28" homebrew cast oven in walled enclosure Belgium

[Kris S]

Unfortunately I also cracked the oven. Now I know every oven is 'supposed' to have cracks so I'm not freaking out, just wondering if this a normal crack or not. It spans about half the dome, starting from the base at the back and running to the apex.

I might have overdone it on it's third firing, especially as I wsas just wanting to cook a chicken, but I read that you first need to clear the dome, so I did just that, and than let it cool down to put the chicken in.

I guess for a chicken I could just as well have stopped firing with half the dome cleared? That could have prevented the oven from cracking... Can someone confirm if it's a good practice to only clear part of the dome if you don't need pizza temps?

Anyway, pleased overall, these are the rough temperature data as measured by a laser temp gun:

45 mins: dome starting to clear
90 mins: dome completely clear, first sighting of crack
150 mins: glowing embers and floor temps around 320°C - 600 F
8 hours after firing I still had two big pans of tomato sauce (2.5kg approx) softly simmering inside
18 hours after firing (this morning) floor temp 80°C - 176 F and dome apex temp 93°C - 200 F.

it's only the third firing, and the first full temp one, so probably still some moisture in the oven/insulation and I only have a thin uninsulated ghetto alluminum door atm, so I excpect heat retaining capacity to still improve a little in the future.

[Kris S]

Time to throw in a chicken. Great work, you’ve done a nice job.

Done.

[Kris S]

Undecided whether to use a hebel block or P/V-crete as insulation for my door.

Any preference for on or the other? Same insulation characteristics?

[Kris S]

OK thanks David!

[david s]

No, because (top drawing method) if you make the door to fit well when the oven has cooled, it won’t fit and seal properly when the oven is hot and has expanded, unless you have a deep rebate in the door.

Bottom drawing method results in a better seal with oven hot or cold because it accommodates the thermal expansion better.

[Kris S]

Allright, thanks, I thought about expansion and shrinking too...
The bottom drawing is a simpler build as well.
Just out of curiousity, are both door styles equivalent in terms of keeping heat inside?

[david s]

If you do it according to the top drawing, as the dome cools and contracts it is likely to jam against the cooler door making it impossible to remove. Do it according to the bottom drawing.

[Kris S]

Stupid question about the oven door: does the door go AGAINST or INSIDE the dome opening?
I would think inside and the bigger decorative plate facing outwards to us shutting against the reveal.
Just want to be sure before I start.

So I think the top drawing is the correct way.

Thanks!

[Kris S]

Making steady progress now because of good weather last week and the coming weeks

Put the chimney pipe in place with some 7:1 perlite/vermiculite-crete (also using 3 tabs method). I put a 1" ceramic blanket around the pipe as heat barrier for where the pipe passes the wood roof beams. Gonna add some alufoil as well.

Laid the roof tiles

Attached the cement based boards on 3 sides. The front will remain open for a while untill I've fully fired the oven at least once so I can inspect better what's going on inside the enclosure and the vent/ chimney area.

The chimney will of course also be covered with the cement board.

[david s]

I don't think it matters that much. I use a 900mm flue pipe and have for one reason or another made them shorter or longer with no perceptible difference in the way the ovens fired. The diameter or cross sectional area of the flue is the far more important factor.

[Kris S]

That's a good idea about putting some alufoil between the blankets around the pipe, thanks David!

Any rough guide about how much free space would be needed between the end of the pipe and the chimney cap stone? (post #81)
Reason I ask is I think aesthetically , if I were to use the full length of the pipe, AND add say 4" of free space above that and put my cap stone, the complete chimney heigth will look a bit weird, too long for my tasting... So if 4" air space is what's called for, I might cut the pipe 4" shorter...

[david s]

That looks fine, but you may as well use the full length of that pipe. The draw from the flue pipe comes from both the diameter and the length of the pipe, with the diameter having a more powerful effect. When doing the drying fires it is safer for your casting to do it with insulation, but more insulation layers will make it more difficult for the moisture to escape to the atmosphere, so one layer would be better, then pop the other layers back once the black has all burnt off. I think 3" of blanket between the pipe and the timber should be sufficient, but I'd be placing some alfoil between the blanket layers (you should be able to stuff two layers between the pipe and the timber) in that area so it will reflect some heat, but don't place the foil directly against the pipe because, being conductive it wouldn't work.

[Kris S]

This is where I'm at now...

[Kris S]

And follow up question: how much free space is required as minimum to have a good draft above the pipe, under the cover stone?

I need to know in order to determine my chimney height.

maybe this image (not my oven) helps to explains what I mean:

[Kris S]

Question time again!

So I've covered the dome with at least 3" (7,5 cm) ceramic wool, and I've still got some left + some rockwool, glasswool and perlite and vermiculite to cover the whole thing with even more insulation.

That's reassuring to me because I'm building a wooden enclosure around it, to which I will add hardiebacker board. I know it's advised to build a metal stud enclosure (firehazard) but on the other hand I've also read that some people have done it with wood without problems so... we'll see.

The only place of a little concern is the space between the first roof truss and my 6" single wall chimney pipe: I have about 3" or 7 cm air gap. Now for as far as I've heated the oven even a single 1" layer of ceramic insulation does a tremendous job to keep the heat inside, so I'm gonna put one 1" layer around the pipe, that should protect the roof truss from the worst heat.

That's my gut feeling, but I wonder what some more expirienced builders think...