Glenn Herbert

Worry about horizontal movement of hot air is a red herring. As long as there is a good push or pull of draft from a vertical element in the system, a certain amount of horizontal movement is easy to reliably maintain. These factors have been experimentally established; early trials often pushed too close to the limits for reliability, and there are always amateurs who think they are going to do something different and better who end up smoking themselves out, and those who don't understand that certain elements of a system have to be done just so in order to work. I think publicity from those people has done great harm to the RMH movement.

A good draft can be established in ordinary conditions with only a 150-200F stack temperature. A rocket mass heater combustion core is specially designed to burn essentially all combustible gases so that there is no creosote at all, and thus no need to keep a chimney very hot. A hot fire in a wood stove is good, but does not approach the performance of a J-tube or batch box core built according to published specs.

Making light straw clay panels that can be dried, then put up and plastered immediately, sounds like a good method. Any finished panels need to be strong enough to not crack in handling, and joints need to be precise or designed to be covered.

Since the sides of the pot are presumably open to air and shedding heat that way, I would suggest insulating the sides. Also, a way of transferring more of the stove's heat to the pot would help... maybe thick aluminum sheet pieces in an L from the rest of the stove top extending up the sides of the pot, with insulation above (not touching) the stove top and enclosing the pot. An old aluminum cookie sheet cut up might work.

I also found that with my building situation giving good positive draft all the time, a 6" stovepipe chimney works well. Paul also tried a 6" chimney in one of his 8" pebble J heaters and liked it, as I recall. A 4" chimney did not work.

My 8" J-tube in a bell (built in 2015-16) happened to use similar ISA as the 6" batch box recommendations and works excellently without wasting extra heat up the chimney.

I have used 2x4s drilled every couple of feet with threaded rod to hold them in place and clamp them to the top of dry-laid block walls. They make a fine bond beam when the cores are also filled with concrete. I wrapped the threaded rods with duct tape inside the forms so I could tap them out when done and reuse them for another wall top. If you want to match the height of a block course, 2x10s would allow clamping to the course below to maintain flat surfaces as you go up.

If you want tile facing, I would fix them into the forms so they are cast monolithically with the concrete.

I agree that a ducted mass would continuously lose heat with any amount of residual draft. I think a stratification chamber would be more dependent on details of the configuration. If the combustion core exit (riser top or secondary chamber exit port) is near the top of the stratification chamber, then it will be introducing cooler air and diluting all the stratified hot air remaining. If the core exit is low in the stratification chamber, it can move more directly to the chimney exit and leave the hot upper part of the chamber undisturbed. This is assuming that draft is restricted from full burn-time flow and does not "jet" at all.

Insulating part of the stovepipe leading from the stove is not going to make it burn fully. Depending on the stove, the exhaust will only be hot enough to keep burning when the stove is burning at its highest rate, and stovepipe is not made to endure fire inside. It will quickly corrode through and create a hazard.

Pulling heat from the stovepipe without burning everything first is a recipe for disaster, as creosote will condense and build up, and eventually during a hot fire start burning and destroy your chimney and maybe your house.

Adding lots of thermal mass around but not touching the stove is the only safe way to improve performance, aside from making sure you always use dry seasoned wood and keeping the fire burning briskly rather than smoldering slowly.

With nearly 7' of difference in height between the two options, I would greatly favor the higher exit point. Even if you don't end up quite as high with the termination from the lower point, you would have a free stack exposure large enough that bracing is probably needed, which adds complexity and potential leak locations.

There probably is a chimney length that is too much, but probably not within reasonable actual conditions. Since a properly functioning RMH will have no creosote in the exhaust, the only real need is to have the exhaust warm enough to rise through the chimney. If it is noticeably warmer inside than out, that handles itself. In shoulder seasons where the house is cold but the air happens to be warm outside, a tall chimney might be a problem.

I definitely suggest angling the exhaust as needed to exit high in the dome, wherever the best arrangement is for all functional considerations.