Design Advice

[ssoccerguy]

I am getting ready to build an oven. I plan to build a barrel oven. I am an avid bread baker (also enjoy pizzas and other cooking) We want a versatile oven that holds heat well and has sufficient thermal mass to bake 2-3 batches of bread and stay hot for other cooking.

I created my own modified Alan Scott design (see attached PDF design docs) to improve stability and thermal mass and would like advice. Specifically on thermal mass and the amount of time I can expect my oven to stay hot / temperature drop per time. I have an inner substructure made of 9x4x2.5" med duty fire bricks then an overlaying structure of 9x4x1.5" fire brick splits.

The inner dimensions are 41" deep, 28" wide, 18" high at dome, The door opening is 17" wide and 12.5" high (I plan to build a custom door with rope to seat, iron with ceramic fiber insulation)

I plan to set the floor on sand slurry on top of ceramic fiber board then wrap the oven structure with 3" of ceramic fiber then cover with stucco. After insulation I will add the front with flue.

I designed on google sketch up and after finalized I can upload to 3D warehouse to share with with community.

Thanks for the advice in advance.

[david s]

1. Barrel ovens require either or both buttressing and steel bracing.
2. End walls should be built under rather than beside the vault.
Both of these requirements make the barrel oven probably more difficult to build than a dome.
There are plenty of barrel oven builds on this site and researching them first would be a good idea before you commit or commence.

[SableSprings]

Welcome SSoccerguy! How many loaves are you planning/hoping to bake with a single firing? I have a modified Pompeii and have baked over 30 loaves (several batch loads in the oven) with a single firing. My oven is not nearly as well insulated as it could (should ) have been, but it has served me well since completion in 2009. After working with both styles (barrel & Pompeii), I really believe the barrel design is better for larger batches (>10 loaves each load) primarily because the better access. When I load bread in my oven, the loaves in the back are done first and frankly it's a pain to work "over" the bread in front. With the barrel shape, you're dealing with an entire row that is loaded and unloaded as a unit.

That said, baking 10 loaves (max) in a batch works very well for me. I'd make sure that you can fit a full sheet pan through that opening...you will want to use it occasionally for Focaccia and biscotti. My oven is 39" wide and 45" deep, so again...the number and size of loaves you are thinking about is going to be a pretty significant factor in deciding if this size is right for you.

The advice and design points from David S are really good ones. If you are only thinking about 15-20 loaves for a bake (done in 2-4 loads), a Pompeii might be a more versatile design for you. Either design form will give you plenty of heat storage for lots of bread and follow-up cooking as long as it's well insulated. I'd also stress, that if you're going to bake much...don't forget to consider access to your kitchen, cooling racks, and cover for you (and the bread) when it rains.

[ssoccerguy]

Thanks for the responses. I am hoping to bake as much as possible for a home baker (sourdough and whole grain) and would like the oven to work with multiple large batches and hold 200+ for 2-3 days post firing. I do 3-10 lb flour batches currently in my indoor convection oven, but would like to take the heat outside and have additional heat to cook dinners.

I like the Buttress idea and have been considering adding that (see attached - it will also add more thermal mass).

Davis S - I do have the back wall built into the vault. The front is integrated in the base but not the vault. (In an engineering sense the pressure from the heat and steam should escape through the door in the front and not put excess pressure on the front barrel joint.)

Mike - How did you design your thermal mass / what is the thickness of your oven and what is the loss of temperature per hour.

Also, this is being designed to be inside a covered outdoor kitchen I am adding so plenty of covered space with fridge ect. Also is the reason for space constraints and wanting the most efficient small oven possible.

[SableSprings]

I used full sized firebricks (9" x 4.5" x 2.5") for my oven. I laid bricks in a herringbone pattern on the cooking floor, so it's 2.5" thick. I cut the bricks in half for the dome, so that's 4.5" thick. I used about 4" of perlcrete (5:1 perlite:cement) under the cooking floor as my insulation and 5"-6" of 10:1 perlcrete over the dome primarily to save money. There have been a lot of construction innovations and certainly an improvement of materials (at much lower costs) that are being used by builders on the forum these days. That's the main reason that David S and I both agree that spending time reading on the forum is a REALLY IMPORTANT investment of your time before committing to a build style.

I believe that if I would have used 2" of foam glass under 2" of ceramic board below my cooking floor and 4"-6" of ceramic batting over the dome, the oven would have retained heat far longer than is the case for me now. In fact there are several builds in the forum that easily hold/keep temps above 400F for 3-5 days and the main difference between their ovens and mine is the improved insulation. I've attached a temperature graph of my heat loss as a low end of the potential retention scale. Once I'm done baking and close up with my insulated door, the oven loses heat at a much lower rate as it drops below 350F or so. After two days the oven has usually dropped into the 200-250F range and on the third day I pile in wood to prep for the next firing.

Actually taking the cooking outside in the summer was one of the reasons my wife really got on board with building the oven. Hope this helps...

[david s]

Most refractory material, castable or brick takes around 1 hr/ inch of thickness for heat to move through the material, depending of course on the intensity of the heat applied to the hot face. So, adding additional thermal mass via increased wall or floor thickness will add extra time and fuel to get to temperature. Most cast ovens are around 2" thick, but most brick ovens have to be thicker to retain integrity because of the mortar joins between the bricks and are therefore usually 4" thick. Given the internal dimensions of your oven I think one layer of brick really well insulated should be plenty. A double layer of brick as you propose, 2.5+1.5 will give you the same result, but double the bricklaying labour. A simple way to add more thermal mass for a bread oven is to turn the floor bricks on edge, and it appears that this is what you intend by looking at the diagram, but a single layer on edge gives you 4" so you may want to add a second 1.5" layer. If you intend to bake every day, then the double layer of brick will be fine as the oven will never really cool down. Which ever way you go plenty of insulation is the key to heat retention.

[SableSprings]

SSoccerGuy, since it sounds like you're looking at almost daily baking (or at least several times per week), have you considered changing from your proposed black oven to a white oven design? The white oven is certainly more complicated to build, but being able to recharge the retained heat without having to fire and clean out the cooking chamber could be worth it in your situation.