Glenn Herbert

You wouldn't need to completely seal off the bench space; when the direct path to the chimney is open, most of the gas will flow through that. For positive control, I would put a flap hinged on the angle between 2 and 3, so it could direct flow either into the bench or straight to the chimney. One control instead of three is much better.

I think it depends on the particular installation. If you are depending on a heated chimney to get enough draft, and you had a way oversized bell that shed heat quickly, you might never get really hot or a really good burn. If you have a massive bell sized just right for a 6" BB and good natural draft, a 6" J-tube would burn fine and would probably take 3 or 4 times as long to heat up the bell.

My 8" J-tube burns fiercely as soon as the kindling is lit even if the bell (about 50 ft2 ISA, 9" thick brick + cob) is stone cold, no assist from a warm chimney.

A J-tube the same system size as a batch box will eventually heat up the same bell fully. How long it would take depends on the specifics of the situation.

Okay, so you are thinking half of a hypothetical duplex, not a particular house. One half would be in the neighborhood of 1000-1500 s.f. A RMH could be built to take care of that in about a 3' x 4' x 7' high footprint, with double skin for safety and mass.

You would typically put a RMH near the center of the house, so that no part is too far away from it. It needs an actual floor plan to get any more specific about size, shape and location.

Welcome to Permies, Colleen! Those are spectacular works of art.

Traditional masonry heaters are quite complex internally, never mind the external decoration, and require an experienced builder to complete safely. In this forum we concentrate on much simpler to build versions, originally with ducts running through a mass and currently with stratification chambers or "bells", with fireboxes that are even more efficient than traditional masonry heaters.

We can give you advice and specifications that will let you safely and effectively build your own rocket masonry heater and be confident it will work without hiring a professional stove mason. The external decoration is up to you.

A masonry heater works best in the space to be heated, but I understand the reasons you may want to build in the basement. This can work if all conditions are right. A masonry heater does not need to be as big as the ones shown, depending on your climate and the size and character of your house. So how big is the house, how much insulation does it have, how drafty? What is the floor plan like, open or closed off rooms?

It is possible to put a heater on the ground floor with piers in the basement supporting it. This may not even be too complicated depending on your house.

Some information we need to assess the situation would be the size and character of the house, its age, insulation, weathertightness, a description of the chimney...

What exactly do you mean by a baffle? How close to the floor of the final bell barrel is the exhaust to the chimney? How tightly do you cover the J-tube feed between fires? Do you have natural draft when the heater is cold? How strong is the draft when it is operating?

As long as the main floor is not leaky (letting cold air percolate directly up), I think the underfloor insulation is not the main issue (but good to get it fully insulated). A 6" duct in the mass would possibly be too quick of a reduction from 8", though the extra large barrel(s) above the riser may shed enough heat to make that not a problem. My experience with a 6" stovepipe and chimney from the bell of an 8" J-tube is good, but that has all the heat extraction in the zero-friction bell.

Welcome to Permies, Carson! That is a very long list of questions, too many to respond to in one post. I will hit a few highlights to start.

Masonry heater code calls for 8" minimum clearance to a ceiling.

If the basement slab was poured by a commercial house builder, it is probably nominal 4" thick. It may be strong enough, but as you have no height restriction I would start with a 4" reinforced concrete slab a bit larger than the bell footprint, on top of the existing slab.

Professionals generally use straight hard firebrick for firebox walls, without added coatings. As the door will generally be a bit smaller than the firebox, you should not have a lot of sliding wood, and you should not throw logs in such that they might hit the back wall or ceiling. The load should have a couple of inches clear to the back wall.

I have not heard of anyone using an expansion joint between core and structural base, and I do not think one is needed between lower common brick and upper firebrick in the inner bell. The part most likely to be affected by heat is the top, and firebrick expands less than common brick when heated. The heat gradient is fairly uniform from top to bottom, especially in a first gen batch box.

I would definitely allow for expansion if using angle supports for the roof; less so with firebrick slabs.

As long as you maintain clearances to the ceiling and possibly a heat shield on the ceiling, I would not worry about insulating between skins.

For water soluble mortar, you want a clay-sand mix. Runny and very fine sand for thin joints, conventional mortar consistency and don't worry about the sand size for wide joints. This is what stove masons commonly use, so I don't think you need to worry about longevity.

I think your flue routing would be safe for your use, but if somebody ever took out the RMH and put in a standard wood stove, it would be very dangerous, so I think it is best to follow code for the flue and roof penetration.

Smooth, white, interlocking and lightweight sounds like styrofoam form block, designed to be filled with concrete and rebar to give a continuous waffle-like concrete structure with minimal heavy concrete and maximum insulation.

As for the 3-2-10 rule, I ran a temporary 6" metal chimney up 6', out the wall, and up another 4' because I didn't want to build the stone chimney into the loft space above the heater until I finished the master bedroom addition. The chimney top ended up 3' below the roof edge and I didn't want to work on the roof three stories up with nobody else around... That configuration worked fine most of the year because the chimney on the east side was always in the lee of prevailing winds. A few days each spring we get east winds and those made it impossible to run the RMH without backdrafting. Since I raised the chimney 5' I have had zero issues with backdrafting. It is only 2' above the adjacent roof edge and 1' above the other side of that piece of shed roof 10' away.

On dead flat land, except where there is no possibility of flooding such as a desert, this would definitely be the best version of the general concept.

If there is any slope to the land, it would be possible to make some or most of the finished floor below original grade and be even more sheltered.

In most cases, I expect the undisturbed subsoil would be better compacted than any mechanically placed soil, so the more excavation compared to raising the better.