Our Second Rocket Stove

It is almost the end of December and winter is well upon us. It arrived much earlier then last year (we are now experiencing snow and temperatures that arrived in late January last year). I am relieved that we got the second rocket stove done in time … it means this year we can enjoy life not just in the bedroom but also in the living room. We were able to find a barrel which means it looks (and works) more like a standard rocket stove. Circumstances still did not invite building a proper bench for thermal mass. So we opted to build another “bell” like chamber to retain more of the heat inside the space. We also experimented by building a small metal oven into the bell.

Despite a successful rocket construction during our first build, I was hesitant about this second build. While rocket stoves are a relatively simple, do-it-yourself technology, they do require a certain level of precision and accuracy in design. I am not really worried about efficiency (it’s so much better then standard stove technologies – that you really can’t go wrong with it. I am worried about smoke and poisonous gases leaking into the room. Two things can prevent that (1) a proper design (proportions of different elements) and (2) good finishing. I have proper design pretty much under control. Finishing was, and continues to be somewhat of a mystery. Our experiences with cob and earth finishes have been … well … mysterious. We are still not confident about it.

I was able to delay the project itself by two preparations that needed to be made. The first was to build some kind of small platform on wheels that would enable me to get the existing metal stove out of the way. Though this picture comes later in the time-line … this is the platform in action. Due to two wheels with brakes I was able to get the metal stove onto it and out of the way on my own (though the story of getting it out of the house took a funny turn):

The second preparation was to prepare the barrel. First I had to cut it open and then I had to burn the paint off it (so that no poisonous paint fumes would be emitted from it as it got hot on the rocket stove):

Because of my hesitance I started the project slowly, giving myself time to get back into the “rocket vibe” and to explore what I wanted to build. It began with a rough model that was constructed in the garage. I completely took apart and rebuilt the model a few times over many weeks. I spent quite a bit of time staring it, letting questions appear, letting solutions appear, moving parts around … I took my time with it … until I had a reasonable model … and restored confidence to start actual construction. I lit the model once to check for good draft … but given its design (round barrel meets square bell) it was kind of pointless since it was difficult to temporarily seal.

When I took the model apart I took a few images to document the different layers. I used those images to recollect and reconstruct dimensions during the actual construction. The construction began with a “subfloor” upon which I could build the floor of the rocket itself. The subfloor is built with mostly used adobe bricks. There are two ash-pits (the one in front and on the right is just under the feed chamber and the one on the left is under the future chimney exit) which are built with firebrick.

The floor itself is made of half-thickness firebricks.

Then came the first layer of the core. Though at the end of the day I decided that this would create a burn tunnel that was too deep so the next day I ended up taking apart most of what I built the previous day and removing this layer.

So this second layer was actually the first layer of the burn tunnel (though the picture still has the above pictured layer before I took it out)::

… and then on with the core including a (this time) brick riser (the oven is just set in place to measure precise location optimized for brick sizes), not yet built in):

… then a test fitting of the insulation container – rounded sheet metal tied in place with thick wires:

… and then a test fitting of the barrel itself:

with the core complete it was time to start building the heat-storage bell that contains the oven:

I then realized that it would be easier to continue building up the bell with the barrel in place (so that the quirky round-square meeting could be properly built). But to do that I had to first put in the insulation. The insulation is a mix of perlite and clay slip. It went all around the heat riser and almost all the way around the burn tunnel (no insulation was put in on the bell side of the burn tunnel).

All insulation openings were then sealed with a thick clay (cob-ish) mortar to keep the light and airy perlite from flying around.

Then it was time to complete the bell walls.

… and a concrete-slab we had lying around (of which there are more) was placed on as a cover (it was already fitted in place in the model) providing a lot of thermal mass (it was very heavy – a job for two) and an easy solution for bridging the wide opening of the bell:

The last part that was built was the ash collection pit/chimney exit chamber (on the left):

A few more cut firebricks were used to close the gaps between the barrel and the bell … including the installation of another clean-out opening that gives access directly to the passage-way between the two. Then all that was left to do was to seal all the opening with cob:

and install the chimney:

… and we fired it up and it worked like a charm. The immediate heating effect is new to us (in the first rocket we built where we didn’t have a barrel to radiate heat it takes time to heat up on the inside before that heat is radiated into the space. With this one the barrel gets hot within minutes (with still just the initial kindling wood burning) and quickly becomes too hot to touch. The room it was in was very cold since we had not heated it at all this season. We had a little smoke during the first firing (natural since the entire stove core is cold and damp) so a window was open … and the door to the entry hall was open and the hall itself was open to the outside … and still there was a very fast and noticeable heat throughout the entire space.

I never get tired of watching a hissing fire fire get sucked into the burn tunnel:

 

Then came the finishing stage. Despite numerous soil composition tests we seem to have ended up with cox mix that was clay rich. We were starting to run out of time (=running into extra cold) and drying the cob takes a good firing up of the rocket over two or three days … so I decided to risk it and applied the cob to the entire stove. Being clay rich meant that it contracted a lot … leaving a lot of cracks … which we could have dealt with … but is also pulled away from the body of the stove itself … and fell of in large chunks.

This is Ricky (in one of her winter outfits) making good use of the straw-bale we used to create the cob mix:

So we ended up pulling it all off and creating an alternate mix … a formula we learned of when we re-finished the north wall of the house. The base was a different clay earth … very sandy (10-15% clay and the rest a fine silt). At first we added to it gypsum as a binder (instead of aiming for a more precise clay-betonite mix). The resulting mix dried way too fast, so we added to it some hydrated lime to slow the drying. We ended working with a formula of 1 part gypsum, 1 part lime and 4-5 parts sandy clay. It gives a hard finish that had much better adhesion and seems to be heat-resistant. It did crack a bit, but that did not compromise adhesion. We will probably try to add another finish coat and maybe some color to it in the spring (all the soil is frozen now).

Initially we had to keep the rocket going for longer periods to really drive the freezing cold out of the room and the walls. The more regularly we use it the less we need to keep it going to enjoy a warm room. When the room is already warm it takes one feeding of the rocket to drastically boost the temperature in the room. It is crazy efficient.

It works amazingly well (to my surprise) as a cooking surface. The only limitation is that you can only cook on it when there is wood burning inside (and for a short time after the fire dies … while the barrel is still hot enough) which, because it’s really efficient, is not a lot of time. So to use it we need to consciously plan to do our cooking while we light and feed it.

The oven in the bell does not work. The rocket is so efficient in heating up the space that it simply does not run long enough to heat up the bell enough to get the stove warm enough to be useful. That’s the nature of this super-efficient stove!

There is more to be said about its performance, but that will come in a later post and after we’ve had some experience living with it. So far we are very happy 🙂

 

 

Rocket Stove Water Heater

I’ve been looking at lots of applications for Rocket Stoves. It is a beautiful and simple DIY technology I want to use as much as possible in our new house. This includes heating water. We currently have a simple (purchased) wood-based boiler that does an OK job and I suspect even has an inherent “rocket” effect … and mostly prooves that it can be done. So I’ve been looking around for hot water solutions based on Rocket Stoves.

Before I go into the details of the one I found I would like to point out one piece of advice about Rocket Stoves that I came across, stuck with me and is exemplified by it. Rockets work best when they are designed with one primary purpose in mind. This means that a rocket designated for hot water will probably be more feasible and work better then a rocket that is used to heat a space AND heat water. I can testify that it can be very tempting to build a supreme-do-it-all rocket … but it just doesn’ work.

With that in mind I came across the following Rocket-based hot water system. The project is documented with a set of images and a blog post.

Rocket Stove Hot Water Schematic

It is a rocket-stove dedicated to heating water and does nothing else. Because it is a single task rocket it is actually simpler then the basic rocket-stove since there is no heat exchange barrel and the water tank itself is the thermal mass. The key element is a heat-exchanger that sits on top of the heat-riser. It is a metal box within a box – where the heat from the rocket is transferred into the water. Depending on the position of the water tank flow is either achieved either passively via thermo-siphoning or with a pump.

The heart of this solution is the heat exchanger and at the heart of the heat-exchanger are small plates of metal welded into it, which increase the contact surface between the hot exhaust and the metal itself. With these plates a 1 meter tall heat exchanger can be designed to have 6+ meters of contact surface (as if the exchanger itself was 6 meters tall!). The trick is to size the internals in such a way that the surface area of air-flow will not become smaller then that of the heat-riser so that the exhaust can flow smoothly out. If designed optimally then, as with most good Rocket Stoves, there should be very little exhaust heat left in the chimney pipe.

We don’t have (yet) the skills to make this kind of heat exchanger but I am confident we can find a metal-worker who can create one for us. My thoughts are to connect a 6 inch rocket to a 300 liter tank of water and that should provide a simple and efficient and backup for days where the sun does not provide enough heat for the solar hot water panels to kick in.