2nd oven, want faster heat-up times: 4.5" vs. 3" vs. 2.5" thickness tradeoffs

[kebwi]

I built the oven shown in 2009 in Seattle (I added the extra tall stovepipe to try to get an abusive neighbor to leave me along about the smoke). I have since moved, and consequently lost my beloved oven (I honestly considered hiring a flatbed and a crane to move it; I loved it that much). So I'm gingerly researching options for building a second oven, although I admit that if I do this again, I will want to keep it pretty simple and try to get the job done in a timely and effort-minimizing manner. There are some things I would like to improve upon in a second oven however, despite my primary goal of getting the job done quickly and easily. I was quite discouraged with the heat-up times of my first oven. I had to stand out there feeding it wood for two hours before I could use it. My floor was face-up bricks (so 2.5" thick") resting on three layers of InsBlock 19 (3"). My dome was the standard Pompeii design: width-wise bricks (4.5") wrapped in bulk (not blanket) InsWool HP, 3" thick around the base and 6" on the top, then encased several more inches in 8:1 vermicrete around the base and 10:1 vermicrete over the top (or 50/50 vermiculite/perlite in later stages, when I ran out of vermiculate).

Here's my webpage for the build, if you're curious: http://keithwiley.com/brickPizzaOven.shtml and http://keithwiley.com/brickPizzaOvenPictures.shtml

Despite what I interpret as pretty generous insulation, it took a long time to heat up the oven, which I attribute to the 4.5" walls soaking up the heat, not necessarily the insulation leaking the heat away, so I would like to improve on that design if I build a second oven.

I have seen some discussions of using thirds-lengthwise-bricks for the walls, so 3" thick instead of 4.5", but didn't feel that those threads came to much of a conclusion on the benefits and tradeoffs. I'm also curious about the option applying bricks face-on (so 2.5" thick walls). The goal would be to reduce the thermal mass and speed up heat-up times. But of course, I don't want to lose so much mass that I can't achieve and maintain pizza temps, and I obviously don't want the dome to structurally fail.

It seems to me that 2.5" surely ought to heat up to 1000F without any trouble, and then stay there so long as I keep a fire going. Why does it need to be an incredibly thick wall? That makes no thermal sense to me, unless the insulation is so poor that heat is escaping out the back faster than it is coming in the front from the fire and a thicker wall simply slows down the heat escape, but surely more insulation would do a better job at that task than more thermal mass, right?

The metal commercial ovens that have become quite the rage obviously have very little thermal mass or insulation. So, it seems possible to build a brick oven with 2.5" thick walls, but no one on Forno Bravo ever does that. Is the only reason 2.5" walls aren't encouraged the fact that it makes the dome a less stable arch, risking collapse, or does the Pompeii design actually encourage 4.5" walls for thermal and cooking reasons, despite the frustrating wait times? Obviously, I don't want my dome to collapse, but what is the reason for encouraging people to use 4.5" walls? Is it merely a structural/strength argument or is there a firing/cooking reason that I am not properly appreciating?

What about 3" walls, achieved by applying thirds-bricks along the long axis? Is there a strong argument for doing that instead of 2.5" face-on bricks? I feel like face-on bricks would be less cutting. I could even use full-size bricks facing into the oven instead of cutting them into smaller pieces. Yes, the dome would look less pretty from the inside with rectangular bricks facing inward, but like I said, since this would be my second oven, I really don't want it to be a time-consuming project. I would want to get it built pretty quickly. Wrapping face-on bricks into a dome seems like an incredibly fast design. I could even shave thin triangles off the ends to give them a little bit of a taper so they wouldn't literally "fall out" of the ceiling. Just a touch of wedge-shaping would alleviate that risk, I believe.

Thoughts?

Thanks.

[david s]

You are correct in your observation about thermal mass and wall thickness and its relation to heat up times. The problem with a brick build is the integrity of the width of the mortar joint. Over hundreds of years it has been established that a brick construction for houses or walls needs to be at least 4” Many have tried laying bricks on edge and it often leads to failure. With an oven in the form of a hemisphere the structure is obviously way stronger than a flat wall, but the thermal expansion problems cancel out that advantage. You wouldn’t be the first to think that thinner brick walls would be an improvement, but in practice they don’t hold up wel. Cat builds however do not have the same problem and most manufacturers have settled on dome thickness’ of 2”, some going thicker at 3”. This provides sufficient strength at the same time as reducing thermal mass to provide a good balance of oven performance regarding retained heat, heat up time and fuel consumption Cast builds can also deviate from the hemisphere by having low domes without the steel bracing or buttressing required in low dome brick ovens.
My advice, if you really want a brick oven, then don’t reduce thickness, but if you do, consider a cast build.

[kebwi]

Thanks. That is disheartening. I found the two hour (minimum) wood-feeding frenzy of my first oven extremely discouraging. I really don't know if I want to build another oven if using it will involve attentively hanging out in my backyard for several hours first (and hour, maybe 90 minutes, sure, but 2-3 hours is just a huge time killer). As you can see, I already over-insulated my first oven (3" inches of InsBlock, 3"-6" of InsWool plus several inches of v-crete). I don't think insulation was my problem. I think the problem was waiting for 4.5" of brick to soak up the heat. The thermal mass can only take the heat on so fast; there's a limit to how quickly you can push heat into the bricks. Maybe I'll try building with 3" walls (bricks divided into thirds and aligned with their cut 3" dimension oriented along the radial axis) and see how it goes. If I actually cut the bricks into wedges (I cut them very tightly for my first oven) then it will be physically impossible for them to actually fall out of the ceiling. I was hoping to avoid making hundreds of precise cuts this time around, but the tradeoff makes sense: thick walls and casual cubically-shaped bricks or thinner walls and precisely wedged bricks that form a mostly-self-supporting arch. The only issue then will be whether the shape of the dome, obviously not a true catenary, will be sufficiently stable to essentially free stand, along with a little help from the mortar and any buttressing around the base.

Grrr.

[david s]

Unfortunately a catenary arch that has its height smaller than its base leads to walls at base far too horizontal for practicality and far too much outward thrust.
Your conclusions about the high thermal mass and wall thickness are correct and are the contributor to the slow heat up.This one reason why I'm a fan of smaller ovens. I wouldn't think twice about firing my little oven up on a Friday night to cook three pizzas, with very little fuel consumed.
As a rule of thumb heat takes around an hour to travel for each inch of wall thickness in brick or refractory.

[kebwi]

Yeah, thanks again. That sounds like my experience with my first oven. Obviously, true catenaries are ridiculous shapes for ovens. But they have the mathematical beauty of being perfectly self-supporting. I'm enamored with the notion of the mortar being practically unnecessary, but true arches don't work that way. Ooof course I would also like to go for a lower, flatter roof than the Pompeii design, which obviously doesn't go well with my thin-wall motive and would put a lot of outward pressure on a soldier course. I saw a bunch of threads where people wrap the soldiers with metal bands or cable, or pour a v-crete ring around the outside of the soldiers. There seem to be lots of ideas in this domain. I don't know what I'm going to settle on. I really haven't committed to this project. I would like it to be an unfancy, simple, quick build, which of course the standard Pompeii was designed for: just bricks cut square in two, done and done. But I was frustrated with my wait times on my previous oven (which btw was only 36").

Anyway, thanks for your input.

[kebwi]

Pretty simple question: Has anyone ever poured a ring of weight-supporting 5:1 v-crete under the dome footing, but then filled the interior of the ring with 8:1 or 10:1 v-crete where it will only have to hold up the floor? Seems kinda clever but am I overlooking something obvious?

[kebwi]

Does anyone know if these are definitely firebricks? The seller is selling them as such, but they appear to have been installed as ground pavers, which is rather odd. I realize Idaho is a brand that makes firebricks, but they might make ordinary bricks (or insulating firebricks) as well as heavy firebricks. Any ideas?

Thanks.

[kebwi]

I don't intend the following expression of confusion my part to be contentious (it obviously is, but I don't intend it that way), but I genuinely don't understand how the forum is being used now, admittedly having returned after a fourteen-year hiatus, so if others could help me use the forum in its contemporary fashion, I would appreciate that.

Fourteen years ago I greatly benefited from the FornoBravo forums and community to build my first oven, which was pretty popular on the site at the time, if I may say so. But this time around, after a mere three posts on my part, I've been asked by a moderator (who joined three years after me and my original oven build, mind you) to not use any of the subforums for topically relevant questions anymore (such as posting questions about materials to the materials forum or about techniques to the techniques forum or about designs to the design forum). I've been asked to consolidate my entire set of inquiries to a single monolithic "build" thread, tucked away somewhere in a single location on the forums. I realize "build" threads are popular on this site. My own such thread from fourteen years ago ran to thirty-three pages and was a pretty popular project at the time. I suppose the moderator in question has no idea about that since it predated him by three years, but I am quite familiar with the concept of personal build threads on the site.

But I started that thread after I had a solid project established and underway. Heck, my first post included CAD drawings for heaven's sake. I didn't create a personal thread after my third broadly general information gathering post.

At the time, I eventually started a build thread under Good Background Information / Brick Oven Photos / 36" in Seattle. So be it, but now I see such threads scattered all over the site under different forums. In fact, that appears to be the one thread grouping for which there is no unified subforum, oddly enough. Should I start a new "all my general questions" thread instead of posting questions to relevant subforums, as the moderator has indicated?

It doesn't organize well either. Answers and ongoing discussions about different questions will become intermingled in the single over-arching thread. It is obviously better to confine each question to its own unique thread of discussion. Heck, that's what the subforums are for: they facilitate precisely that sort of organization.

If I'm not supposed to be using the subforums for anything, then who is? The moderator said my questions were part of my "upcoming build" (that is incorrect, I have no plans to build anything yet, just vague conceptual questions) and therefore belong in a consolidated "personal build" thread. But doesn't that logic applies to everyone else too? Any question anyone asks pertains to that person's potential and eventual "upcoming build" as much as my questions do. By that logic, none of the subforums would ever receive any posts. Everyone would simply start a personalized thread (somewhere) as soon as they join and write all their messages and questions in their own thread from that point on. Why were my questions deemed more "about my build" than other people's questions are "about their own builds"? I genuinely don't see the distinction here.

I won't deny that this somehow feels unfair, as if other people are permitted to post questions in categorically logical areas of the forum(which makes them easier to find, along with attention-grabbing relevant subject lines) while I am being told to squirrel all my posts away into one corner, with no subject lines for individual questions, making them harder to find, harder to organize, and harder to track, for no apparent reason.

In my efforts to gain clarity on this issue, the moderator has suggested I leave the forum entirely, my having only just posted my first question in fourteen years just one day ago. Good grief, this for the violation of posting questions in relevant categories of the forum? I even asked the moderator which subforum he recommended I put a personal thread in and he didn't tell me. I don't think I should start another thread where I put my last build thread, in the photos subforum. I have no photos. I have no build. I have nothing resembling a plan to build an oven. I have posted a few cursory questions about wall thickness tradeoffs, seemingly novel vermicrete designs, and firebrick identification -- thus far.

I think a personal "build" thread would be for questions like "So, I'm part way into this thing and I have a particular unique situation due to my circumstances and I would like help with it". That seems like a good question for a personalized "build" thread. But asking "Say, what do people think of the tradeoffs pertaining to wall thickness?" feels like a discussion for the techniques subforum, not some personal catch-all-questions endlessly paging thread.

At this point, I'm honestly not sure how to ask my next question. Obviously I have been all but prohibited from posting to a subforum of the relevant category (although others can use them apparently). But should I really start a thread titled "All my various generalized oven-building questions" and then just dump my questions in there one by one (each new question posed without any descriptive title to make it easier to find and classify and organize)? If that's how the community uses the forum fourteen years later, so be it. Things change. Maybe the forum just isn't used the same way anymore. Just clarify it for me please (and I still don't see what the topical subforums are for in that case).

Do the moderators not want people to ask questions in subforums whose topics are relevant to those questions -- and where each question can be accompanied by a title that cleanly summarizes and "searchifies" that question? Should all questions just be subjectless replies to insanely long singular threads the scroll on for 30 or 40 pages? I will concede on this issue and use the forum however everyone else thinks it ought to be used. I've been away for a long time, so I get it. But I'm astounded, so I'm asking. Is that really how the forum should be used now?

Thank you.