I've wanted to heat my home with a rocket stove since I first read about them around 6 years ago. I started tinkering with them almost immediately, designing a tin can stove out of the cans you get when purchasing ingredients for chili. Though it was a success for what it was, you certainly couldn't heat a house with it. I moved on to a larger tin can unit, made out of metal 5 gallon pails and 4" dryer vent pipe. With that build I learned about the stack effect, and the problems that arise when you try to build a rocket stove in the basement of an older 2 story home. It also didn't put out enough heat for a whole home, but could have been used as a good room sized heater.
That led to the build featured here, which was my first serious build with clay and cob. Below is a picture of the finished build. I don't really like to start at the end, but the first picture in my post is the one that goes in the feed, and the first picture for this post isn't much of an attention grabber.
It all starts here, with my local mud. The spot of land I live on is about 70% clay by volume, 25% various sized bits of shale, and 5% organic matter in the top 6 inches. As I dig deeper, there's no organic matter, and a slightly higher volume of shale. The clay screens out easy and forms well, but has been very prone to cracking while drying and firing. For this build, I tried two things to help alleviate this, and to make the clay more insulative: perlite and straw.
My understanding was that the perlite would help make the clay significantly more insulative, so I used this for the bottom layer. I did the build on this osb sheet so it would be easier to move, but it quickly gained too much weight for the plywood and had to be set on a sturdier base. The perlite also helped to reduce the weight, but not by nearly enough.
For the rest of this build I used a fairly standard cob, which I made by adding pieces of my decorative grass leaves into the clay mix, cut to about 1/4" long. The left side of the guitar-shaped form below is the burn chamber, and the round part near the middle is where the riser will be. The rest is just base to support the 15 gallon drum that will cover the riser, and room to attach an exhaust pipe.
Here you can see the base of the heat riser. I should have used cob for the whole riser, but I had a chunk of 4" cast iron pipe laying around that was almost the perfect size, so I used that to save time on my test build.
The pieces above did not crack as bad while drying as some of my earlier clay builds, but the did still develop a few that needed closing up during the drying process. Because I did this in my off season and don't have a special unit for drying greenware, this took about 2 months to dry enough to be test fired. Once dry, the piece in the picture above weighed about 100 lbs.
Below you can see the stove on its new home atop our Aerocart. I put some old birch cupboard doors under the osb to help support the weight. The cast iron heat riser is installed, with the bell end down.
To seal around the bottom of the heat riser, I used more cob. It was just above freezing outdoors while I worked on this, which was less than ideal. Because it was so cold, I was firing the stove as I added this cob, which led to the cracking seen below. Despite these cracks, it did not leak any smoke, and it kept the riser from tipping over when the stove was moved.
All the pictures I took of the fire came out blurry like this, but you get the idea in this one. When the barrel is over the heat riser, it naturally becomes very hot as it absorbs all the heat coming up the riser. It also traps any unburned smoke or other gases above the riser, where they also get burned. Once the riser reaches temperature, the only exhaust that comes from under the barrel is steam.
I realized quite early on that the stove was not getting enough air to the fuel, so I put this little raised floor on the bottom of the burn chamber. This took care of my combustion problems, and it roared like a dragon after that. You can see in this picture that I have a crack on the right side of the burn chamber, and all the way around where the cast iron heat riser sits on the clay base. They look bad, but it held together for several hundred hours of burning, and was still in usable condition when it got disassembled in the spring.
The old school J-tube stove is great for testing, and useful for outdoor cooking, but for heat, I want something I can fill with wood and forget about. By building an upward facing mouth at the fuel end, you can feed the fuel in standing up, so that it self feeds as the bottom gets burned up.
The theory was okay, but with very dry fuel, which the rocket stove greatly prefers, sometimes the flames would creep up the feed mouth and consume the fuel outside of the burn chamber. Naturally, this was very undesirable.
It was useful information though, and I got a good deal of that problem sorted out, as you'll see in future posts. I learned enough from this build to move on to the house sized build, but it had a few problem as a room size heater. For one, my base layer was not nearly thick enough. Over the course of the winter, the heat that moved through the base burned a hole all the way through the osb and halfway through the birch. It probably would have been okay indoors if it had sat on a double layer of bricks.
The weight was also a bit prohibitive, finishing up at around 250lbs. Obviously not unusable, but quite a bit heavier than I'd hoped. When I revisit this, I'd like to get the total weight under 150lbs. I've been working with some other clay mixes that I think will make this possible.
The biggest problem with this build is that you have to feed it pretty much constantly. Even with the self-feeding mouth, it only burns small fuel, so it needs to be tended about every 20 minutes. This is a lot better than the straight J-tube, which had to have the fuel pushed forward every 5-10 minutes.
It definitely put out enough heat, being about equivalent to a 23,500 BTU kerosene heater, which is what I heat my home with currently. This means that I may also be able to scale it down a bit for use as a room heater, which would also help cut down on the weight. I won't get to much into my speculation here, because I could go on forever.
Well, I hope I taught a little, or at least inspired someone to try their own hand at one of these fascinating little devices.
If you have any questions at all, put them in the comments. This time of year, I should be able to deliver fairly prompt responses. I'm still new to posting into communities, so I hope I'm not cluttering things up anywhere. Feel free to flame me for breaking any rules of posting etiquette ;)