Specifically the important parts:
“a mass of at least 800 kg. (1760 lbs.)”, “an overall average wall thickness not exceeding 250 mm (10 in.)”, “external surface of the masonry heater … does not exceed 110 C. (230 F.)”, “at least one 180 degree change in flow direction” of the released gas.
Essentially, it’s a rocket stove and a large thermal mass of masonry, and the masonry has a few design restrictions to meet certain performance characteristics (i.e. it should absorb most of the heat released by the stove and it shouldn’t release that heat too quickly from any particular point on the mass). Mostly I’m surprised it’s that easy; I would have thought anything that can radiate heat slowly and consistently would by necessity also absorb heat slowly and consistently, making it difficult to capture most of the heat from a fast-burning stove.
Specifically the important parts: “a mass of at least 800 kg. (1760 lbs.)”, “an overall average wall thickness not exceeding 250 mm (10 in.)”, “external surface of the masonry heater … does not exceed 110 C. (230 F.)”, “at least one 180 degree change in flow direction” of the released gas.
Essentially, it’s a rocket stove and a large thermal mass of masonry, and the masonry has a few design restrictions to meet certain performance characteristics (i.e. it should absorb most of the heat released by the stove and it shouldn’t release that heat too quickly from any particular point on the mass). Mostly I’m surprised it’s that easy; I would have thought anything that can radiate heat slowly and consistently would by necessity also absorb heat slowly and consistently, making it difficult to capture most of the heat from a fast-burning stove.