Rocket Mass Stove

[Renaissance]

Rocket mass stoves are apparently an extremely efficient way of heating a space, which could be useful for cold winters ahead! (or cold springs, summers and falls for that matter) They also seem like they can be built fairly cheaply and with just a little know how. They use a J shaped combustion chamber that utilizes an intense sideways burn that gets so hot that it has very little ash and exhausts moisture rather than smoke after it gets going. The smoke and heat from the burn chamber smoke gets pulled up into a secondary barrel that will hold much of the heat while the fire is going. I watched some videos of a guy who made one, and he said the outer barrel can get as hot as 700 degrees Fahrenheit / 370 Celsius but the normal temperature is around 400 degrees Fahrenheit / 204 Celsius. The exhaust can be made so that it passes through ducts covered by cob so that it acts as a thermal mass which will radiate heat long after the fire goes out. The thermal mass is often made out of cob (sand, clay and straw) which is apparently a great material for thermal storage.

He's a pic of a completed stove with the thermal mass:

One of the main informational sources online seems to be at the Permies forum:

http://www.permies.com/forums/f-125/rocket-stoves

The people running that forum also have some videos on youtube. The couple lives in the Okanagan Highlands in British Columbia, Canada. So it gets cold there! They don't talk much about the mechanism of the rocket stove in the below video, but they do talk about it's efficiency. They said they go through about cord of wood in about four months, which is pretty impressive.

http://www.youtube.com/watch?v=4usXIAoy9us

Here's another video that shows the steps a guy took in building one of these (he doesn't do the thermal mass though). It's the first video where he gets to it's construction and is part of a series:

http://www.youtube.com/watch?v=-rcJ_lp-bYI

 

[clerck de bonk]

Thanks for the info Shane!

 

[JonnyRadar]

Very cool, thanks for sharing Shane :D Gonna have to try to make a small prototype...

 

[theos]

Rocket mass stoves are apparently an extremely efficient way of heating a space, which could be useful for cold winters ahead! (or cold springs, summers and falls for that matter)

Absolutely. This could be a good group project.

This _http://www.youtube.com/watch?v=P6ValmUnjz4 rocket stove can be made very cheaply and easily. It can come in handy to cook on in case of a power outage. I made one last summer and it worked fairly well though I only used it a few times to test it.

 

[monotonic]

We made a rocket grill by stacking 8 cinder blocks and hoisting them on a few bricks to provide intake at the bottom, and just placing a grill on top. It's very temperamental and will shoot rocket flame out the top. It sounds like a dragon breathing fire. It's not efficient at all and requires constant vigilance to keep the fire going. A very sub-optimal setup. But does grill VERY fast when you do. I've always wondered about ways to improve the design, IE gravity-feed mechanism, and increased efficiency. It looks like we could make something a lot like this using cinder blocks. Of course, cinder blocks are convenient but shouldn't be used for a lasting option. They eventually crack, warp, and fall apart. Still, they may work great for rapid-fire experimentation. I expect for a grill in our case, sliding some oven pipe down the exhaust would help channel the heat upwards into the meat rather than being absorbed by the cinder blocks. Having the exhaust cook the meat directly does seem to impart some nice smokey flavors, except that the meat dries out while cooking due to the constant searing air current.

 

[monotonic]

Inspired by this post, I took apart the cinder block grill and reworked it into a gravity-feed setup. It had become obsolete anyways due to a nifty grill we got recently, so it was about time to try some improvements.

I drew the below image to illustrate what I'm talking about. These are cinder blocks. The tall stack is the convection chamber, the covered stack is the gravity-feed chamber. All of this was gotten from the local Lowes store. The red bricks at the bottom aren't conventional red bricks, they are about as thick as the walls of a cinder block.

Cinder blocks aren't precise so if you just stack them they will rock. Therefore I put sand between them, and move the block back and forth over the one below it so that the sand distributes between them and stops the rocking. Clay or just mud may work just as well. There are TWO large slabs on the bottom. Before I built it I ripped up the plants and leveled the area of dirt I had decided to put it on. The slabs went down on top of that. It is important to use TWO slabs because the fire directly contacts the upper slab, and it WILL crack. If you don't put a second slab below it, the cracked slab will bow into the dirt and the entire thing will rock and become unstable.

The cinder blocks also crack and warp somewhat, but I've found usually two sides crack and then there's no more cracking. This was intended to be as cheap as possible, and that is okay to start with I guess. Take note that large cracks can undermine the convection, so don't put cracked blocks on the bottom if you rebuild it.

It's neat to look into the outlet holes while it's burning because you can see the flame shooting into the convection chamber; sometimes it makes little tornados.

This setup does seem like it will be less tedious to work with. Still, we have to poke a stick into the inlet occasionally to break up the embers so the sticks will fall down. You could easily use a higher stack for the fuel chamber so the sticks would be heavier.

One interesting idea I had is that the fuel chamber would work perfectly as a smoking chamber, because smoke collects in it. I am not sure how to construct the chamber to allow this though.

To start it going, you need to drop a bunch of leaves into the convection chamber. Then drop some leaves into the gravity feed chambers, and then drop your bundle of sticks on top of them. Light matches and drop them into the convection chambers. This establishes a convection current, and should sound like a roar. Now drop matches into the fuel chambers to light the leaves there. Then put the top on the fuel chamber.

You can grill meat over the outlets if you have a grill, or use skewers. We haven't tested it this way yet however.

 

[monotonic]

Well, we tested it out yesterday and it worked much better! Heat output was more consistent with much less prodding and trifles. One problem is that the embers can gather at the bottom and restrict air flow, so I'm not sure how to deal with that yet. However after you finish grilling and seal the thing off, whatever's inside continues to burn down to ash, even if you left some big chunks of wood in. That was a big surprise for us.

The meat we cooked stayed moist on the inside, despite being a bit charred on the outside, and tasted great. So it's an improvement over the previous arrangement and even over our fold-up grill.

What would be great is if there was a way to adjust the amount of heat so that charring could be eliminated and it could replace an indoor stovetop. I believe the red bricks on the side opposite to the air intake could be slid out to provide adjustment to the convection flow.

If we start a community project why don't we set some goals:

1: The stove should be able to replace an indoor stovetop and maybe even oven.
2: It should be as efficient as possible.
3: it should need little maintenance and last indefinitely.

If the furnace could replace an indoor stove, there would be no reason not to get your sunlight every day!