I've designed my own stoves and outdoor boilers and met with considerable success with my boiler projects in particular.
My first outdoor boiler was a free "jack" boiler, very small, but with some mods, heated my thousand sq. Home and domestic hot water on less than half the wood my friends with $5k-$10k boilers used. The rocket stove mass heaters are essentially the result I achieved with my boiler mods. I first moved my outdoor boiler indoors, building a small, steel panel lined boiler shed 30ft (9m) from my home.
I built custom rigid foam boxed liners for my supply and return lines, insulated the shed with salvaged fiberglass to R36 in both walls and ceilings, and included a dedicated fresh air intake for combustion air...on the coldest days, -15F (-26C), that shed was never below 80F (27C). At first I kept the stock water jacket, and merely fabled up an 8ft (2.5m) water jacketed heat exchanger out of 100lb (45kg) LP cylinders with 6in (15cm) well casing as the exhaust pipe.
Burning absolutely dry, seasoned wood, plus always having a week's worth stacked in that hot dry shed to ensure it being both dry and warm, really helped ensure a clean hot burn. I only fired the stove up twice a day, two longish hot complete burns, and my exchanger pumped the 190F (88C) + water into a dozen more salvaged LP cylinders, modified to encourage good flow, stored in my cellar. I also used a non functioning gas water heater as my heat exchange from my boilers water supply to heat separate water for domestic hot water.
My biggest leap was when I decided to fill the water jacket with refractory cement, thus insulating my actual firebox, creating a much, much hotter fire temp, and put a stop gate with silicone seal in the top exhaust, routed the new exhaust out the bottom with in line combustion air, and realized I had me a downdraft gasifier! Essentially, the downdraft gasifier operates like the so called rocket stove, but with more controlled combustion/forced combustion, thus I could double my already awesome exchanger length built into the exhaust stream, and still never worry about gassing myself while in the shed. By this time, my tiny boiler was heating my home, endless hot water for family uses, and the in-floor heat I put in my two story (800sqft/74sqm each) shop... on even less wood for fuel than before.