Glenn Herbert

Peter recently built a modified sidewinder Shorty core with the opposite exit port configuration you describe, and it worked. The fill in the lower part of the riser needs to be configured in a particular way, as he described.

A layer of polyiso board on top of the existing wall is perfectly reasonable. It will need to be covered with drywall for fire resistance, and some minimal wood framing at drywall joints at least so that the joints will not buckle or shift. It is possible that gluing the polyiso to the wall and the new drywall to the polyiso can make the centers of drywall sheets rigid enough for good looks. If you will want to put any shelves on the walls, you will need regular (2x2) framing behind the new surface.

As you have an attic, putting more insulation and bigger vents in it would definitely be a good idea. The ideal would be vents that allow breezes to flow freely through the attic space in summer, so heat does not build up there.

I think you would be best served long term by taking measures that will reduce heat buildup in your upper floor. Things like a ventilated roof skin above the existing one, vents high and low and taking advantage of prevailing breezes, screening of walls and windows that are exposed to direct sunshine...

The existing floor slab is probably uninsulated and fairly well coupled to the earth, so I don't see much benefit to adding another mass on top of that.

You have more heat than you want all summer, day and night; adding thermal mass to the exposed wall will just ensure that the heat is held longer. What you want is more insulation on that exposed wall so heat doesn't get through as fast and the outside has a chance to cool to ambient temperature at night. An extra thickness of insulation will add a negligible amount to the weight of your structure.

Is that a crack in the slab running from the circle to the back? Is it supported where the crack is, if that is what it is?

The bottles were totally embedded in cob. The mouths should be able to breathe a bit to avoid explosions. The floor of a bell is not likely to get hot enough fast enough to be a problem.

The floor of a proper bell would not get near as hot as the ceiling or even walls, but could get too hot for long-term wood contact in some situations. I can't say more than that.

I don't see any benefit to having the bottles exposed at the base of the brick wall; I would just have the gaps in the bricks at the base with the bottles inboard of the bricks. No interference or accommodation needed. If you had one of those bottle cutters, you could cut off the bottoms of all the bottles, put them end to end, and have free air circulation which would reduce heat to the floor much more than enclosed bottles.

I don't see including some 1" iron pipe in the floor as doing enough to be worthwhile.

If you can get insurance with a RMH in the house at $150 extra per year, the extreme efficiency, simplicity of build, plus ease of use may well outweigh the added cost. How is your house laid out? Is a central mass heat source going to work in it? An open water heating method that avoids boom-squish and connects to your existing circulation may be a worthwhile enhancement. If planned for from the start, it could be added at any time.

That looks like a reasonable schematic. Don't confuse "bell" with "batch box"; one is a heat extraction chamber, the other is a combustion core. They are generally used together as batch boxes do not like the friction of duct-in-mass, but bells can equally be used with J-tubes. A J-tube is simple to build and reliable if you get close to the right dimensions, while a batch box is more finicky and would probably better be saved for your next build once you have some experience... unless you are already an experienced mason or builder.

Cinder block should work fine for the walls of your bell. The higher heat areas need to be at least old red brick, or firebrick or other refractory material to stand the heat.

I built bottles into the cob surrounding the core of my rocket fired oven... can't say how well they work, though one near the riser base exploded in the first firing as the cob was wet and sealed the bottle neck.

I think they would work fine in the floor of a bell. I would bed them in some perlite-clay and space them enough to have perlite-clay struts between them, and a continuous layer above. It would probably be wisest to keep some space open at the base of the bottles so that air could flow from side to side under the bell to prevent heat buildup in a long firing.

I would want the brick walls to be supported on cob or on the floor with some gaps at the base; the base of the bell will probably not get too hot, but better safe than sorry.

A standard method of flooring a bell is to space some cement board up on bricks so air can flow freely underneath. This requires buying cement board, but is fast and easy, and takes little space from the height of the bell cavity.

The dragon's mouth is the exhaust, though fire never makes it that far. Flames seldom reach into the oven cavity. It smokes impressively at startup, or a bit when loading a big log.