exhaust is 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
the verbose details
First, here is my lame drawing showing how I think a rocket mass heater works:
Before attempting to explain anything, let me say that I attended a rocket mass heater workshop and made three videos that cover a lot of the explanation.
This first video is a general tour of 12 rocket mass heaters:
This second video is a day and a half workshop on building rocket mass heaters compressed into ten minutes:
Here is a close look at the exhaust from the rocket mass heater we built:
My first attempt at building a rocket mass heater without somebody to guide me. This is an attempt at a semi-portable rocket mass heater. Plus a nice burn demo.
The book is really great! And look! Right there on the cover you can see a really nice rocket mass heater!
In the discussion thread I said "Within the next eight months, I wish to see several of these in operation. And I wish to be involved in making at least one." And about two hours later came the response "There's a Pyromania workshop in January down at Cob Cottage Company where you learn to build these things."
So I went! It was far better than I expected! I think I covered most of the info in the videos. After posting the videos I was peppered with questions in the discussion thread. And one of the class instructors got involved and answered heaps of questions!
bonus rocket mass heater info ....
This picture shows an example of the innards of a rocket mass heater that does most of the work On friday night of the workshop it was very dark. We built a fire in the right side of this thing and it acted like any ole camp fire. Pop and crack and the the flames and smoke go straight up. Then one of the instructors took this insulated chimney thing and warmed the insides over the fire. And then placed it on the left side. Instantly, the fire stopped coming out of the top on the right side. Instead, we watched in amazement as the fire burned sideways. All of the flames and smoke were hell bent on going to the left. And in just a few seconds, it was as if there was a powerful wind go into the right hole and coming out of the top of the chimney thing. So then the instructor lifted the chimney thing and the fire went back to normal. Down again and then sideways firestorm thing happened again. Amazing!
So we moved the bricks into their final resting place and set up the bare bones skeleton of what will be our completed rocket mass heater. The metal insulated chimney thing that we were tinkering with before was just for demonstration purposes. In this pic you can see the ducts that will carry the exhaust through the bench mass before going outside.
This picture shows the rocket mass heater combustion chamber This is the workhorse of the contraption. The metal interior will get really hot thanks to the surrounding insulation. That super heat will force the air to move hard and fast! What we see in this picture will be covered by a barrel in the next picture.
Now we see the freshly completed rocket mass heater It will take a long time and a few fires to get it dry and hard.
Here is another rocket mass heater workshop. This one is building the rocket mass heater into a greenhouse
And this one shows the rocket mass heater barrel prep. Complete with an excellent demonstration of the rockety stuff - check out that smoke reburn!
More on Rocket Mass Heaters
Now, go take a look at the discussion thread in the permaculture forums where there is gobs more info, and people who know far more than me can answer your questions!
Since the first pass at this article, I've recorded a bit more rocket mass heater footage and pushed it up to youtube. There is one about heating water, another about ... well .... see all of my youtube videos here.
On friday night of the workshop it was very dark. We built a fire in the right side of this thing and it acted like any ole camp fire. Pop and crack and the the flames and smoke go straight up. Then one of the instructors took this insulated chimney thing and warmed the insides over the fire. And then placed it on the left side. Instantly, the fire stopped coming out of the top on the right side. Instead, we watched in amazement as the fire burned sideways. All of the flames and smoke were hell bent on going to the left. And in just a few seconds, it was as if there was a powerful wind go into the right hole and coming out of the top of the chimney thing. So then the instructor lifted the chimney thing and the fire went back to normal. Down again and then sideways firestorm thing happened again. Amazing!
The metal insulated chimney thing that we were tinkering with before was just for demonstration purposes. In this pic you can see the ducts that will carry the exhaust through the bench mass before going outside. 
This is the workhorse of the contraption. The metal interior will get really hot thanks to the surrounding insulation. That super heat will force the air to move hard and fast! What we see in this picture will be covered by a barrel in the next picture.
It will take a long time and a few fires to get it dry and hard. 
