Yea, that title deserves to be in all caps. Rocket Mass Heaters ROCK, hands down they are the BEST method for burning solid fuels. It is like a gasifier and a wood stove in one. Now if you wanted to run an internal combustion engine then by all means go with a gasifier. But if you are in the business of extracting every ounce of heat from a particular carbon based solid fuel...like wood, then the RMH is for you. Want to know the best part? It's REALLY easy to build.
All you need is a pile of fire bricks [solid bricks that have zero moisture will work, if your bricks have moisture in them, when you light your RMH, it may explode. But fire bricks are very porous and allow for the moisture to escape without exploding, which is good. Some people use metal stove pipe instead of the bricks, so if you have stove pipe, you could use that too. [well you are going to need stove pipe anyway...so if you want, just get a bit more.]
You will also need stove pipe [aren't I just the funniest?] You are going to need about 30 feet of 6 inch pipe and about 3 feet of 10 inch pipe. The 10 inch pipe is going to go over a section of 6 inch pipe to make the insulated burn chamber This burn chamber is the heart of the RMH. It allows temperatures to rise to extremely high temperatures facilitating complete combustion of the unburned gasses. Temperatures inside this chamber will reach 700+ degrees so I would recommend that you not use galvanized pipe for this section because of off gassing poisonous gasses and melting.
You need a 55 gallon drum or similar. This acts as further insulation for the burn chamber as well as acting as a heat emitter for the house. a water heater tank would work well if you had one.
And you need earth, mass, sand, rocks, cob, etc...something that you can build around the exhaust of this heater to act as a heat battery. The more mass you have, the longer you will have heat after your fire, but the longer you will have to wait to get heat when you start your fire. be reasonable. a foot of mass on all sides of the pipe is a good point to start. If the mass gets too hot for you, add more mass. Simple right? Why don't more people have these in their homes?
Construction is pretty simple you line the floor where the RMH hopper and burn chamber is going to be built in fire bricks [the displacement area of the inside of the hopper/burn chamber area should be around 19 square inches [2piR of 6 inch exhaust pipe is almost 19 square inches] if using bricks, a 4"x5" area will give you 20 square inches. Now make this tube about 3 feet long.
At one end of the tube build up a hopper, the hopper is where you put your wood to be burned you want it to be a good foot tall. On the other end of the tube build up your insulated burn chamber [you can use fire bricks for this if you have them and dont want to buy the pieces of pipe for the burn chamber. But make sure that the chamber is sealed or the air will escape and your stove wont work. sealing it isn't hard, just mortar it all together, you can use cob mud for this if you please.
You want your burn chamber about three feet tall to get the most out of your combustion while still giving you heat for your home. Over the combustion chamber you place your barrel/drum/whathaveyou to act as your heat sink for your house. [you can also cook on it!] The barrel is upside down [opening facing down]
The bottom of the barrel should be about 1-2 inches from the top of the combustion chamber. The bottom of the barrel should be sealed except for the opening for the exhaust pipe. Once you insert your exhaust pipe, you want to seal that too so no smoke can come out into your house. the exhaust pipe is just a long length of pipe buried in earth. It can be long and slender, or short and wide if you put a U union and turn your pipe around. The very end of the exhaust should be ducted outside.
THATS IT! to start your RMH just take a few pieces of newspaper and stuff them down the hopper al the way over to the combustion chamber and light them on fire. Once lit, you want to stick some more newspaper in the hole to pull the fire back to the hopper where you have your sticks and branches and such. The draft created by the heat in the combustion chamber will pull air down through the hopper making the fire burn sideways and not letting any smoke come out of the top of the hopper.
Once in the combustion chamber the exhaust will heat up to the point of complete combustion giving you the most heat for your wood.
The exhaust will come out of the combustion chamber and hit the barrel, dumping alot of that heat. It will then turn around and head down towards the opening of the barrel [the barrel is upside down on the chamber], dumping more heat. Finally the exhaust will flow through the stove pipe that is surrounded by earth dumping all the remaining heat and ending up with just water vapor and co2.
These heaters/stoves are ~90% efficient in field testing which is WAY better than the 90%theoretical or lab tested efficiency of a conventional wood stove, which would be considerably less in field testing.
Here are some videos about people with Rocket Mass Heaters. And if you are into this kind of stuff, feel free to come join us at the forum over at http://permies.com :D