Sorry I was just going off of what Erica W described here:
The other option I often suggest is an open pot or tank directly above the barrel, like the top position shown in your drawing - but with a large reservoir instead of pipes.
One of the other forum threads shows an Australian hot-water heater where they built the vertical insulated firebox and then suspended a whole (open) barrel above the fire. Then run the coil through this larger barrel. The large barrel can come to a complete boil (it's open to atmosphere) without damaging anything except maybe splashing the fire. The pressurized pipes running through the large hot water reservoir are protected from boiling as long as there is water in the big tank. It makes something like an on-demand heater, at a skill level that many DIYers (do it yourself-ers) could handle. I believe geoff lawton was involved with that design.
I would use the bottom half, fill it with water and have a copper coil in the water, which would be part of a closed system, to heat water in a 1000 litre liquid tote for an aquaponic system.
I also could circulate the heated water in other 55 gallon metal drums, with a pump, to create a heat battery in my greenhouse.
Does this make sense?
Is it too hot on top of the riser to put a copper coil right on the drum? The top of my heat riser was glowing last night in the greenhouse, with constant feeding of pine 2x4 scraps. I thought it would be too hot to place the coil right on it. If not, then forget the 1/2 barrel filled with water and the coil placed inside the water.
I just want assure people that I would never setup this system without the proper pressure release valves, a bit paranoid of that and Legionnaire's disease. Have to do a bit more research on that.