read THE book a while ago, and been interested in RMH for a while.
Also done a couple of portable Rocket Stove while on a nomad bicycle community (www.ecotopiabiketour.net) and loved to cook on them, fast to lit and cook and less wood burnt (less to pick )
me and my partner are now building a tiny house, around 4mx4m space and always fancied the idea of a Rocket mass heater bench ... starting now to plan mentaly where it would fit and start looking for materials.
i never saw one live, and realized all the videos and pics i saw the barrel stays very high, and never anyone praises it for the cooking hability on top of the barrel...
is it possible to design a RMH that also is used for cooking regularly (so the top of the barrel shouldn't be too high)? could i follow the 15cm (6'') design of the book and shorten the barrel? what would be that limit? and what's the consequence of a shorter barrel? i remember reading about making the barrel longer would make it burn more efficient, does a shorter one not be able to draft?
our plan is to build the RMH on a second floor wooden, part of the weight will be over the stone wall of the first floor, it's all still hypothetical, as the house is not that big (check https://permies.com/t/43534/timber/Ruin-structure-sketch-Roundwood-tiny) for holding a full sized RMH 6'' (15 cm, non european standards are still hard for me to follow)
i wonder if a 5'' (12cm) RMH would be an option, it would require a lot of attention to start, and keep firing, and down-scaling to 5'' could leed to which other problems?...
and if one would downscale (in weight and size) the RMH 6'', what could be done?
6"/15cm is about the smallest size standard RMH that is said to be easy to build right - heat output scales down with at least the square of the diameter, and air friction effects don't scale down with diameter but become more significant. A 4" system is possible for experts or careful and lucky beginners, smaller than that does not seem to work.
However, a batch box combustion system has been tested to 4" diameter (possibly smaller), and as long as you follow the published specifications it should work well.
http://donkey32.proboards.com/thread/734/peterberg-batch-box-dimensions You can add whatever thermal storage arrangement you desire after that, and in the Portuguese climate it will probably be sufficient to heat your small home. A 4" batch box would be much easier to tend than a 4" J-tube which would need sticks added every few minutes.