I love what you're doing! I built my wooden house by myself and it has cob innerwalls with heatingpipes , (fed by a downgraded heater at this moment) a RMH is the next thing to do. My one and only concern
is redoing things over and over again for the benefit of fun, changing forms and experimenting. So wouldn't be nice if that is possible with less effort as possible? I'll go for the sand and stone filling, but I've a question about the accumulation. Is it enough? Is there a big difference in comparison with cob in terms of radiation?
Instead of a wooden case, I'll build a hollow bench from cob and want to make some connection with the walls in future, so it has to be changed easier than dried cob all around the duct will be. The second benefit will be the weight on my wooden floor. I already made some concrete poles for extra foundation last easter (forced to stay at home a couple of days in Holland:-)
I also produced a few cubes 150x150x150mm from Curon 120S (famous Dutch refractory mortar, but very expensive) with vermiculite in different mixes to find out which ratio will be most efficient without degrading to fast. I want to produce the burntunnel and heatriser with drumstand and connections for the duct, ash pit and cleanout in 3 molds, regarding to the dimensions mentioned in Ianto's (PDF)book. This way I hope I'll have less concerns when rebuilding.
Keep up the good work,
Regards, Boy - Holland.