28" homebrew cast oven in walled enclosure Belgium

[Kris S]

Mullster (Ha ! finally figured out how to do that)

Thank you, I'll start the 5-7 days curing fires tomorrow as we have just had 2 rain filled weeks , so I'll see if the cracks worsen or not.

In the meantime I've been busy with the decorative arch, I stole the idea from mesoiam: https://community.fornobravo.com/for...ge4#post425083

I used some brick splits, bedded them in mortar, added some wire mesh and rebar, covered it with concrete and added some loops of thick wire to tie the arch on to something later.

this weekend I plan to start building the enclosure, I'm gonna go with what I know and feel comfortable working with: wood instead of metal studs. I know it's considered a big no no but since I'll be covering the oven with 2" ceramic wool insulation and some left over rockwool on top of that, I'm confident I can avoid exposing any wood close enough to hot surfaces, we'll see...

[Kris S]

Just a quick update: yesterday I covered the dome with 3" of ceramic insulation and started the curing process.

I was really gonna try to go the slow way and try not to go beyond 150 C on day 1, 200 on day 2, 250 on day 3... but I found it difficult to maintain a 150 C fire. I ended up curing 6 hours around 200 C with spikes of 300 C.

after a while I saw steam and to help the steam escape I removed 1" of insulation but backed of in temp.

On day 2 I could see no visible cracks and tried to not go beyond 300 C. After a few hours around 250 - 300 C there was still no steam, so i thought I might as well push it a little harder to 350 C. Still no steam or cracks so I kept it at 350 for another hour or so.

a little later I noticed a small area at the top of the dome starting to clear. I took the temp and I spiked to almost 400 C

Still no steam, but immediately backed of.

Is it normal that I don't see any steam anymore on day 2 and that the very top of the dome is already starting to clear a little? The cast itself got 3 weeks to dry out....

At what internal dome temp does the dome becomes completely white, I.e. what's the final temp i should aim for?

[Kris S]

Here's a pic of the apex starting to clear a little. That top log may look like it's too big for curing fires, but it's not, it's the camera angle and the oven diameter itself is just 70cm - 28".

I haven't yet fixed the definitive chimney, so used a 15" long 6" diameter pipe, and most of the smoke takes the chimney route instead of coming out of the front arch, pretty pleased with that.

I hope to have put a roof over the oven within the next 2 weeks.

[david s]

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

[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...

[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]

This is where I'm at now...

[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]

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]

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]

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.

[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!

[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]

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]

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.