I basically followed the rocket mass heaters builders guide book exactly and it has been working flawlessly with only regular cleaning necessary. I've made a few minor upgrades over the years, like using a fiberglass blanket instead of rock wool for the riser insulation. I plan on converting it to a batch box burn chamber eventually.

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The size of the firebox is very important. I've seen RMH builds that have made the firebox too large and it wouldn't draft without a booster fan. This is one of the downsides to RMH. They require more feeding or need to be very large to heat large spaces or large masses. Batch box designs are possible but I've heard it's challenging to get right without experience with standard j tube designs.