The Koreans and Chinese used to use these:
adapted here and renamed “rocket stove mass heater”:
https://richsoil.com/rocket-stove-mass-heater.jsp
It is very efficient.
ETA: I just uploaded the image above (hard-coded) to the bOING-controlled server(s) because I’m seeing “broken link” in my own browser. When in doubt, code it out…
ETA2: Image original size must be too big so I ensmalled it. Display looks ok now from here. Gosh that was involved.
