Frozen in Chicago: Refining my sketches while I wait for spring (design check?)

[Olivia Wilson]

Hi everyone, Olivia here again!

It’s been a few months since I first introduced myself. I was really hoping to break ground this autumn, but life got busy and then that Thanksgiving weekend snowstorm in Chicago pretty much sealed my fate.

I woke up on Sunday, looked at the snow piled up on the patio furniture, and realized there is absolutely no way I’m pouring concrete or laying mortar until at least March.

So, I’m trying to be at least a little productive while everything outside is frozen. Instead of building, I’m cleaning up my scribbles and trying to pin down a “master plan”. I took my messy notebook and turned it into a simple project journal to keep all the sketches and notes in one place.

You can see my winter planning journal and sketches here.

I have a specific question about the sketch (it’s the third image on that page):

I’m trying to hit the classic 63% door-to-dome height ratio. In the drawing, I’ve got the dome at roughly 18–19 inches high. Does the entry arch look too low in proportion to the landing and the rest of the entry? I’m a little paranoid about building the whole thing and then having smoke rolling out the front because I messed up the geometry.

Also, for any Chicago-area folks: I started a draft shopping list at the bottom of that page. If you have a favorite masonry supply yard that’s friendly to DIYers (and won’t laugh at a small order), I’d really love your recommendations.

Thanks in advance for any thoughts, and stay warm out there!

Olivia

[UtahBeehiver]

Looks like you have the winter to mull over your design. Looked over you sketches and the only real issue is what is the sand bed for? It is not suitable for insulation. Ideally CaSi board is the best but also the most expensive, a min of 2" needed. If under budget constraints, 4" of 5 to1 p/v crete is needed. On dome, ceramic blanket is best but you can get by with 8-10 p/v crete or a combination of both.These are mins. 36" is a goods size, good insulation is key to extended cooking. Smoking of ovens causes, wet ovens, wet wood, overloading oven with wood, too small of chimney (8" ID needed for 36" ID oven). Check out Habitat for Humanity Restore stores, sometime you can find items for you oven construction or Facebook, ie firebrick, chimney parts, etc. Ideally a cover over the oven is best, I live in Utah and the freeze/thaw cycle are hard on the oven and adjacent finish materials. Oven opening looks good, to do not have to exactly get 63% just close.

[david s]

Further to Utah's comments, you need to indicate on your drawing the proposed thicknesses of each layer.
Having the oven floor below the level of the supporting slab introduces the problem of water draining down under the floor and soaking the underfloor insulation.If you must go this route there are simple drainage solutions.
Your sketch seems to also indicate that the inner face of the oven is insulation blanket. The inner dome needs to be either brick, castable refractory (or homebrew) that is thick enough to provide sufficient strength and have sufficient thermal mass for retained heat cooking without creating excessive fuel consumption and extended heat up times. The usual for brick ovens is 4" thick for sufficient structural stability and 2" for cast ovens. The area of the oven mouth needs to be wide enough for good access and high enough to take your largest roast. The larger the area the greater the heat loss, so some compromise may be needed for either really large or really small ovens. The 68% rule works well for a 36" oven, but that rule was only arrived at by an historical Canadian study which looked at the average oven mouth height to internal height ratio. Because most ovens were of a similar size the 68% was the number arrived at, but has little to do with the ideal oxygen to fuel ratio or ideal combustion circulation, so don't worry about it. Use your time to explore this forum and look at the many well documented builds. You'll get plenty of valuable feedback from folk who prefer to share rather than keep secrets.

[UtahBeehiver]

Dave, Good catch on interior material in oven, I missed that. Olivia, go to the Newbie Section and under Treasure Archive you will find links to builds that were well documented.

[mongota]

The door proportions, you have the formula to use as a height buideline. So draw that opening out, then compare the dimensions against what you want to use the oven for. Will a tuscan grill fit through the opening. A dutch oven. A XX" wide pizza peel. If the 63% works, then it works. The shape of the opening can be altered. Mine is a continuously curved arch, others buttress for more vertical sides to the opening.

"Smoke machine". The start of most fires generates a little smoke when first lit, but the smoke dissipates pretty quickly once the fire gets going. Unless you are buring wood that's too green. Size your flue to the dome size, have a chomney that's tall enough for the flue diameter and the chimney should draw just fine.

You've gotten good feedback regarding the recessed sand box. Design and build to prevent moisture from getting into the structure. Also design and build with a path for moisture to escape when it does get in.

Keep the oven structure dry and your fires will smoke less. The heat from the fire will go towards heating up the dry masonry versus the dome temperature stagnating while the heat goes towards cooking off moisture in the structure. A dry oven, good insulation, and seasoned firewood will give you a dome that clears itself and is ready for cooking sooner rather than later.

[JRPizza]

Hi Olivia, It's good to see you are putting in the work planning - it will pay off when you start building.
I don't see any mention of door opening style (sorry if you mentioned this in an earlier post). I prefer a hemispherical-circular opening, but some builders want to go for a flatter arch. The flatter arch needs some extra support for the side loads and I personally don't think it looks as good. If you are going to see if your cooking utensils fit through the opening the type will matter.
I'm guessing you have a copy of the Forno Bravo Pompeii oven plans? They show opening sizes for 36" and 42" ovens, neither of which hit the 63% ratio (I'm not totally sure how much that matters but it is in literature). When I started looking at numbers I ended up building a 39" oven. That size using standard fire brick made my door opening and arches come out very nice and if you are really thinking of a 19" Interior diameter, you might want to go an extra 1/2 inch and make it a 39'er. You can look at my build for some of the details, but below is the Forno Bravo numbers for door height along with my 39, which are pretty much in the middle, and almost hit the 63% on the nose.