Massive Thermo Mass Battery in my round house Suggestions

Ok here the dillieo… Looking for aditional Ideas and or advice :

I am in the middle (ok 1/3) of my round house build.

I really want a rocket stove mass heater in it but no way of getting it approved!
So
I am going to make the most efficient rocket stove boiler the world has ever seen! well maybe just my county anyways? Ha!
It will be self circulating, tankless/thermo mass, multi fueled & or chip feed & it can be manual fed normally when powers not available!
I have my detailed plans for boiler all figured out! I’ll have time next winter to build it,

I am getting close to putting in the hydronic heated basement floor, Soon! Spring is near!

I want to install a “massive” boiler heated Mass put in the center of my 36’ round floor! My ideas so far;

Dig say a 5’ deep and 10 x 10 ft hole, Would that be big enough? or even bigger maybe? 3 storey, 3k sq ft

Insulate with ?? heavy straw cob or Styrofoam or ? Not sure what could handle that much weight? Any suggestions?

Then maybe the bottom (if needed) and all sides with an additional insulation of 4” perlite mix that I’v read you guys talk about! Would that be good enough for a slow drain off a mass of that size, ideally @ 200 degrees?

My biggest question is what would be the best & cheapest materials to “pack” in the mass around the Pex tubing? I’m thinking crushed concrete (not next to pex), river rocks and & sand maybe? I need typical materials form a gravel pit! My land is mostly sand…

I do have a pond down the hill?

Future look... maybe?

I love this idea of thermal batteries.

I've been preoccupied with this idea for about a year now. I've been searching videos, articles on rocket mass heaters, building construction using rammed earth, passive cooling -- stuff like that. Sadly, the reality is that I will probably never be able to implement these ideas from the ground up, as we've just purchased a home that was built about 30 years ago. But, I was thinking I could build a couple of heavily insulated sheds each filled with a large thermal mass (concrete, metal, stone, sand, oil, water) which I selectively charge during the off-season -- circulate hot summer air in one shed to be used during the winter and cold winter air in the other shed to be used during the summer (condensate could be collected to supplement water to the garden). If I were particularly zealous, I could boost the effective thermal charge of each shed by engineering a supplemental charging system -- perhaps a network of subterranean cooling tubes for the cold shed or perhaps a top-mounted parabolic solar trough on the hot shed to heat a heat-transfer fluid. I live in the south, so I would probably want to have my cold shed three to four times larger than my hot shed since the summer-time air will contain lots of moisture and my winters will be relatively mild. If I lived near the coast, a river or a stream, I might consider pumping the available water through my cold shed depending on the the water temperature.

I hate to temper anyone's enthusiasm for thermal mass but I dont think its as important as many would lead you to believe. Thermal mass walls, best used in the rarest of desert climates, are different than what youre proposing so I wont elaborate on those. Conditioning a home's air is less about how much thermal mass storage capacity you have than creating an air-tight and thermal-bridge-free building envelope.

I feel your bigger challenge is going to be air sealing and insulating your building envelope enough to efficiently keep the BTUs you create inside your home for those chilly MI winters. If you did this to Passive House levels of insulation then you could arguably heat your home with a small hair dryer. You seem to be taking a different approach entirely by blasting a bunch of BTUS into a massive amount of centrally located thermal mass. I certainly think it could work and Iam all for experimenting when it makes sense but dont ignore the power of airtightness and eliminating thermal bridges through the insulation. If you do it right, then your thermal mass will need far fewer BTUs and your rocket mass heater or woodstove will need significantly less wood.

Do you have any sketches of you boiler to share? Iam a little confused about the location of things. If youre thermosiphing, then your woodstove/RMH needs to be below your thermal mass which of course complicates the structural support the heavier you go. Some of what you describe sounds like an old carolina water stove. They dont make them any more but they are giant water tanks surrounded by a woodstove and the water was pumped through radiant pipes. Some of them were even equipped with solar thermal assist.

Have you considered an outdoor, wood fired boiler? Or applying the rocket mass heater to this form of heating?

Assuming that the windows you drew are facing South for Passive Solar gain, dont make the deck more than about 18" deep or it will shade the windows too much.