We’ve briefly mentioned rocket stove mass heaters before. Here’s an excellent site that compiles a lot of free info about them and has free plans. (Detailed drawings are for sale, but anyone should be able to figure out the process by looking at the illustrations, reading the text and watching the videos.)
“Rocket mass heaters in a nutshell:
– heat your home with 80% to 90% less wood
– exhaust is nearly pure steam and CO2 (a little smoke at the beginning)
– the heat from one fire can last for days
– you can build one in a day and half
– folks have built them spending less than $20