What is the most efficient outdoors water heater design?

Hi

Looking to build a wood fired water heater outside to heat hot water for hydronic heating and domestic use. What is the most efficient way to do this. It would be great if I could fill it up with at least 30 kg of wood and let it burn. Also if possible a short flue run so I don't have to make a long chimney for it. It would have to be over 10 meters long to clear the house. I don't mind running a small fan but preferably no electric consumption because we are off grid and have limited energy in the winter.

Thank you

Hi Yen,    Although I don't have any personal experience with what you are looking for or can confirm whether they are the most efficient heaters but can at least direct you to some possibilities to start with. You probably already know about wood boilers. A quick internet search will show many commercial varieties out there.
One version of this is Matt Walkers water heater : webpage

A walker Boiler as listed above. I would insulate it and put it in a lean to greenhouse, so that you can vegetables can use the exhaust/extra heat and and carbon dioxide.

I agree with placing it in a greenhouse because there is a lot of waste heat. Ive been trying to design a good hydronic system for years and have built a couple. But they both ran on propane. I’m still working on the design for a efficient wood burning hydronic system. The Geoff Lawton one has potential. They use it for a shower house I think. It’s  a safe  design because it is a “open system” and when it starts producing steam will keep itself at 212 degrees if I understand it correctly. Be safe.