rich tetlow wrote:hi there big al
thanks for your input. i've had a good look through 'rocket mass heaters' (only 2nd edition though) & started trying to put things together. on the subject of dimensions, my stove is 10 inches across (widening to 11 at the top, which i love), 17 deep & 14.5 high, & the riser, which i drew quite short on my diagram for space reasons, is 20 inches high. i'm hoping this will be enough to burn up most of my gases, if not i guess i can extend it up, which given your comments about temperature might also be a good time to let the extension go clear of the water jacket so a bit of heat is dissipated there. my problem here is that i really want as much heat as possible transferred to the water rather than the surrounding air...but without boiling it. i guess the volume of the water will be quite significant here, haven't made the outer part of the water jacket yet but i'm currently looking at about 1.5 inches of water between inner & outer, which feels like quite a bit to just flash into steam. i guess i'll probably put it all together & fire it up in the garden where i can run for cover easily if things go wrong! then proceed based on the results from that, i'm hoping the reduced size of my system will mean a bit less heat than the somewhat daunting temps most rockets seem to hit, but i'm changing so many parameters its hard to estimate, specially as a complete amateur!
also, thanks for the heads up about diverters. i'd kind of wondered if i might need something like that & based on your opinion have decided i do. fortunately the way things work out, as seen in diagram 2, the top of the horizontal bit of burn tunnel just comes up above the centre of the rear flue hole, so what i'm thinking is to cut a slit in the insulation at the back there & a hole in the top, replace the flue blanking plate with a suitably cut piece of steel (so i don't have to cut the blanking plate itself) & poke a steel plate through the slit so it just comes up to the edge of the hole. that way my exhaust starts by going straight up the chimney, then when things have heated up i can just push the plate in, it goes across the hole & the gases are diverted round. leaving a break in my insulation where heat can still pass from just above the firebox to the cooler part of the flue, but not to a significant extent hopefully.
by the by, the stuff i'm using for insulation is an inch thick ceramic fibreboard sold for lining kilns & things. absolutely horrible to work with, like a dry, dusty version of wet mdf but apparently very good at resisting erosion by hot gas. just about structural enough that i think i can just stick 4 sheets of it together to make the riser as well. i mention this for information & interest. thanks again, i'll now carry on, watching this space in case anyone wants to tell me if i'm about to do something dumb, & report back on any results that seem worth it.