My 27" build in Albuq

[JDub36"]

Hi,

I left the forum for a while, after buying some initial materials and initiating a post (JDub36") and getting stuck on design issues. The 36" has been reduce to 27" due to following the Oven Plans v. 2.0, p.17 stand/hearth dimensions without reading anything on the Forum. Built the stand and the hearth as spec'd on the Plans only to realize a 36" oven won't really fit once you factor in the walls (4.5"), insullatioin (3") and render (1"-2" depending). So... have come up with a 27" floor diameter and have been laying out bricks to figure out how much of a landing, if any, I can get away with. Will probably just end up with no landing but have the flue gallery right up to the front edge of my corner build hearth (thank you rsandler!).

A little background: the idea for a WFO came probably 6-7 years ago and was to be part of a whole backyard kitchen. I laid a brick patio, borrowing a really nice wet saw that is now mine. The saw will be very nice to have for the oven bricks. The kitchen incorporates an old battleship of a charcoal grill/smoker that was my Dad's and also a Weber charcoal grill, That's right, homey don't do gas grills! A new job and other life commitments have delayed this project, but the WFO is now getting started. I poured the hearth last winter and am now getting going on the oven.

The corner hearth turned out with dimensions of 47" on the long sides by 31" on the short sides with the front face 22-3/4". I built the stand with a bunch of cinder blocks I picked up off of craigslist. The blocks weren't all necessarily the same size but they were spaced to achieve the desired outside dimension, hence some good-sized gaps that will need to be filled in when final finish is done.

A drawing on the Minnesots 36" build site helped me finally decide on floor dimension of 27". In the attached photo, I'm playing around with bricks to come up with a 16"opening although I realized that I have 16" at the inside arch and the largher opening, towards the front is 18". Probably too big for size of oven? Is 16" even too big? Also, what is standard/typical lip on inside arch to accommodate a door? I've got about 3/4".

Because of limited length for landing, and the plan to have a flue gallery/landing (if that makes sense?) I plan on having a 5" chimney. That sound about right?

I'm so far only planning on having the oven sit on 2" of FB. Firstly because the source I used was so darn expensive (~$170 incl shipping for 2"x 24" x 36"-2 pieces) and also not sure I really need the oven to stay hot for 3 days after backing pizza. I absolutely do want to bake some bread, but how soon will it loose sufficient heat to not be able to bake, say the day after, with 2" FB bottom?

I tried to buy some more Insblock 1900, but the source kind of wigged out, said they didn't really like shipping it because it's brittle and breaks into pieces....

Anyone have a reliable source for more FB, should I decide to increase bottom insulation from 2"?

[UtahBeehiver]

You can always pour a 3" 5 to 1 pcrete first You need 5 to 1 for strength, topped with the 2" CaSi. You need to consider drilling in some weep holes in the base for egress of water. It will get in there and wet insulation is one of the main reasons of poor over performance. 3/4" revel is okay, bare minimum. Arch opening height should be 63-65% of dome height although I have seen a little fudging on the this rule of thump. Width is builder's discretion.Dome insulation is just as important as floor insulation. You can always do a cast oven which will gain you a larger ID since cast ovens are abt 2" plus in thickness. David S builds really nice and small ovens. See his threads.

[mongota]

I highly recommend that you take a second look at the functionality of your 27" oven. From the size of the opening and how it'll intersect the 27" dome to how much (or how little) floor space there will be too cook pies with a fire off to the side.

You're so early in the build. So early. I'd consider taking a step back and considering how you can modify the base to get a larger oven. It'd be painful short-term, but if you can't pour a second cantilevered slab over your existing slab due to height, I'd even consider demolishing your existing slab then reforming and pouring a cantilevered slab over your current base.

Dang, if I lived nearby I'd come over and lend a hand.

So much effort goes into building one of these things, I'd hate for the end product to not live up to your needs.

[david s]

In defence of smaller ovens because of the reduced fuel consumption it means they get used more frequently. Fuel consumption is related to chamber volume which means a 20% diameter increase leads to 75% volume increase. A small oven means less labour to build, less weight to support, less materials required, which also means lower costs. Large oven owners won’t feel inclined to fire up their ovens on a Friday night just to cook 2 or 3 pizzas for a small family. Generally most oven owners cook only one pizza at a time anyhow. My oven is only 21” internal, yet I comfortably cater for 30 guests and have done for over 50. What you do need to do is make people share whatever comes out of the oven rather than cook individual pizzas for each guest. This is a more enjoyable eating experience anyway.

[rsandler]

I've only ever had 36" ovens, so can't speak to how easy a smaller one would be to use, but Davis S knows his stuff, so likely you'd be fine with a 27" cooking diameter. Our hosts FB sell a 24" oven, after all. That said, a couple wild ideas to gain a bit more space, and some further design thoughts after that:

If you're willing to do an enclosure instead of an igloo shape, you could screw the steel studs into the sides of the stand, rather than the usual approach of laying track on top of the stand. Would buy you a couple inches of stand space at least. Adds to the cost though--more studs than you might otherwise use, and studs and cement board are probably more expensive than render anyhow.

Another idea might be to form up and pour a cantilevered extension for the front of the oven, to support the gallery. Shifting that forward would give you more usable interior space. You'd want to drill holes in the front edge of your slab to accept rebar from the cantilevered bit (but defer to anyone with more concrete experience who things that's a dumb idea!)

As far as door design, you just want to have the reveal/lip big enough that your door has something to sit against, and still room to remove the door. My current oven has around 3/4 inch, or maybe 1".

The recommended ratio for door height is 63% of dome height. Assuming you're doing a hemisphere with 27" diameter, dome height will be 13.5", so door heigh should be 8.5". If your door is a full semi crircular arch, that would be 17" wide. So you're in the right ballpark from a exhaust-size standpoint, but mongota is right that as you've laid it out in the picture, you're cutting off a lot of the usable cooking space with your door arch.

You might think about shortening your flue gallery so that the door intersects further out on the arc of the floor. It looks like you've laid out a one-brick length gallery, 9" deep, which usually works well to fit a 6" diameter flue, which is needed for a 36" oven. But, if you've got a smaller dome, you might get away with a smaller flue, and could shorten the gallery as a result.

[UtahBeehiver]

There is not one size works for all but I can tell you from my end, I would have built and smaller oven. My current oven is a 42" ID oven and it is a monster party oven but not very practical to fire up for just the two of us, but great for large parties. David's ovens are on the opposite end of the spectrum but from his experiences he manages to host fairly large get togethers. You are constrained by the hearth size but you could gain ID by going the cast method which is what David's ovens are.

[mongota]

Certainly there is nothing wrong with a smaller oven. I thought he wanted a 36" but due to a design mishap he was settling for a 27" oven. That's all.
My best to all...
Mongo

[JDub36"]

Gentlemen, thank you all very much for taking the time to offer your critiques and comments-very much appreciated. I've had to be off the build for a while, dealing with major family stuff... I've been talked out of, and back in to the 27"! I think it will be fine based on the weekend couple of pizzas/one-pizza-at-a-time scenario. I have indeed hosted pizza parties cooking on the charcoal grill w/pizza stone for 10 people or so, everyone sharing whatever comes out and it does work just fine. It will mostly be cooking a pizza for me and the wife, with the occasional bread baking afterwards. I did set out to build a 36", but, as I mentioned, I built the stand and hearth before ever logging on to the forum, just going off the FB Plans...

Thank you, rsandler, for spelling out those dimensions, very helpful and saves me from very time-consuming searching on the forum, which I am not good at. I guess Bacause of the reduced size, I do plan on using a 5" flue. Not sure how long the gallery needs to be, to build the inner and outer arch, for a 5" oven. I guess this could mean another long search on the forum, but I get going down so many rabbit holes on other aspects of builds....! Is 1/2 brick about the minimum thickness for an arch?

Upon the previous advice of UtahBeehiver, I've brought home a hammer drill from work, with assorted long bits to drill some weep holes in the hearth. Is there a rule of thumb as to how many (for a now 27" dia over)? Thanks again ya'll.

[JDub36"]

Drilled some weep holes and laid out some scrap porcelain tiles on which to lay FB insultation (I have more porcelain tiles I can cut up and lay out, do I need more...?). Still not sure whether to buy another 2" of FB, whether or not I'll really need it. With the 2" of FB I have, will I only be able to bake pizza on Day 1 but not have enough residual heat to bake bread on Day 2? I know there are a lot of variables, like how well insulated the overall oven is, but with 2" FB, the fire brick and 3" insulating blanket and 1" or so coating (I know the stucco will not count towards insulating layer), basically the formula laid out in the FB Plans, ... The FB has proven to be hard to come by and very expensive. Spending the $$ to purchase another bundle of FB is not a deal breaker, but just not sure I really need to spend that money.

The other cost/benefit question I have is the hi temp mortar. My local (and I think only) source of WFO material has a commercial mortar, Sairset. Anyone have any good or bad to say about this material? Going the homebrew mortar route, this supplier says they have fire clay and quoted me $50 for a bag, which seems exorbitant for basically a scrap material. Not sure how much the Sairset is, but by the time I scrounge portland from here, fire clay from there, the right sand from another source.. I might just buy a bag of Sairset and be done with it.

[rsandler]

Do a search on this forum for Sairset--it's not the right stuff. I was tempted since my supplier had some and was talked out of it by perusing this site. Basically, the wet pre-mixed mortars aren't the right thing for this application. A general problem with non-homebrew is that it's engineered to have very small (like <1/8") joints, which is challenging to infeasible in a brick oven (many people, myself included, get close to that on the sides by beveling, but top and bottom you're looking at a minimum 1/2" joint). Also, you will likely find the Sairset is pretty expensive too (I got quoted $82 per bag, and expected to need about 3 bags). A dry proprietary refractory mortar like HeatStop ought to work okay if you can find it, but expect it to be expensive as well.

For homebrew supplies, you might check with your local brick yard? Even if they don't have refractory materials, they ought to have basics like clay, lime, portland and sand for a reasonable rate. A quick google search turns up Kinney Brick Co. in the Albuquerque area, which looks like the right kind of place. Might even stock firebricks. There may be others as well; the local restaurant "brickyard pizza" causes some difficulties with the search :-P. The big box stores will also have portland, lime and sand; you shouldn't pay much more than $50 for enough to make the whole oven. I might have even seen clay at my local Home Depot, but my memory is hazy.

[UtahBeehiver]

You will go through much more mortar than you think and it is worth the effort to use homebrew with fire clay being the hardest item to find. You need to go to brick/mason store to find. It will not be at the big box stores.

[Giovanni Rossi]

Drilled some weep holes and laid out some scrap porcelain tiles on which to lay FB insultation (I have more porcelain tiles I can cut up and lay out, do I need more...?).

I copied Joe Watson's ( Gulf ) technique for the weep holes - see post #7 in his Simmental Farm Build

Post # 8 in that same build shows an example of using sheets of mosaic tiles. There are other good examples out there, but I'm pressed for time to dig them out for you.

Many others have gone the broken tile route as you have. The compression strength of CaSil board is good compared to other types of floor insulation, but I would be inclined to add more pieces for greater support.

[JDub36"]

Thanks for the advice on the Sairset guys. It is officially nixed. My local brickyard said they have fireclay, the price quoted just seemed excessive. But what isn't these days..? The cost per pizza just went up, but who's counting? And thank you, Giovanni, for the links and advise on broken tiles. I'll check those out-

Well, it is summer, and the boat is out of the shop so going to the lake this weekend. Maybe in the meantime, someone will talk some sense in to me regarding my questin about 2" FB floor insulation vs. 4" and is 4" would be compulsory if, say, I wanted to bake a loaf of bread the day after firring up the oven for pizza.. I know the question isn't quite a yes or no, but just looking for some opinions if any one has any. Thanks again!

[Toiletman]

Thanks for the advice on the Sairset guys. It is officially nixed. My local brickyard said they have fireclay, the price quoted just seemed excessive. But what isn't these days..? The cost per pizza just went up, but who's counting? And thank you, Giovanni, for the links and advise on broken tiles. I'll check those out-

Well, it is summer, and the boat is out of the shop so going to the lake this weekend. Maybe in the meantime, someone will talk some sense in to me regarding my questin about 2" FB floor insulation vs. 4" and is 4" would be compulsory if, say, I wanted to bake a loaf of bread the day after firring up the oven for pizza.. I know the question isn't quite a yes or no, but just looking for some opinions if any one has any. Thanks again!

As an exploration for my build I created a little spreadsheet to compare insulation thicknesses. It's quick and dirty and all in metric and just explores some of the variables I was looking at. Once my build is finished I'll check it properly, add 'non-sensical' units and turn it into a google sheet that everyone can use. It considers the insulation thickness to be similar around the whole build (ie. CalSil thickness = blanket thickness as well), takes my iD of 800mm as the main diameter and neglects any heat loss through the door. It does however give a good impression of the physics at play, and the relative loss of insulation function above a certain thickness:
Compare the red (1/2" insulation), blue (2" insulation) and purple lines (4" insulation): the difference between red and blue is huge, but between blue and purple you're basically not gaining a lot: after 12 hours there's maybe a +15C difference in oven temperature between 2" and 4", vs -40C for the 1/2" variant.

The brown and green lines explore variation of some other parameters: brown: using full bricks instead of half bricks on the dome, while keeping internal diameter and insulation thickness the same (adds a huge amount of thermal mass but also significantly increases the heat loss area and the thermal gradient inside of the bricks). Green: using an iD of 1000mm instead of 80mm and keeping the other parameters the same (adds thermal mass, but also significantly increases the heat loss area).

[tmoskos]

For the 27" oven, a 16" opening should work fine; 18" might be a bit wide but should be okay if you prefer. For the arch lip, 3/4" is a bit tight; aim for at least 1" to accommodate a door better. A 5" chimney sounds reasonable for your setup.