I'm currently working with a fairly typical basic design, 6" heat riser which dictates the size of j tube and fuel feed----very small, so the first conclusion i have come to, since there is not much difference in external size of the stove which is dictated by the bell, not the heat riser, i have decided to go to an 8" stove---the 6" stove could heat all my floors and hotwater but i would have to run it 6-8 hrs per day. so initial btu estimates were spot on but the intense management required to feed the fuel limits the amount of time it can actually be burning since i have other things to do beside tending the stove--that was something i didn't think about--with a firebox nearly twice the size -using larger chunks of wood and more of it will make tending the fire far less labor intensive

Heating hot water with a 100' length of 1/2" copper wrapped around the top 1/2 of the 55 gallon drum / bell of the stove has been very effective, and i believe i have pushed the safety limits to failure about 3 or 4 times and in every case failure was due to not circulating the water soon enough allowing heat and pressure to build up in the system.

The failures were characterized by blowouts through the pex tubing close to where it connected to the copper coil, and this  could produce scalding if one was unlucky enough to be close by

now that i have installed a thermostat to turn the circulating pump on and off there have been no further issues, but the possibility of circulating pump failure means the system should always be operated in an unpressurized state, in my case  that meant turning off the water supply to the hot water system and opening the hotwater valve to the shower head

keeping the system unpressurized produced no failures, the pex complained a bit when active steam was running through the tubing, but the venting prevented bursting, so  i would say the danger in an unpressurized system is negligible until one might subject the system to numerous circulation failures resulting in repeated venting of steam

please note that at no time have any of the copper components failed, even to the point of boiling dry, if something were to happen to actually clog both ends of the copper pipe which is next to impossible, i believe the failure would be a burst pipe with steam but no shrapnel as might occur with cast iron water jackets

the attempts at creating a thermal siphon failed as i assumed they would, distances and elevations were not conducive to an automatic flow, which necessitated using a small circulating pump to heat both the hotwater tank and the radiant floor.

the plan is to set the hot water tank next to and higher than the rocket stove, and use larger tubing that is insulated to enhance flow, automatically circulating water through the coil and eliminating pump failure as a danger to the system.

also, use of a temp/pressure relief valve that vents to a drain is highly recommended, as is keeping the system unpressurized during stove operation

The goal of course is to not push the system to failure in the final iteration. Keeping things safe and foolproof under all situations is the design goal. having a reliable effective thermosiphon should be the first priority, whatever might happen, having a storage tank of water keeping temps below flashing in the coil eliminates most if not all danger. relief valves would be an effective way of insuring that untrained operators couldn't hurt themselves or the system by inadvertently keeping it pressurized while running a fire continuously  in the stove. also i plan to wrap the coil with a final layer of thin steel like the type enclosing most water heaters, in the event of a burst pipe this would divert the immediate rush of steam long enough for anyone present to escape injury

NO SYSTEM IS TOTALLY SAFE WHEN WORKING WITH THE POTENTIAL OF STEAM, PLEASE USE THESE CONCLUSIONS  WITH A MEASURE OF CAUTION, AND IF YOU ARE INEXPERIENCED WITH PLUMBING AND STEAM HEAT, GET SOME HELP/ADVICE FROM SOMEONE WHO IS

as a final conclusion/ observation let me note that the rocket heater performed well, very low smoke/ odor although there is some, indicating a not quite perfect combustion.

vent temps are in the 100-110 degree F range with very little heat going into the cob. perhaps with an 8 inch system more heat will go to the cob, but that remains to be seen.

there is much less radiant heat from the barrel into the room, the copper coil removes the largest sorce of radiant heat at the top side of the barrel, although the top surface of the barrel will glow red hot and i have taken to keeping a large container of water over the hot spot in an attempt to reduce the rate the barrel might burn through

oh, one other conclusion might be to the use of double wall insulated stainless steel for the heat riser--the inner wall had burned through after about two months of operation, but the insulation and outer wall were in tact,, at first glance i thought it was perfect, but looking inside the pipe showed the effect of the high temps even on stainless--it was still functioning well at the time i changed it over, replacing it with a heat riser cast of perlite and fireclay-2" thick-after installing the new heat riser the top of the barrel started to glow red indicating higher temps were being achieved