Scupper look into thermal mass wood heating or russian stoves , i may get the numbers wrong but a thermal mas or russian stove seems to be the best method to extrace heat from wood, from my understanding it is a narrow long fire box at the top it narrows down and a second chamber opens up where more outside air is brought in, when the lower fire gets hot enough the smoke ( unburned gas) ignites or burns something like 2000 degrees i think they say the efficiency is something like 90 percent WAY above any other type of wood burning and produces very very little smoke ( clean air for the neighbors ), used in houses they use masonary i think at least 2 tons. fire this thing up once per day and it releases heat for 24 hours, you could use dirt for thermal mass but the dirt has to be kept dry, i think this would work very good in cold climat . but then there are those that use the smoke from a low temp burn to run car motors which can run generators ???lol