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I attended the rocket mass heater workshop over the weekend and it was really warm.
We built a rocket mass heater in and for Mike Long's greenhouse.

The foundation for the stove was built out of gravel and concrete chunks with a layer of cob on top.

The basic fire box and the extra support ring were built out of fire bricks stuck together with cob, which is a mixture of 1 bucket of clay + 1 1/2 buckets of masonry sand + a little water if needed. The cob is ground together (mixed) by rolling the mixture on a tarp back and forth and then pounded and smooshed together by stomping and rubbing the mixture with your shoe. The bricks are dipped in a slip (mixture of clay and water) and then mortared together with the cob.

The firebox is rectangular with two openings on top. One is where you feed the wood in to burn it, and the other is the outlet port and it gets fitted with a length of insulated 8" stove pipe. The pipe is mortared to the opening with cob and stands straight up with the stove pipe hole up. The smoke and products of combustion come up out of the stove pipe. The insulated stove pipe was made using one piece of 8" pipe and one piece of 10" pipe. A mixture of Pearlite and slip is jammed between the two pieces of pipe to make the insulation layer.

A 55 gallon drum with one open end is then placed over the stove pipe resting on the extra support ring. This forces the smoke to go down and out the horizontal duct pipe that directs it out of the building. There is a 2" space between the top of the stove pipe and the inside of the closed end of the drum. On the trip outside the heat from the fire warms up the cob coated horizontal duct pipe, which acts as a heat radiator to warm the greenhouse. It would be a very nice warm place to sit on the warm cob bench.The 55 gallon drum is also a radiator and sometimes gets coated with cob to add more mass to help radiate heat. If you were to get the stove really cooking, the top of the drum can get very hot.

Once the duct pipe gets to the outside it turns up and becomes a standard chimney with a cap on top.

I had never worked with cob before and I found it to be a very pleasing experience. It kind of took me back to my childhood and playing in the mud. Straw is added to the cob for any structural applications such as building a home. When the cob dries it is hard like concrete but dusty if brushed with your hand. So for a sealing finish linseed oil is sometimes used. I'm sure there are other finishes also.

The workshop was a great experience where I met more people who are on the same page that I am on.

One of the highlights of the workshop for me was the dip that Mike Long’s house tenant made. It was “Baba Ghanoush”, a dish made with fire-roasted eggplant. It was to die for. I will have to make some soon.

Many thanks to David Zhang for setting up the workshop and to Rick Flug for posting pictures of the event. I may build one of these stoves someday, and for sure I will build a rocket stove for cooking during emergency times.
Thanks to all who attended for making the event such a great experience.

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Thanks, David for the report; sorry I missed the workshop. The stove sounds like a great idea for my greenhouse... I'll get right on it after...! Ro
Thanks David C. for the excellent summary, and to Rick for uploading great photos.

The photo album can be seen here:
Great write up David! There are a couple books out there too, specifically on building the Rocket Stove.

Did you all get the cob work done? . . . My body said 'Stay Home' on that weekend :-(
The rocket stove I helped build this spring with David Z. friend out in the Laurel? area did not get finished during the one work day. I really wish I could get back and see the finished stove & experienced the warmth.

I hope Mike invites folks over to help with greenhouse gardening & transplanting in the future and to see the stove after its finally coating & hardening!
We didn't get all of the cob installed during that day. It needed a lot more to make the cob bench and it is very labor intensive and time consuming to make.

I keep thinking about making a stove like that to heat water for my radiant floor system. If the grid goes down I will need a way to make hot water for the house floor. Maybe a coil of copper tubing wrapped around the stove pipe (heat riser) and inside of the 55 gallon drum. Now I just need a 12V pump to circulate the water from the stove into the holding tank.

Henry Stanley said that some people completely covered the 55 gallon drum with cob also. The more cob you install the more mass you have to radiate the heat that gets built up in the cob. Then it's the thickness of the cob that determines how long it will radiate the heat. For each inch of thickness the heat radiates for about an hour. So if it is 6" thick it will radiate for about 6 hours. He said that you would build a fire in the stove once per day in the late afternoon or evening and feed it for about 2 to 3 hours and then let it radiate overnight to keep the greenhouse warm.

Here is another idea. How about building the stove like I described with the copper tubing inside the drum and on top of the drum add a short section of another drum and cover it all with cob and use the short section of drum as an oven. I think I am getting hooked on COB!

What a great idea this mass heater is. I will have to get the book "Rocket Mass Heaters" by Ianto Evans. We were told that he is the man who brought this technology to the US.

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