28" Oven Design Feedback - First-time builder

[JBA]

Hi my name is Joona and for the past year I have entertained the idea of building a wood-fired oven in my backyard.

After making the pitch to my better half I have been able to get approval to build one. I have spent the past few weeks or so reading the excellent Forno Bravo guide, as well as watching numerous videos on youtube on how to go about.

The biggest I am able to go is 28" for the cooking floor. The total footprint of the oven is going to be about 4x4 feet, or 48x48 inches. Over the past week I have designed one using a 3D modeling tool that I'm familiar with. This is to help me visualize how it will look, and also to estimate the number or bricks and other material that I will need.

As this is the first time designing and building a wood-fired oven, and not being able to rely on designs for the more typical 36" or larger ovens, I'm hoping that I can get feedback on the custom design of my 28" oven.

Notable features include an additional 1" concrete heatsink between the fire brick floor and the FB board insulation. I have read of people having issues holding high floor temps for long periods of time, and this is my idea for mitigating that. Pre-heat might take longer, but I rather wait for that than not having enough heat for baking pizza.

Another feature of the oven is the brick chimney, where I'm not sure if the opening is large enough. The total area is 22 square inches, which I'm hoping would be sufficient for a 28" oven.

Finally, the dome is built around the oven floor rather than on top of it, in the unusual even that I would ever have to rebuild the floor. I am hoping this will also reduce the risk of cracks in the dome as the floor and dome should be able to expand and contract pretty much independently (?)

I expect to build the stand in the coming weeks, and begin building the floor and the dome some time in May. Any feedback on the design before then would be greatly appreciated!

[david s]

Hi Joona,
Most oven builds, especially those that are brick rather than cast, require extra room at the front to accomodate the flue gallery. Make sure your plans will allow the 48” supporting slab to accomodate the 28”internal diameter oven.

Most builders opt for a round stainless flue because it provides a smoother surface for smoke flow than a brick chimney as well as being far lighter. Heavy bricks place more force on the gallery and outer decorative arches.22 sq in for the chimney will be sufficient for a 28” oven. You can always increase the chimney height for even more draw, but should not be necessary and consider the extra weight.

If you want more thermal mass in the floor it is easier to lay the bricks on edge giving you 4” rather than 3”
Although it’s almost impossible to remove a floor brick if it’s under the dome wall, in practice it’s only the bricks in the centre of the floor that are susceptible to damage from cracking or spalling, not the bricks at the perimeter. A1/4” gap between the floor perimeter and the dome wall is usually left to allow for expansion. This requires some precise cutting of the floor bricks. My own view is that it’s far easier to cut the bricks that will sit under the dome roughly as their edges will be covered with insulation anyway. Either way you do it, the floor bricks are best laid loose to allow for individual free expansion.

[Kvanbael]

Hey JBA,

You clearly have a grasp of the main principles. So it is a matter of details. Some thoughts about your design, mainly the proportions.

- in proportion, oven walls are pretty thick, suggesting you want to cook long with retained heat. But... then 3” of insulation is not a lot.
- flue arch lack a bit of overhang to catch the smoke. I suspect a lot of smoke will leak out of the front.
- flue entry is pretty shallow, which means front of dome will be not so well insulated (if you need more depth, then you could omit that landing)

PS: extra layer under Floor functions as heat bank, not heat sink. I did something similar for baking multiple batches of bread. Note that any floor will eventualiteit cool down. For large pizza party, you can also swap fire to other side every 10 pizza’s or so.

[JBA]

David & Kvanbael, thank you both for your feedback! My takeaways from what you've both said is:

- Lay the floor bricks on the side rather than "flat" to gain more thermal mass in the floor instead of going through the process of casting an extra layer of refractory concrete underneath.
- Design for a stainless steel chimney to reduce weight on flue gallery and to improve smoke flow out of the oven
- Building the dome on top of floor rather than on the perimeter of the floor will make it easier and require less precision when cutting the floor bricks. I am assuming that I should not mortar the dome against the floor, to allow the floor to expand and slide underneath the dome if necessary?
- Increase dome insulation to enable cooking with retained heat. Currently the oven walls would be about 4.5" inch thick. How much insulation would you recommend? 5"?
- Extend the flue entry
- Add more overhang to flue arch to catch smoke.

Will the 2" FB board be enough insulation for the floor or should I increase that?

I hope to share an updated design in a few days.

It's important to me to be able to cook using retained heat. If I just wanted to bake pizza it would be much easier to just buy a stainless steel pizza oven and be done.

[JBA]

Latest design with changes as suggested. Additional feedback would be very much appreciated!

[david s]

The more layers of blanket you use the more uneven the surface becomes. Apart from the cost issue, more layers are subject to the law of diminishing returns. A good solution is to apply a layer of 10:1 vermicrete over the blanket layers which evens out the form back to a perfect dome surface, provides a firm enough substrate to apply the render on to and is also has around the same insulation value as the same thickness of blanket.

I still think you need to go back to your numbers, because a 28" oven with 3" of insulation layers and a 2" outer render would leave you with 1/2' from the outside of your 48" square supporting slab. Given that you need extra room at the front to accomodate the flue gallery, you're highly unlikely to be able to fit the oven on the square 48" supporting slab.There would be nothing worse than finding your supporting slab is too small when building the flue gallery and doing the outer layers.

2" of underfloor insulation is considered minimum, if you don't want to go to two layers of calcium silicate board you could consider an inch or even two of 5:1 vermicrete under the calcium silicate board.

A 2" thick outer render is overkill IMHO unless you plan to park your car on the top of the oven. I use 12-15mm thick layer reinforced with AR fibreglass fibres.

One huge advantage of small ovens is their economy of fuel use. It's dead easy to flash up the temperature quickly with very little fuel. Not sure how you want to use your oven, but if you were to use it everyday to bake bread for instance, then really well insulated may be of a lower priority.

[JBA]

Thanks again David for your feedback and advice. I feel confident it is helping me build a much better oven!

I'll be updating my design to include 2" of 5:1 vermicrete under the calcium silicate board.

As for the measurements: The supporting base is going to be around 48"x48" but the hearth I'm planning to build will be about 48"x52" (4" overhang at the front) which should support the flue gallery and a small landing. I've attached a picture of the measurements that I have. Let me know if I'm missing something, but to me it looks like everything will fit, albeit with tight margins. With your recommended thickness for outer render that's less I was planning, but also an added layer of 10:1 vermicrete, the margins are hopefully just about on my side!

14" floor radius
4.5" brick dome thickness
3" insulating blanket
~1" vermicrete layer
<1" outer render

Roughly 23.5" total radius/47" total diameter for the oven, not including the flue gallery. Maybe I should increase the hearth size to 50"x52" just to give me that extra bit of design margin.

As for how I want to use my oven: For as many things as possible! First and foremost, pizza. The morning after a pizza party I'd like to be able to bake bread, or at least do some kind of roasting or baking in the residual heat on the day after. I want to get as much out of the oven as possible, even if its on the smaller side.
I'm likely not going to be using the oven more than once a week on average, probably less. If it was possible I'd go bigger than 28" but my backyard is small and the only place where I can put a wood-fired oven I'm limited to a footprint of roughly 4x4 feet (see picture)

[david s]

That sounds pretty good. If you are laying a 2" 5:1 insulating slab you'll find it takes a lot of water, around 3.5 parts of water for every 10 parts of vermiculite by volume. This means that after the hydration process there's still lots of free water that needs to be eliminated before building over it. This is better explained in the study attached.

Vermicrete insulating slab copy.doc.zip

[JBA]

That sounds pretty good. If you are laying a 2" 5:1 insulating slab you'll find it takes a lot of water, around 3.5 parts of water for every 10 parts of vermiculite by volume. This means that after the hydration process there's still lots of free water that needs to be eliminated before building over it. This is better explained in the study attached.

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Thank you for sharing this study. It suggests to allow for a really long curing (drying?) time compared other guides/tutorials that I've read or watched. Should my takeaway really be to let it dry for 4 weeks before starting to build on top of it? Will 1 week not be sufficient? (Given fair spring weather in Seattle)

I've seen others starting to build only a few days after, and that's with a 5" slab of 5:1 vermicrete.

Also, does a mixture or perlite and vermiculite have significant advantages over only vermiculite?