Benjamin Dinkel

I'm with Mart here.
I use that set up for Tempeh at 30C.

Hi L,
you say a large thermal mass might be problematic. To me that means I would also dissuade you from using under floor heating, as that uses all the floor as thermal mass and is usually a system with a lot of inertia.
You could supply hot water heaters or the heat exchanger of an HVAC with the hot water.

Hey Midge.
The ratios depend on the type of clay and also sand.
And how thick you want and need to plaster.
If you need to cover uneven blocks and want to unify it all, I would suggest a first coat of almost 1“ using clay, sand and fiber.
Depending on the weather, you'll need to let it dry a bit.
And then add a second,finer coat. With sifted sand and no fiber. Maybe 1/4“ thickness.

You need to do some tests for finding the right ratios.
I would start with 1 part (volume) clay and 3 parts sand and/or fiber.

I don’t know if waterproofing is a good idea. Can you protect the plaster from direct rain?

Also don’t let the plaster touch the ground but build a little stone wall, like 3“ high, as a moisture barrier.

Hey Dave,
what is it that you're trying to learn?
There's a lot of different "masonry" style heating systems in the world. The goal always being to extract most of the heat from the exhaust gases.
To me the RMH is just the modern, more low tech take on that. The key being a complete combustion and a later heat extraction.
A lot of contraptions and inventions I see don't have that in mind and extract heat from incomplete combustion, which leads to creosote and potential disaster.

Hey Mark,

I don't see why the brick chimney makes it more complicated. It's whithin the bell, so as long as you find a solution for the bell it's going to be fine. Also you could build a 4 walled chimney whithin the bell, just to be sure.

As for the clay mortar, a brick wall doesn't get the stability form being "glued" together, but from gravity and friction. So as long as you interlace the brick correctly, an L-shaped wall should be very sturdy.

Now the biggest challenge, the "seal". Super wool and/or other gaskets would be an option. But a simple solution, which would also provide more stability, could be to chisel the existing wall, where old and new meet. Creating a space for the new wall to go a couple of cm into the old wall. Aplly plaster and done.

And as a precaution I would definitely go get a CO and smoke detector. With any woodburning device. They're only good for 10 years, not very permaculture, but a good safety measure.

Hi Yen, I would love to see some photos and hear about the details.

Hey Mark.

Mark Roelofs wrote:
Or the chimney could be along the entire depth of the bell

That was what I was thinking. I don't think it would be too big and much easier to build.

The bypass needs to be up I think. Otherwise it won't have much effect.
Depending on the type of bypass I think they can close quite well. Check out the one I used (attached).

The CFB sounds like a good idea. Is it the same as calcium silicate? I've seen that available here

Hi Richard,
I think this richsoil article about Wofati would be a good start for your search.

Hey Mark,

Materials: they have standard sized firebrick in most building material stores. I don’t mind Leroy Merlín, but I think the FB there was expensive. I buy mine at BigMat, Pérez Lázaro or the local “material de construcción”.
Splits might be harder to find. Also I find them harder to stick together. I usually just use clay slip.

ISA, insulation and bell depths:
I was thinking whether building a brick chimney against the outside wall, whithin the bell, would be an option to include a bypass, solve connection problems metal/brick and take care of the insulation problem.

William Bronson wrote: I'm pretty sure the "juice box straw" design is a metal chimney in a bell.
I'm not sure if it has ever left the design stage.

Yes, it technically is a metal pipe in a bell. But the ones I saw were situated in a bench, following a barrel. In those setups the barrel would shed a significant amount of heat and thus the exhaust pipe can survive the temperatures.
In a bell, above the height of the riser, it might be too hot for normal steel pipe.
But as Peter mentioned, stainless steel pipe should be able to deal with it.