31"/800mm in Eindhoven, the Netherlands: Design critique and build topic

[Toiletman]

My dome is very symmetrical in shape so the v/crete is the same thickness through out the dome, compressed just enough to keep shape, farther up you go the less compression needed since you are not fighting gravity as much. David is right on the porosity of the v/pcrete but in you install a stucco render the steam pressure could affect this layer (mine was 2" of ceramic blanket, 2" of v/pcrete and 1" stucco layer. Heat/steam rises to the highest point so I am not sure if the tiles at the base will relieve pressure build up. David vents his ovens through a uniques chimney ceramic collar.

Thanks both for chipping in. Since then I made a double batch and am getting higher up the dome, also finding the effect you're describing UtahBeehiver . I've got two days off tomorrow and the day after, so if the weather is fair I hope to get to fully finish the vermicretin' this week. Hope to be able to shoot some pictures your way then.

It's a good remark about the porosity of the vermicrete. I am planning on covering the outer layer with plaster and then a sealing layer, so after that it won't be able to breathe anymore. However, I did also fit the chimney connection with David's design of having some vent holes there. I was thinking of getting the insulation wool to touch up to that, but worried a little about fibres getting loose. Now I think I'll just put some very loose vermicrete around the openings and get the best of both worlds.

[UtahBeehiver]

My dome is very symmetrical in shape so the v/crete is the same thickness through out the dome, compressed just enough to keep shape, farther up you go the less compression needed since you are not fighting gravity as much. David is right on the porosity of the v/pcrete but in you install a stucco render the steam pressure could affect this layer (mine was 2" of ceramic blanket, 2" of v/pcrete and 1" stucco layer. Heat/steam rises to the highest point so I am not sure if the tiles at the base will relieve pressure build up. David vents his ovens through a uniques chimney ceramic collar.

[david s]

Steam will pass fairly readily through the blanket and a lean vermicrete mix, so I don’t believe an additional path is necessary. At 10:1 a lean vermicrete mix has approximately the same insulation value as blanket, with the weight of cement being a little more than the weight of vermicrete or perlite.(see attached table). This leaves the mix fairly open when set, negating the need for any steam channels. The mix sets up strong enough to act as a substrate onto which a cement render can be applied. If you do want it stronger, the addition of more cement will work, but also drastically reduce its insulation value as well as impeding steam passage. If the mix is made too wet it will wash the cement off the grains leading to an inconsistent mix. I’ve found 4 parts water to every 10 parts of vermiculite or perlite by volume, works well for the stuff I get, but it also depends of the grade of the grains. The finer the grade the more water is required. Another way is to add water to the dry mixed material until it just starts to pool in the bottom of the barrow.
Additionally the mix can be made stickier by adding some powdered clay. I use one handful for every litre of cement. Alternatively some acrylic could be added for this purpose.
After 24 hrs if there are any high spots, they can be easily carved back, any low spots can be easily filled with more vermicrete.

[Toiletman]

I used a homemade curved float and compressed the v/pcrete mix to assist in keeping the dome shape from slumping.

That's a nice solution. Your courses look very flat on the top? Did you compress it down by hand? I'm assuming you compressed mainly from the top? Or also towards the dome? Any idea of how much? Ie. one or two inches inwards or sth? Judging from the picture it also looks slightly wetter than mine.

Have you considered install a vent in the v/pcrete to allow steam from any water that may get into the dome? When water sublimates to steam, the volume increase by 1500 times and can cause internal dome pressure that can potentially crack the outer dome material.

I extended the mosaic tiles underneath the blanket and the vermicrete, thinking that that will be a good path for any pressure to escape around the whole circumference of the dome. Also from seeing pictures where the moisture/steam is actually being driven downwards due to the temperature gradient. I might still add a small U-shaped pipe on top, but with the channels in the mosaic tiles quite 'open' to gas pressure I'm pretty confident that no significant pressure could build up like that.

[UtahBeehiver]

I used a homemade curved float and compressed the v/pcrete mix to assist in keeping the dome shape from slumping. Have you considered install a vent in the v/pcrete to allow steam from any water that may get into the dome? When water sublimates to steam, the volume increase by 1500 times and can cause internal dome pressure that can potentially crack the outer dome material.

[Toiletman]

The good weather is back! The oven is now fitted with chicken wire to fixate the ceramic fabric a little bit more, and the first two courses of vermicrete are in! Really feeling like the last steps towards the curing fires.

A couple of questions on the vermicrete mix:
By volume I'm using 5 parts vermiculite, 5 parts perlite, 1 part portland cement. Waterwise I add until it starts loosely sticking together.

When applying I have to compress the mix a little bit for it to stick properly and I was wondering if that's indeed the way to go, or if I should make the mix wetter for it to stick immediately? The first course I compressed quite a lot, but now I'm just loosely pressing it with a float, so that it stays together and doesn't crack loose again. Is that what I'm looking for? Does anyone know of any good video's that show the consistency?
Also quantity wise: I'm now mixing up batches of 10-12L (roughly a bucket), which fills roughly one course of 8cm high (5cm thickness). Applying takes roughly an hour or so. Should I mix more in one go? I'm not sure if I have enough time to apply and if it will stay in tact with higher courses?

Pictures attached.

[Toiletman]

Pffft... it was the 22nd of february since I last posted... the pace is again glacial. Yesterday I managed to get the insulation blankets on, as the weather is currently fantastic here. The insulation is 2"/50mm all around, and 3"/75mm on top. I still have to make some kind of a steam path to the chimney, for which I left some cut-offs. See the pictures.

Upcoming work: Add some chickenwire and then go for a layer of vermicrete (2", 10:1). There's still quite a bit of movement in the blanket, so I'm not trusting that I can apply such a thin layer of vermicrete without it moving/breaking due to blanket movement (from for instance applying vermicrete 30cm along the circumference). Especially after seeing how the material behaves, when I was making the plug for the chimney cast. So I will go for adding the chicken wire around first, to fixate it it place more.
To make sure that any steam that builds up inside the insulation I've extended the mosaic tile floor to also be underneath it, as well as providing above mentioned path towards the chimney outlets. I'll probably add a U-shaped pipe on top as well.

I'm still on the fence on how to finish my table... natural stone would be really nice, but it'll cost a lot to have sth cut to size, and also hell to put it in place. The kitchen counter concrete finish might be a good option, but I'm currently leaning more towards tiles. Anyone has any great examples to consider or use as inspiration?

Any further remarks or things to consider before starting the vermicrete?

[Toiletman]

The sleeve I make for accepting a removable flue is made from 0.55mm 304 stainless. Being thin it is easy to shape by hand. As shown in post #53 there is a space so that it does not complete a full circle. This helps in providing some room for expansion as well as providing a space to accomodate the lapped joint of the flue pipe that fits into it. To make the pipe fit inside it I wrap some plastic around the pipe, then fit the sleeve over it and tighten the wires up so they are just firm, but not really tight. The plastic allows for easy enough removal as well as providing enough of a gap so the pipe is a nice sliding fit.
Also added a couple of pics showing the fitted sleeve and the weather cap I use when the flue pipe is not in place. Hope this helps, maybe too late.

Thx! I'll also have to think about how to make a nice edge and a cap around the final product. The thinnest sheet metal that I could get was 1mm, so bending it in shape is a lot more difficult than 0.55mm!
Don't worry about the gap: instead of a gap I left an overlap, so that the metal can easily slide on itself when expanding.

Besides that, we have some non-freezing at night temperatures, so I woke up from hibernation and mortared the chimney connection in place. See attached pictures. After mortaring it I put wet cloths around it, so hopefully by tomorrow there's a nice strong connection. I'll re-wet the cloths again for the coming days, and then it's praying for properly good weather to start working on the insulation.

[david s]

The sleeve I make for accepting a removable flue is made from 0.55mm 304 stainless. Being thin it is easy to shape by hand. As shown in post #53 there is a space so that it does not complete a full circle. This helps in providing some room for expansion as well as providing a space to accomodate the lapped joint of the flue pipe that fits into it. To make the pipe fit inside it I wrap some plastic around the pipe, then fit the sleeve over it and tighten the wires up so they are just firm, but not really tight. The plastic allows for easy enough removal as well as providing enough of a gap so the pipe is a nice sliding fit.
Also added a couple of pics showing the fitted sleeve and the weather cap I use when the flue pipe is not in place. Hope this helps, maybe too late.

[Toiletman]

Difficulty removing the plug may have been caused by the lack of a slight taper (called a "release" in mould making parlance).
Also as the casting sets it shrinks slightly, especoally if there's slightly too much water in the mix. Removal from the mould at around 48hrs usually works best so the casting has sufficient strengrh, but a reduced amount of shrinkage.

You're fully right. That's quite stupid of me, because 3D printing would basically have given me the taper for free... ah well, we live and learn.

You have in effect a gallery on top of a gallery, so the smoke flow will not be as efficient as it could be.

Yes, a fully open path directly into the chimney would be more efficient. Luckily the actual profile is not as bad as in your sketch. The width of the gallery arch is much larger than the width of the vent arch. The chimney inlet width is roughly 90% of the vent arch width --> so no significant restriction. Depth-wise the position of the chimney does present a restriction: as a result of the very deep gallery and having the chimney centered on the gallery there is quite an offset from the face of the vent arch. For the smoke to exit the front of the gallery it would have to travel much further as well, because the gallery is so deep. Therefore I'm not too concerned at the moment, but we'll see how the actual performance is once the fires start burning.
Btw, I've seen several of the ovens I looked at as examples have a similar design. Also the kit from for instance the firebrick co. has a similar design. The big difference is that my oven's gallery is much wider and deeper --> the typical one has roughly the same width as the vent arch, and depthwise might even use the vent arch as the back of the gallery arch.

Luckily there's also some actual progress on the chimney connection to be reported. See the pictures.

Based on your sheet metal adapter I've gone the same way: thanks! I've cut slots into a piece of sheet metal, then bent it into a cylinder (I did it by hand/hammer, I'm assuming you have proper tools david s ?). Once it fit the chimney pipe, as well as the smoke inlet, I fixed some wires around it and bent the tabs inwards and outwards. It's already quite sturdy like this, even without any surrounding vermicrete.
For final fitting I'll have to put some cardboard sheet around the chimney to give it some space to expand, but I'm confident enough in it that it's now basically down to waiting for good weather so that the stuff can go onto the actual oven and I can get away from boring CAD pictures .

[david s]

Difficulty removing the plug may have been caused by the lack of a slight taper (called a "release" in mould making parlance).
Also as the casting sets it shrinks slightly, especoally if there's slightly too much water in the mix. Removal from the mould at around 48hrs usually works best so the casting has sufficient strengrh, but a reduced amount of shrinkage.

You have in effect a gallery on top of a gallery, so the smoke flow will not be as efficient as it could be.

[Toiletman]

The casting should've finished its damp curing by now, so recently I spent some time getting the plug out. See the progress below in pictures. The 'repair' area cracked while doing this and I discovered some more voids. I'm thinking of just filling the voids and adding a little cement here and there. As the whole thing will be loaded in compression and then encased in vermicrete I don't think recasting the complete top is warranted. Let's hope that is indeed the case.

Now it's waiting for temperatures sufficiently above freezing so that I can put the casting onto the gallery arch. In the meantime I'll see if I can prepare the sheet metal connections as proposed by david s in post #53.

[Toiletman]

What happened at the top of the cast?

As mentioned and shown in posts #71/#72: I mixed the concrete too wet and it was slumping down, so flowing out of the top part of the cast. I now have more thickness at the bottom than planned, and had some gaps at the top. I filled them in again, and they're now damp curing... my feeling is that it's good enough to hold up and provide sufficient surface for the chimney connector to sit on.

I am a bit perplexed when going back over this thread. Post#65 shows pics of the top of your flue gallery arch. With your casting sitting on top of it, the slot in the top of the arch becomes redundant and a restriction for smoke flow. I was under the impression that your casting would replace the brick arch. I think it will still work ok as you propose, but not as efficient as it could be.

I did consider a fully cast gallery arch initially, but since the gallery is so wide to accommodate the barbecue area and the arch has such a shallow curvature I thought it would be very challenging. Having a small thickness would probably not have enough strength, and making the cast as high as the brick gallery would make it very unwieldy to manufacture and transport. I also like the look of the brick arch face a lot.

You are right about the shape of the bricks becoming a restriction. Not fully a restriction, as the the smallest surface area is always the chimney pipe, anything upstream is wider. However, the widening of the brick arch will cause a negative pressure gradient which might cause some recirculation. Overall though, from the cross section I think it's still smooth enough. Especially if you consider the lengthwise cross-section: the vertical path in the middle of oven is very straight and smooth, it's only at the sides that the recirculation might cause a decrease in draft.

Attached some pictures of the cross sections, as well as an alternative design I considered fully covering the brick arch with a casting --> in the end I felt it would only add to the complexity of construction and make the casted part less robust --> the only material that's really needed is the material to sit around the smoke opening. My main worry was about the brick arch being able to support the weight of the casted part, that's why I thought of extending it to be supported on the gallery walls. In the end my feeling is the gallery arch should be able to support the weight, as well as that once the vermicrete plaster is in, it will provide a nice stiff surrounding shell, with a lot of height giving it also a lot of stiffness, probably eliminating the problem completely. Feel free to chime in with your thoughts...

[david s]

What happened at the top of the cast?

I am a bit perplexed when going back over this thread. Post#65 shows pics of the top of your flue gallery arch. With your casting sitting on top of it, the slot in the top of the arch becomes redundant and a restriction for smoke flow. I was under the impression that your casting would replace the brick arch. I think it will still work ok as you propose, but not as efficient as it could be.

[Toiletman]

Demolded, inspected and filled the cracks and holes: