Ope-dog's 36" Oven .. (fingers crossed!)

[Ope-dog]

Hello Gang! I have been VERY impressed with all the hard work and ingenuity I've been reading on this site. As I'm still a month away before I can get building my oven, I figure I should make the best of my down-time to finalize my design and maybe get some feedback from the folks that have lived and breathed the trials I am up against.

Last summer, I wanted to build a BBQ at my weekend property in WA state. Business delayed it a year, and so this last month I was able to get going on the BBQ. Well, as I sketched up my final BBQ design, I came to the conclusion that putting in a smoker sounded pretty cool. Never smoked anything before, but why not have the option, right? So the design got tweaked and a smoker incorporated. After I went down a massive rabbit hole on youtube learning how to smoke and my first brisket came out surprisingly juicy and edible, I threw out the idea to my wife about having a pizza oven on the end of the BBQ would be fun. Her eyes lit up and she squealed like she was back-stage at a Neil Diamond concert. (Ok, I'm dating myself a bit here.. but you get the idea..) So it was so. We were going to have a pizza oven...

Down another rabbit hole.. but the science behind a pizza oven makes smoking look like child's play. No matter. I'm too committed to the idea. A friend of mine from SoCal purchased a pizza oven from this site a few years back and he's never shut up about it. But I've never been the guy that just buys it and calls it good. I have to always try and build it myself. (I also have to try trails in 2WD before I lock in the 4WD, but that's another forum..)

But I'm also a bit of a purist and I think it would be cool to have an oven that is a bit more "authentic" to how they were built centuries ago. In other words, if I can use organic (ie. cheap..) materials I think it would give it a bit of uniqueness and backstory. So I have a few questions out of the gate and if anyone has any input to offer I'd be more than happy to hear it.

1. Looking to do a 36" ID on the oven. (The base is 47"x47") I'm reading that the height of the dome should be 60-75% of the base ID. Does anyone have any specifics they can add to this? It seems the arch height is 63% and that has a long tried-and-true history to it.
2. I have read conflicting reports of people using glass bottles and sand as an insulation under the base of the oven floor. Most folks here look to use manufactured materials for the insulation. While I can imagine it's much easier to work with, I go back to my thought process of trying to replicate a build from 200+ years ago and materials that would've been used as such. Any thoughts / info would be great to hear..
3. Sticking with the insulation theme, I was planning to have an outer shell of a manufactured brick/paver material which I have already used to clad my existing BBQ. The thought process is that the entire unit, when finished, will all have the same façade. While these are not fire bricks, I trust that if I insulate the dome sufficiently, they will be able to handle the heat. (ID will be built with firebrick..) So will a 3-4" layer of v-crete provide enough insulation to then clad with a concrete paver? Also, I have read some of the earth builders like to use a cob mixture for ovens, render, etc. I have plenty of horse manure on the property, along with sand, clay, and water. While this cob mixture has been used in earth oven applications, I'm not sure of the insulative properties next to a 900 degree oven.
4. Has anyone built their oven using forms, sand, or anything other than an IT? I have seen arguments made for all types and can see the pros and cons of all types. Just curious of anyone else's 1st hand experiences.

I know I dwell a bit on the insulation subject. While I can certainly agree that today's manufactured materials will be much more efficient, I don't know if I need this oven to hold cooking temps for 24+ hours. That being said, I'm also from the mind-set that you always put in more electrical outlets than you think you'll need and over-build a deck for the amount of people you COULD have over at any one time. :-)

[UtahBeehiver]

I am responding to your thread rather than hijack another members thread. I would suggest you post all your question on your build thread that way all the answers are centralized. Dome height for a pompeii is the radius of the oven, IE 32" oven has a 16" dome height. Low domes ovens will have a lower dome height. I am wondering if the 60-75% you mention is actually for the inner arch height. Standard rule of thumb is 63-65% of the dome height.

[Ope-dog]

Thank you for the input, UtahBeehiver. I felt like that is what it "should" be, however another site had mentioned this figure. The arch height is referenced many places, and I know some people have mentioned building lower domes for various reasons. So that is good info. Thanks for the head's up on thread org.

[UtahBeehiver]

I suggest you download, study and read the free eplans from Forno Bravo store site. It is a good overview on oven history, design, and construction.

[Ope-dog]

Hi Russell, I've spent a lot of time on this site.. and others. The "others" provided that conflicting information, thus sparking my curiosity and wanting the input from people that have done actual builds. With all the info available, it's hard sometimes to tell what is a "rule" vs a "preference." This is an example of the "lower" domes referenced on this site as well.. but again, it appears to be a preference more than a rule.

Thanks!

[Gulf]

Hi Ope,

Primitive oven, low dome, high dome, barrell vault are all preferences. All of which have a pros and cons. Most of us on this site have only built one way. My advice is to take the above advice and keep researching the builds until you decide on what you prefer. Then, we may be able to help direct you on your journey to the perfect oven.....for you .

[Ope-dog]

Hi Gulf .. Been reading all your materials on various other threads, not to mention viewing your impressive forum. I'm honored to have you joining mine. :-)

My studies are nowhere near an end, and I suspect I'll continue to review and revamp even as the build goes. To me, the look of a higher dome is more pleasing than a low dome. As I've stated prior, I don't have to have the most efficient oven out there. If it's not holding heat 2 days later like some, I'll probably be ok with that. But as I would just assume not have to redo or repair anything down the road, I am trying to cover all my bases ahead of time.

I do have a question on chimneys / flues and maybe you can point me in the right direction? I've read many threads on this site and other sites regarding sizes of chimneys vs the oven and what is recommended. I saw on your very elaborate WFO that your chimney starts at a pretty good size and it looks like your flue is made with fire brick? I have some old pieces of flue that my father had. Was thinking it might be neat to incorporate them in the build. This flue is 8" x 16" (old clay flue material) and there are 3 pieces, each about 9" or so tall. Do you think these are too large for a 36" oven? I know a 6" or 8" is suggested.. but once again, not able to discern rules vs. preferences on this particular subject.

Thank you for the pointers!

[Gulf]

Yes, I think they are a little large for a 36" build. I know that some will say that you can't over flu an oven but, imo you can. Think of it like this, many primitive ovens have no chimneys/flues. (That is the ultimate "over flued" oven) The smoke is in your face. Your only talking about 27" max with what you have to work with. You may be able to use one of them to transition to an 8"X8",or 7"X7" from a smoke (gathering) chamber. The exhaust heat/smoke from an oven needs to be constricted somewhat and with the proper height to create draw imo.

[Ope-dog]

That makes sense and kind of what I was thinking.. I would use one section to transition it down. After all, if nothing else I don't want the chimney to overshadow (literally) the rest of the build.

[Ope-dog]

Hi Gulf... I have a follow up question that maybe you could lend some input? I've been through many, many threads on the site hoping to get an answer to this. As the clay flue mentioned in the above thread may not work out dimensionally, I am contemplating doing a chimney made out of firebrick. (Similar shape to the little "model" I have put together on my avatar..but a bit higher.) It seems that most builders have used either cast vents/chimneys or opted for a metal flue of some fashion.

Is there any reason why a chimney made solely of firebrick would not be recommended? I know that house chimneys typically have clay flues, then insulation, then brick... But for a pompeii style oven, do you risk your chimney cracking or shifting if made just with firebrick?

Curious to have someone weigh in on this..

Thanks!!!!

[Gulf]

I really can't tell much from your avatar. But, I understand what you are saying. Layed as stretchers there should be no problem. So long as they are not going to be near any combustible materials. Above the smoke chamber you can convert to clay pavers imo.That will save you a little money.

[Chach]

I don't see why it wouldn't work...it would be really heavy but it should work.

Ricky

[mongota]

If I was going to incorporate a 100% masonry chimney with stone cladding on my build, the one significant thing I MIGHT have done differently is I would have CONSIDERED cutting away the 4" thickness of under-floor insulation that currently sits under my vent tunnel bricks (which support the chimney) so the bottom courses of vent tunnel brick, and thus the weight of the chimney itself, would bear directly on the concrete slab and not on the 4" thickness of floor insulation.

With a heavy masonry chimney built directly upon the board insulation I'd have been worried about potential insulation compression from the weight of a masonry chimney.

On the flip side, for a heavy masonry chimney the vent tunnel arch underneath a heavy masonry chimney may require buttressing to prevent lateral spreading and collapse of the landing tunnel. The bricks added by buttressing will increase the footprint of the arch tunnel that sit upon the insulation, and thus decrease the weight per square foot of the chimney on the insulation, reducing the potential of insulation compression. If sufficient, the floor insulation COULD be left intact and the masonry chimney with the larger footprint built directly on top of it.

All told? It's something I considered but did not pursue.

Tangent: If you really want to use the 8x16 flue on an outside oven, since you'd want a precise cut you could use an angle grinder with diamond blade to take an 8" wide strip out of each 16" wide side of the flue. Bring the two 8x4 "U" shaped remnants together for an 8"x8" split flue. Or cut a 9" strip for an 8"x7" flue. Etc. Homebrew mortar the flue joints as you build your chimney. If you use more than one length of flue liner while going vertical, offset the cuts by 90 degrees as you stack the liner.

[Ope-dog]

Hey Gang,

First off, I appreciate all your input. I know there's a lot of threads going on and a lot of builds to weigh in on ( covid has no doubt given a lot of people time to work on their WFOs, I suspect..) so I really appreciate your insight on my question. Certainly Gulf, your chimney is a force to be reckoned with and top notch, so it's good to have your opinion. And Ricky, you basically echoed my exact thoughts.. if the fire brick can handle a 700+ degree fire, I can't imagine the chimney would have any issues. But as you and Mongota bring up, ... the weight.

I'll be perfectly blunt... I like the look of a brick chimney better than a SS or metal flue. It's just a personal preference. And as I'm trying to tie in the look of this with my existing BBQ and smoker (which I still haven't finished the chimney on that yet, either..) the brick look is ideal. So.. it sounds like a few options are on the table..

1. Build the entire thing out of fire brick.. then clad with the matching brick veneer. = HEAVY!!!!
2. Use a metal flue pipe, then clad the metal flue pipe with brick veneer. = NOT so heavy.. maybe a bit more pricey
3. Mongota's idea..which is brilliant and one I had not considered! = a BIT heavier than option 2. MUCH more affordable. 3. More labor intensive and potential for the flues to crack and be back to options 1 or 2 anyway...

To address the weight issue on the isulation base, Mongota.. I hear what you're saying and that makes sense. I'm wondering if I were to buttress a bit on the sides as you're proposing, I could then tie in the 2 "columns" a few rows up which, in my mind, would thus lend support and weight distribution at the same time. Regardless of what above option I end up going with, I appreciate your suggestion as in my mind I'm thinking that will have a nice, robust look to the front of the oven. I've included a quick sketch-up and maybe you can let me know if that's what you were also suggesting?





So after reviewing the sketch, I guess the real question is how much weight I want to have on the front of my base stand? I know I want the finished product to be a brick that matches the existing BBQ. Those cannot be exposed directly to the heat of the chimney. I am thinking that there is no upside to sticking with the firebrick for the entire chimney. I believe between the weight and thickness, it will not be worth the $50 or so difference in buying a metal flue and covering it up.

Sounds like option #1 above is the least desirable after review. Any feedback from the forum is ALWAYS appreciated!


- Scott

ps. In case anyone is wondering, the above scale is 1 square = 1 foot, but it's all a ballpark mock-up for the sake of this chimney discussion... :-)

[mongota]

Scott, yes, carrying the buttress down to the slab is what I was offering as an option so the weight of the chimney would bear on the slab and not the insulation.

Firebrick splits are used for fireplaces all the time. I'd venture they could be used to create the flue liner for a chimney? Half the thickness, half the weight? And the dimensions lend nicely to a roughly 7-3/4" square flue. I'd let others chime in on that.

I certainly understand the desire for the exterior aesthetic to match the other features of your outdoor space.