Glenn Herbert

There have been experiments with compact batch box cores, notably the Double Shoebox Rocket. There have been several distinct variations over the years (DSR1, DSR2 and DSR3). Fox James who posts here has also developed compact cores.

Aluminum flashing is not foil - that is a distinctly thinner product and would not be likely to stop rodents for long. Flashing is used in house construction for weather-tightening to safely shed water or keep moisture away from wood.

I have "dead" space around two sides of my RMH bell, and stack it full of firewood to dry. It can hold at least half a cord, and even wet or green wood split well and stacked for air circulation is bone dry and ready to burn in a month or so of winter burning. I just have to have some already dry to start the season. I can stack quite a bit on top of the bell (which has 6-9" thick roof and sides) and dry it in a few days. Wood stored like this seems to lose almost half of its weight from a damp state, and bursts into flame almost immediately when loaded.

For a very quick and easy demonstration of a batch box, you could try the no brick cutting 4" batch box mockup that I built to test the idea. Just a small basic core with no surroundings, but it gives the sense of how a batch box works, and you can reuse the bricks for a full-size build. The core can be built using the same brick layout, no matter the size of your local bricks.

"Heat engine" is not a regularly used term, though it does describe a RMH.

A whole batch box outside with some mechanism for getting heat indoors is always going to sacrifice a lot of the heating potential. Far better to have the system inside, even if it is fed from a separate space outside the heated rooms. (Not practical for a J-tube, but can work for a batch box which needs to be loaded less often.)

The size and condition of your existing chimney would be important information, as well as its location relative to your proposed addition. Your climate would also help us give the best advice.

I agree with Benjamin that you could put the feed for a batch box in the new room and the rest passing through the wall into a bell/bench in the existing rooms. I think a first generation batch box (with floor channel) would be easiest for a beginner to build. The tall riser is not an issue for feeding a bench. The latest Shorty core depends on a custom fabricated and welded doorframe which needs to be built exactly to spec. A batch box system sized correctly for the load would only need to be fired once or twice a day, not kept burning continuously. Your addition would only need to be big enough to hold a stack of wood for a few days' use and room to comfortably feed the fire. The bell/bench inside the house would likely not take up more space than the safety clearance for your two existing stoves, though you may need to reorganize the interior layout. Again, a sketch would help us give the best advice.

A standard wood stove, even new models, depend on the exhaust being hot enough for creosote to not condense in the chimney, as well as provide sufficient draft. A very nice one my wife bought last year to install in the (ranch style) family home required me to add about 6 feet to the height of the chimney for adequate draft. That was still a foot or so less than the minimum total height in the installation manual, but any higher and it would not have been possible to sweep the chimney. It did need sweeping several times last winter even with a nice insulated stainless chimney (though it would not have been unsafe if swept only once or twice - my wife is a fanatic about keeping chimneys swept since she has heated with wood for decades.)

You can have the efficiency of a rocket mass heater without the extreme mass of a standard design. There is one called the "Cottage Rocket" designed by Chris McClellan (Uncle Mud) who is active on Permies; this is the size of a 55 gallon drum. There is also Matt Walker's "Tiny House Cookstove" which is larger but still reasonably sized for a modest house.

You say you want to use "stones" to support the frame. I think your best bet would be a single boulder under each post with the post bottoms scribed to the boulders. I think making a template or copy of the boulder top after placement to fit the post to would be the best way of managing that. This would start you with a stable base for the posts.

What kind of rock is available in your area? You would likely need one several feet across to support the post without sinking into the subsoil.

I don't know why you would need 3' diameter timbers, even at 30' high, unless the wood was exceptionally weak or a few timbers were supporting the whole structure. If you just want a colossal look, fine. In that case, there would be plenty of scope for lots of mortised joints for floors and braces without impacting the strength of the posts.

I would caution that if you are putting beams between those posts, it will be extremely difficult to move the posts around to get both ends tenoned into the posts. You might have to resort to a lot of lap joints cut into the sides of the posts, which will probably not look as nice as properly mortised joints.

Footings for a structure that large and tall, without concrete, would need some careful engineering of properly placed and compacted large rock. If you wanted the posts to be stable through ground placement, they would need to be something like 6' or more deep below the basement floor. It would be considerably simpler to stabilize them with the rest of the structure tied together with diagonals.

With some information about your soil, subsoil and bedrock composition, and your climate, we could give some rudimentary ideas about the structure. You will absolutely need competent professional engineering advice before you start actually building, or you risk overlooking or underestimating something important that could be dangerous at such a scale.