Benjamin Dinkel

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.

Hey Mark,
It's great to have another Rocket enthusiast in the neighborhood! We live in the Valle de Lecrín, maybe 45 min from you.
I'm happy to drop by on Saturday (if the roads are clear) and bring the Wiesner book, I'll be in Cenes de la Vega. Or you could pick it up there and we can chat over a coffee.

Trying to answer some of your questions:

-6" or 8" exhaust: if you have the space going for 8" has no downsides. Draft can always be controlled with a brick on the feed tube. Will you replace the 250mm exhaust or try to fit the 200 mm (8") exhaust in there?

-I've also read in more than one post that the 8" J-tube is comparable to the 6" BBR. So orienting on the ISA for the 6" BBR should be good. Worst case (too much mass, too low exhaust temp) you could always raise the exhaust pipe, making the ISA smaller.

-Definitely insulate the bell against the outer wall. Maybe even build a double wall filled with insultaion. I like to use Arlita (expanded clay), but you might need 15-20 cm as it's not very insulative. That would probably bring your ISA down already.
Insulated surface as well as the floor of the bell don't count towards ISA.

-Core design: You could have another row (course) of flat laying fire brick as the first course to raise the height of the burn tunnel. Which size firebrick are you using/planning with? Do you have a height restriction with your bell?

-I would add both the T and the cleanout. They're easy to integrate now and in case anything should not work as planned they are ways to look inside, measure, clean, etc. The T could also help you prime the chimney on bad days or with a very cold mass.

Jeremy pointed me to the richsoil article about Wofati. That would be a good read

I'm with Conrad here.
Chapter 30 of the Building a Better World Book also deals with a house in a slope, especially pages 244-250.

Some things you could read up on here are "Wofati" and Mike Oehler.