A couple of months ago a fella described for me a cobrocket stove. You put sticks into a small wood burning hole and the draw of the chimney keeps the smoke from rising into the room. And the smoke would ride under the bench warming butts. And then smoke goes out the chimney.
A little bit of wood. A whole lot of butt warming. A suppose that the room gets warmed a lot too along the way. The important thing is that the air that comes out of the chimney isn't very hot. So most of the heat ended up in the room.
So I tried to draw a picture of what this fella described for me.
And I can see how the uphill slant is gonna help with getting the smoke to go that way.
Did you make one of these?
Joined: Jun 26, 2008
why not warm your butt, smoke some salmon and dry your gloves at the same time!? I love goofing around figuring out and drawing stuff like this its jsut my sort of thing. I would think you would need a grate to build your fire on too to help it breathe. forgot to add that. wait I see I loaded the wrong one! the triangular spot above the smoke channel is supposed to be be open on the front for gloves to be stuffed to warm or dry........ or shoes or what not.....
I like it Leah. You're gonna be rich. I think the interior slope is brilliant for smoke movement, but it also makes the butt warming seats different temperatures. Some like it hot...
Joined: Jun 26, 2008
rich. yes, I could be rich .
Paul - are you going to really try and make something like this? what about a circular design with the pipe out the middle and seating around the fire box? like those benches around trees?
Joined: Sep 30, 2008
Location: Western WA
If I had one of those, I know where my cats would be found all winter. As it is, I have to peel them away from the area in front of the wood stove before I can get close enough to add more wood.
Weather Underground says it could go as low as 6F tonight! YIKES! I'll have to drink a lot of water so I can get up frequently enough to feed the stove.
Joined: Jun 26, 2008
that is an excellent strategy sue . I wish I had a wood stove. then maybe my kitties wouldn't be glued to my head all night. I guess they say that 90% of your heat escapes through your head and the cats know it. miss chi chi regularly gets thrown (quite literally) off the bed because she will bit and claw me when I shift around to a position she dislikes. my other kitty moses, is old and he has really stinky breath. I need to turn a heating pad on somwhere or something. love ya kitties but GET AWAY FROM ME! if i lock them out of the room they reach their arms under the door and shake it producing a thunk thunk thunk every 3 seconds through the night. grrrr.
Part of the design is missing from your illustration. The book "Rocket Mass Heaters" is indeed the one you want to read. I'll try to describe the design in brief:
Part one - the heater:
This is basically a 'J' shaped tunnel made out of firebrick. You gravity feed wood into the lower part of the 'J' (the Feed Chamber", the wood burns at the bottom and the gases combust as they travel through the bottom of the 'J' (the "Burn Tunnel", and then those hot gases rise through the tall part of the 'J' (the "heat Riser". You insulate the Heat Riser, and the heat buildup in the rise produces a chimney effect that draws air through the feed chamber (and also prevents smoke from entering the room)
So where does the gas go from the heat riser? What they do is place a 55 gallon steel drum over the heat riser. The hot gases spill over the top of the heat riser and are forced down the inside of the drum. You then route the gases through stovepipe that runs though whatever you want to use as your thermal battery - a cob bench, a cob bed platform, an earthen floor, etc... and eventually that stovepipe exits the building. Note that you also get some radiant heat from the steel barrel while you have a fire going, in addition to all the heat you store in your battery.
There's a Pyromania workshop in January down at Cob Cottage Company where you learn to build these things. I might be interested in going if we could get a car full of folks that want to go.
There was a Pyromania workshop in November, IIRC it was about $240. That's before any discounts. At this point you can get 10% for paying 30 days in advance and another 10% for going down with a friend. That would bring it down to below $200.
Here's the contact info:
Best times to call are between 8 am and 1 pm PST. Enroll early, the courses fill up! (541)396-1825
You'll probably talk to a guy named Max if you call.
Joined: Jun 26, 2008
holycow! workshops that cost that much actually get participants up there? I don't think there is any way something like that would get off the ground around here. Maybe they are out there and I just haven't found them, I'm not one to be a member of groups and such. sorry off subject.
my thinking cap is still on about this.........thanks for the further explanation todd man some more indepth research is in order,I have never seen one of these rocket stoves before.
Pictures of one like you describe at http://ilovecob.com/archive/solunit-rocket Uses standard rocket stove (which feed from side) embedded in cob Dancing Rabbit's second and shows it. Looks doable. Getting the channeling of the smoke is interesting and worth learning the physics. I helped with one which instead of one smoke shaft used 5. Great if you can get the draw but tricky. There was a guy building them with such elaborate channels it actually came out cool and clean (a Serbian design)
I really like the part where they get the air draw from outside. That has always been a concern to me: you get the air all warm and then you consume it in the fire.
Most dryer and fan stuff has an interesting contraption that will move tiny little flaps when there is air movement - and those flaps will close when the air is not moving. Something like that could be worthwhile to install.
The lid over the burn space seems smart too. Some sort of BBQ lid thingy?
Joined: Nov 09, 2008
Location: Missoula, MT
Paul will have a cool response, I'm sure...until then, I'll offer a quick low-down.
It is amazing how efficient these are and how anyone can build them. The people at Cob Cottage were warm and welcoming and quite interesting.
From the EPA's website, normal wood stoves are 40-60% efficient, and the newer certified stoves are 60-80% efficient. Well, in case this wasn't clear before, when rocket stoves work correctly, they are over 90% efficient.
The stove we built put out mostly steam and CO2. When you stood next to the exhaust outside the building, it was more like a dryer vent that smelled slightly like wood smoke. Phenomenal.
The very first demonstration was a mock-up of the base built with fire bricks. A small fire was built in one half of the base and a chimney was held over the second half. The chimney creates this siphon effect that pulls all the fire, smoke, heat and steam over to the side and up and out the chimney. It might not sound that amazing, but you don't often see all the flames and smoke from a fire pulled completely sideways--totally horizontal.
This mock up was to demonstrate just part of what the heat riser does. Besides the siphon effect, the heat riser also re-burns the smoke to create incredibly efficient heating with low emissions. There's a bit of geometry in making sure your heat riser is at the correct height inside the barrel to ensure the smoke moves with enough turbulence in the right places (see the book for this). I gotta say that even among the Cob Cottage folks there was some disagreement on what this should be. In the end, the one we built worked efficiently immediately - even before the insulation and cob were dry. The beauty is that even if you goofed a bit, you can test it, readjust by moving some cob or bricks, and you're only about an hour off track.
HUGE value in the hands-on learning about the correct cob and insulation textures and application. HUGE value in sitting on a cob bench heated by a rocket stove - and feeling that heat still there almost a full day after a fire was last burned. Huge value, too, in hearing the different opinions and seeing all the different approaches to rocket mass heaters, pocket rockets, Rumford fireplaces/stoves and even a cob oven.
Will that hold a bit?
Oh, and I should say the cob cottages were picturesque - Paul has boatloads of pictures. And that despite yesterday where I could hardly move, my muscles have just about recovered from all the hauling and mixing of sand, clay and cob. (I am SO out of shape! Less accounting work and more gardening please!)
I took far more video than pics, but pics are the easiest to upload ...
So here we are (first pic) roughing out the design. We actually did this friday night. You can really see the flames at night. You put the fire in the right side, but because the left side has metal innards and lots of insulation, it gets a powerful draw going and the fire roars!
On saturday morning we took it apart and moved it to its new home. Only the insulated chimney thing was for instructional purposes only - and too big for our purposes, so it got left behind.
The second pic shows the bricks restacked in the new home. The idea was that the stack of bricks would be the combustion chamber (like the insulated chimney thing). A barrel would later be placed over the bricks and it would be rigged so that the smoke would leave the barrel and travel to the tubes to the left - which would heat a cob bench. The the smoke would travel up the vertical tube. And there would be a cleanout spot to the far left.
The desired barrel would not fit over the brick stack, so the stack of bricks got replaced with two tubes with clay and perlite in between. Also, the cleanout was moved to be closer to the combustion chamber.
The last picture is the finished product.
Joined: Sep 30, 2008
Location: Western WA
Are there special criteria for installing one of these in a house? How about in a mobile home?
Joined: Nov 09, 2008
Location: Missoula, MT
Two of the instructors showed pictures of a rocket stove and cob bench they installed in the home they are renting. It's either a frame, manufactured or mobile home - can't recall - and the landlord was all for it. They're documenting how to use it for the landlord and subsequent renters. They mentioned something about making sure the cob dries thoroughly for our damp NW conditions, but I don't think they really did anything much differently.
I haven't read the whole book - it might have more detail on building one in a house or mobile home.
The question reminds me: I once moved into a house where there had been a homemade insert in the fireplace. The owners removed it before selling because they didn't want issues with the sale, or, I'm thinking, a lawsuit down the road.
These rocket stoves can be built so cheaply, that even if you remove it to sell, it's more that you're out the labor than any significant cost.
I'm going to look at this further Paul, it's an interesting concept, and the thought of no smoke or very liitle is intriguing. We are going to have a bunch of spaces that need heating on and off, and such a thing could be real handy and it's easy to build yourself.
Thanks for the info, and in fact, I'm going to put it on our video site on the hobby page.
Laughter is the best medicine. http://www.lawntimes.com
Joined: Feb 10, 2009
Location: Okanogan Highlands, Washington
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.
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.
Hey everybody! This is Erica from the video - one of the rocket stove instructors!
Moments ago I got online and saw an email from Erica saying a lot of what she said above, so I was going to paste it in here. But I guess now I don't need to!
Well .... wait ... there were some other tidbits in the email:
Let's see ... some good links that we tried to point people toward: www.rocketstoves.com - download the book for cheaper than you can buy it. picasaweb.google.com/eritter - pictures from the 2007 Fox Circle workshop, along with most of our private life http://picasaweb.google.com/eawisner - rocket stove pix from our current residence (the Dana annex) and from the early experimentation days.