Glenn Herbert

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.

"horses for courses...deploying technology that is appropriate to the situation."

You would not use a thoroughbred to pull a plow, or a Percheron to win a race.

You are correct, a heated mass when airflow is allowed will start a decent draft in most cases.

A bypass definitely makes a masonry heater behave like a common wood stove, allowing easy flow of heat to begin warming the chimney.

Some situations, like mine, have good draft through the operating configuration as long as the house is warmer than outside even when the mass is stone cold. This condition necessitates a tight inlet shutoff whenever the fire is not going, to avoid stored heat being drawn up the chimney. I built a bypass opening in my bell next to the chimney, but never needed it so it remains permanently plugged.

Brick has enough texture that even clay-sand mortar should seal it up in bell walls. Stone that has textured surfaces and shapes that would allow stable dry-stacking can also be reliably clay mortared. Smooth round stones would depend entirely on the clay between them remaining sound and not cracking, and I would never make a single-skin enclosure with them. I would feel good about making an inner skin of a couple inches of cob, being sure to seal up any cracks that develop in drying.

If you are in a mild climate, you really don't want a big high-mass heater. A small heater with minimal mass around it would be optimal. If you already have a modern relatively efficient wood stove, it probably doesn't make sense to scrap that and build a new RMH. I would do as you suggested and add a bunch of masonry around but not touching the existing stove.