Just wanted to share this video of the Rocket Mass Heater I built this winter. I've been heating a building here on the west coast of Canada with it for almost 2 months, and so far I'm very happy with it. It is a 6 inch system, and includes a fair bit of radiant surface, which is perfect for me as I don't necessesarily heat the building every day. The mass bench is small, due to size restrictions of my room, and the size of the steel tank I had to work with.
I installed the water coil a couple of weeks ago and it's working very well. If I don't use the hot water from the tank it adds a huge amount of thermal mass over just the bench. Once the bench and tank get warm it will keep the house quite warm all night.
The exhaust (measured outside the house) runs at about 120f, so it looks like I'm stripping most of the heat off the exhaust gasses.