this year I built two 6" RMH according the example Annex 6". First RMH I built in my workshop at the end of winter (fig. 1) and second one (which is in kitchen) is just finished (fig. 2) and we are enjoying the heat just now Kitchen RMH works fine and we heat all house (around 120 m2) while outside temperature is 8-10 °C. Difference between kitchen and workshop RMH is that in kitchen i cut bricks for "shell" on half (fig. 3), rest is pretty much the same. Heat goes just fine through 1/2 bricks while through whole brick in workshop barely came to the surface of the brick.
Kitchen RMH is connected with pipes to chimney which is in another room behind the RMH (fig. 4). Here i want to build one more thermal mass bench but i am little bit afraid that this will affect draft (the end of metal pipe on the fig. 4 is connected to the another pipe in wall which is about 2m long and this pipe lead through wall to the chimney). I was thinking to insert bypass and "turn on" thermal bench in another room only when hard winter come, so the draft should be enough.
Anyway big thanks to authors of the Annex 6", if RMH will continue to heat us same way as now, we will probably save costs more than 4 time