rich tetlow

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.
cheers
rich

sorry, that was a pretty poor diagram, done in a rush as well as in an unfamiliar medium. let me try again;
i've now drawn the stove in black, & my proposed layers of insulation in thick red, this should be treated as forming the whole 'doughnut' of the riser. the hatched area represents insulation face on to the view, hopefully elucidated by the top & end views.
boiler in brown/blue, with pipes not shown, & flame flow in orange. if anything is drawn in proportion, its accidental!
that richsoil site was the first place i came across the rocket stove idea, & i love it, but i don't really have the option to do the whole thing; this is my attempt at a compromise & its intended to be a true hybrid rather than a proper rocket built inside a stove box. my layers of insulation are thin, partly for space reasons but also because i want them to be imperfect- i don't really want gases hitting the boiler at 2000 F, or it will actually boil...i think.
the key problem i want to solve is of having an effective boiler stove that doesn't have the water sitting right by the fire leaching all its heat away before its got started.

thanks, thats going to have all sorts of uses. makes me wonder what other clever things this computer can do that i don't know about...anyway, for the moment i've done a little top & side view sketch of the thing i was trying to describe, which hopefully is now attached.
cheers
rich

hi forum
i'm contemplating a bit of a project, & would value comments/suggestions from some of you more experienced rocketers before i begin...
what i want to do is a boiler for a central heating system, a big insulated water tank is acting as my thermal mass, but the stove is my main subject here. i'm starting with an old jotul woodstove, & what i want to do is
insulate the firebox inside with vermiculite or ceramic fireboard or similar
partition it part way up with more insulation
make a riser of (possibly) standard flue pipe surrounded by more insulation which goes through the partition, & up through the hole where the hotplate would normally sit
cap this off with a metal plate, onto which are welded 2 concentric circular metal sheets, sealed at the bottom to form a round, top mounted version of a backboiler. water enters this by a pipe at the bottom & leaves through another at the top, & this arrangement sits with its inner edge flush with the outside of the hotplate hole.
so the plan is that the fire in the insulated box is smaller but more intense, the riser keeps the gases hot & burning in standard rocket fashion, then i can use the plate on top as a hotplate, & my boilers inside surface transfers heat from the gas to the water & the only place the exhaust has to go is back down the hotplate hole into the stove, where it loses more heat to the area of stove above the partition, then exits through the flue in the normal way, hopefully retaining enough heat to make it up the chimney.
hope i've explained that adequately. a diagram would've helped, but i don't know how i'd do one on here.
so...whats going to go wrong with that? haven't decided the dimensions yet but the hole i've got is just over 9 inches across so i reckon if i allow for a 5 inch inner flue & an inch of insulation all round that just leaves me a slightly wider area of downward 'flue' inside the boiler. i have no idea how high i should make the stack, nor how thick the boiler should be to heat the water effectively without boiling it. current plan is to experiment with some mild steel, which i've got, then try & learn from the results before doing something more durable. any thoughts...?