This is a very interesting way of heating. Paul Wheaton12 shows how a rocket mass heater works and tells how his took him from burning 4 cords of firewood per winter down to one cord per winter and provided warm mornings too. Kind of amazing.
The second video is him showing a couple different builds of rocket mass heaters. I will also post his article if you would like to read it.
The article is at the link below.
wood stove vs. rocket mass heater
12 rocket stove mass heaters – efficient wood heat