Brian (a.k.a Ziggy) recently reported on his blog that his rocket stove is not working. This is a handmade masonry heater he built by hand from cob, firebrick, and steel. It’s a very clever design that has the flu flowing through the cob bed platform. The problem is that it doesn’t draw unless there is a bit of a wind outside. His cob house is located at the Dancing Rabbit Ecovillage.
I took a careful look at all of Brian’s photos and pulled all my old kiln building notes & memories out from my past as a potter and came up with a modification to his design. I’m not sure he’s game to give it a try, but I suspect it would work better with this modification.
Basically I’m simply suggesting that he digs into the cob a little and connects the stove to the rear exit pipe leaving the other two pipes (white) burried in the cob and disconnected from the system. This would make it much easier for the air to flow because there are fewer turns and less horizontal pipe. My other suggestion is to experiment with a taller chimney.
By shortening the horizontal run of pipe, reducing the number of turns, and increasing height of the stack the ability of this stove to draw air should be greatly improved. The secondary firebox below the vertical stack is optional and would only be used to get the stove going when it’s cold. The only trick with this secondary firebox is that it should be sealed up tight once the main stove is running inside because any air leak in the system would reduce the draw. It’s like sucking on a straw with a hole in it.
In a handmade stove like this, like a handmade kiln, it’s a bit tough to calculate the correct size of everything but if you follow a few general rules of thumb you’ll have more luck.
- Air inlet should be the same size as the air exit.
- Air inlet should bring in air from outside the building. If you draw air from inside you suck hot air from the room.
- The chimney height should theoretically be 3-times the horizontal run but build it to be adjustable so you can tune your custom setup to the optimal proportions.
- Higher elevations require more chimney height due to the thin air and lower pressures.
- Air flow should be unrestricted, so the fewer bends in the flu the better.
- Dampers should be included in the design to help regulate the burn rate.
Rocket stoves have the added challenge of getting the fire up the interior brick chamber and back down inside the metal drum. While this helps transfer heat into the room it’s more kiln-like in that it requires the fire to move through a tall firebox. This will also theoretically require the addition of 3-times the height of the firebox to the chimney. This simply means that this kind of stove needs to have a stronger draw than most stoves, so be prepared to add more pipe to the chimney.
Below are some photos from Brian’s house and rocket stove construction. Brian is considering switching to a small Jøtul stove which is also an excellent idea. I’d love to see his rocket stove working but I won’t knock him at all for pulling it out and replacing it with a high-efficiency stove.
Handmade stoves and masonry heaters, like kilns, are a tricky projects. Professionally manufactured EPA certified stoves are extremely efficient, easy to maintain, and offer low emissions. Before trying a project like this I’d suggest doing a lot of research and even experiment with small outdoor setups like heated benches. The math and physics aren’t that hard to figure out, but when the stoves don’t work and it’s cold outside it sucks when it doesn’t work.
Be sure to visit The Year of Mud: Building a cob house as Brian’s adventure continues.
You can see all Brian’s rocket stove photos and his building a cob house photos on Flickr.
The “rocket” actually generates considerable updraft, if the internal chimney isn’t close enough to the top surface of the drum too much heat will stay in the exhaust and it will not draft internally as well. An exhaust blower could be used to fix the problem too but it would have a small electrical cost.