Oh wow that’s a really clever way to store a ton of the heat for longer. Never thought of that, just running the chimney horizontally through a long track of rock. Cool setup 👏🏻
Discussion
When it comes to rocket mass heater the core design is important. This is the rocket that drives the gasses through the ducting in the mass. The bell (barrel) plus insulated riser of the core burn chamber is important, simply running the exhaust horizontally out of something like a wood burning insert will be difficult to get enough draft to the exhaust to make its way out of the house instead of smoke back into the room especially when the mass is cool. The bell combined with the insulated riser inside creates a secondary burn because the smoke gets trapped at the top of the barrel. It combusts and then is rapidly cooled by the large metal surface area of the bell (barrel). The difference in temperature between the riser and bell needs to be extreme which is why insulating the riser is critical. This create a sort of pump action which helps to force the exhaust gasses through the ducting horizontally in the mass and then up out the chimney. Also notice how the ultimate exit of the ducting is very close to the barrel which also helps to increase the draft.