side of your oven and use them as a stand for your peel & other tools.

[bamforp]

I wanted to know before I finalise the design of the oven opening arch, whether a rectangular flue is an issue, it seems that a circular 6" flue pipe is suitable up to 36" and that would have a surface area of 182 cm squared. The alternative would be the rectangular flue which I think works out at 301cm squared so unless I've forgotten my geometry calculations from school the flow of air is at least 50% better if I go down this route - assuming that the height of the flue is the same for a fair comparison of course. Secondly, any strong feelings about suitable chimney height in a relatively sheltered corner of the garden, I was thinking just 400mm, but could afford and build up to 600mm if it would make a huge difference?

Here was my original post which got lost on my main build page as a topic:

1. For the size of oven (37inch diameter) is there any problem with selecting the following middle sized option of these cavity liners (by my calculations it has a larger surface area than a 6 inch circular flue) and would work well. *(see photo) they are 215mm x 140mm https://www.selcobw.com/products/bui...er-215-x-140mm and would be easier to connect to my intended rectangular exit through the oven entrance archway than a 6 inch diameter circular alternative. The 215mm x 215mm flue seems far to big for the purpose.

2. What height should I go to for the chimney, I've read a few thoughts and was planning on having three of these, connected with home brew mortar which would take the height up to 600mm from the point of connection with the arch, could/should I just opt for two and 400mm of height in the chimney?

I too considered those liners from Selco but moved away from the idea because they are cavity liners not flue liners so they are not intended for high temperature use. They are terracotta so they might work but it may be worth testing them to see how they withstand high temps.

[Roger705]

Hi, I'm a bricklayer and wouldn't use the clay flue for a pizza oven. I've done alot of chimney rebuilds and believe me they break down. Spend the money and use a proper insulated chimney pipe, adapter and cap. You know the old saying " good isn't cheap and cheap isn't good" Cheers

[UtahBeehiver]

Thanks, for the comments, since you are a pro who deals with this material all the time this pretty much reinforces the video that David S links to here and there on how clay chimney liners cannot handle the thermal shock of high temps.

[Blairt]

It's interesting what Roger705 says about clay liners. Our building code here (British Columbia) specifically states that, if you are using a masonary fireplace (or oven) you MUST use a clay liner and masonry chimney. You are specifically not allowed to use a steel chimney unless it is attached to a steel firebox. Seems like a stupid rule but it's in our building code.... "Metal liners referred to in Sentence (1) shall only be used in chimneys serving gas- or oil-burning appliances."
Different rules in different areas :-) This is only applicable if the wood burning unit is attached to the house - which mine will be as it goes through the roof of my outdoor kitchen.

[BenKeith]

Not commercial but I can tell you my experience with a 36" cast oven.
I used 10"x4" rectangle to 6" round, metal adapter in mine as form for the chimney. When I first fired it, it was letting a lot of smoke come out of the front of the oven. Since there was not enough heat to generate a strong updraft, more was passing by the opening and coming out the front than going up the chimney. After it got a raging fire in it, it had enough draft to pull it up the chimney. I ended up having to make a door I could just stick over the opening with a metal flange to make a friction fit inside the opening, with a large 3"x10" slot cut out of the bottom. The door ends up being a good thing because the draft though the opening works almost like having a fan blasting the bottom of the fire and it burns better and gets hotter quicker.
If I did it again, I would go with something like a 8"x12" opening in the top feeding into a 8" pipe.
Mine is outside so the smoke wouldn't cause a problem, but I didn't want it making the whole front black like you see so many of them. Mine is a couple years old, has been fired at least 100 times and the brick arch around the opening has very little smoke stain on it.

On other bit of into, I used five inches of #8 matt on the top half and four inches around the sides of my dome. When I was cooking it in the first time to cure it, over an 18 hour period I had mine where it was measuring 1,500f on the inside top of the dome and it reached a maximum of 146f on the outside top of the dome. After several hours of cooking temps, mine never gets more than warm to the touch on the sides and the max I've ever measured on top of the dome is 131f.

One other thing, people talking about metal liners. I used galvanized flue pipe to cast my refractory chimney then a Stainless steel pipe going up from it. BAD idea. Within a year and a half, the galvanized was rusting out and dropping rusted pieces down into the opening. I ended up having to take the SS pipe off and a long bar and scrape the galvanized out of the chimney pipe and the adapter I used for the form. I was so rusted, it came out with no problem and I had three inches or more of refractory around it so it was serving a nothing but a form I used to cast around, but still was making a mess until I removed.