[8bitsrule]

Also mentions the Rocket Stove (new to me), which looks well-worth getting to know about. [https://en.wikipedia.org/wiki/Rocket_mass_heater] (Wiki article describes several designs as well.)

"a rocket mass heater consists of an insulated combustion chamber where fuel is burned with high efficiency at high temperature, and a large thermal mass in contact with the exhaust gases which absorbs most of the generated heat before the gases are released to the atmosphere.... "