Glenn Herbert

The heaters that Thomas has built would be much bigger than you need, but you could certainly use the same methods on a smaller scale. A 125mm (5") size system could be quite compact and not take much floor space, especially as you wouldn't need the large clearances to combustibles that a wood stove needs.

The majority of published builds are 6" systems, so brick layouts for the combustion core would likely need to be worked out from scratch. Batch boxes require close adherence to specified internal proportions. If you do not need full heat all day every day, you might do well with a J-tube build which is very easy to do without welded parts. With an hour or two of part time attention you can have warmth for the day/evening, into the next morning.

One important question for RMH conversion: what is the diameter of the chimney flue? (Or dimensions if it is square flue tile/brick?)

A batch box core needs a chimney of at least the same size to work. I can't imagine your chimney is less than 6" diameter for a fireplace.

It depends on how big your house is, how it is arranged and insulated, and how cold it gets to properly advise on size and configuration needed.

How cold does it get where you are, and for how long? If you do not consistently have weeks of below freezing, you likely do not need much mass for long-term heating, and a cottage rocket or similar would probably work without the need for structural enhancement.

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.