Hi everybody,
I just saw Paul's video, then I looked around this discussion. I loved seeing how the rumor spreads!
The first pictures on this discussion are kinda like a kiln, furnace, or cooking-type stove. I've seen similar designs for "rocket" cooking stoves on websites from India.
But if you sit on the "burn tunnel" of most stoves, your butt will get cooked! And you'd need a chimney more than twice as tall as the tunnel is long, to make the draft work. Those first "butt warmer" sketches are like drawing something from a dream... you see it ... yet it doesn't quite mesh together into a reality.
The rocket mass heater from the workshop (and the design drawing from the book) has an internal "heat riser" instead of a chimney. It gets extra hot because of the insulation, so it doesn't need to be as tall to create a strong draft and clean burn. Kinda like an upside-down siphon.
That's the "combustion unit," where the fire happens.
The "butt warmer" part happens afterward.
After burning, the "flue gases" go through a masonry mass. (The part under your butt is not actually on fire.) It's like an Eastern European masonry stove, but cheaper and cleaner-burning.
In traditional masonry stoves, they do a vertical column, send smoke around baffles inside it, and build shelves around the outside for sleeping, bread rising, cats, etc.
In rocket mass heaters, we burn the smoke completely, then use ducting to channel the hot gases almost any direction. Most popular are benches and beds, but walls or floors can also be heated, and I've heard of rocket-heated garden beds, smelting furnaces, and wood kilns.
For more pictures of the process, check the links on this page:
http://www.ernieanderica.info/rocketstoves
You can build a stove core like we did out of bricks in your (non-flammable) driveway or on a sandy part of your yard. Just like in Paul's pictures.
If you want to hear about really weird things people do with these stoves, or ask questions as you try it out, there's a Rocket Stoves corner here:
http://donkey32.proboards.com
Thanks for coming to the Pyromania workshop!
Regarding the isolation of thermal mass from walls:
Yes, it's a good idea.
In our house (a stick-frame rental unit on a concrete slab floor/foundation), we put the combustion unit about 2 feet from the wall, with a heat shield like you would on a woodstove. Then we put the flue gas ducts at least 5" from the wall, surrounded in 4" of cob and masonry. We put an inch of perlite between it and the wall. I'm pretty happy with the results. The bench definitely gets much hotter than the wall.
The one in Paul's video, Paul helped add about 6" of perlite insulation between the firebox and the wall. In that case, the wall is masonry, so it wouldn't be damaged, but it would still soak up a lot of heat and transfer it outdoors, wasting it.
But the clean combustion and thermal mass "heat storage" wastes so much less wood than even the best fireplace or woodstove.
Once I've got the basic design in hand, I sometimes put efficiency on the "back burner" and emphasize safety, aesthetics, and practicality. It's easy to get caught up in "maximizing" and forget to optimize.
For example, the ideal location for a chimney is the center of the house, so the heat is stored in the masonry and radiated evenly to all areas of the house. But our house was built in a cross shape through multiple additions, and there is no good place to put a stove. Where it could go, we have a kitchen that was just remodeled, and we're not quite ready to give up our electric range and refrigerator. Even if we were, it's a rental, and the next tenants might be understandably confused about the whole thing.
So we put it on one of the many exterior walls, where it can be a good couch and where we could vent it out the window during prototyping. It works fine, and still heats the house pretty well. (We've got it vented through the ceiling now like a "normal" stove, where it gets good draft no matter the wind.)
I hope you don't mind me signing up to jump in with just one long post.
Let us know how you get along.
Yours,
Erica Wisner
www.ErnieAndErica.info