Category: Rocket Stoves

It was on my mind for many months … rebuilding the first rocket stove. It worked good – though not great. There were some design errors and compromises and during the last winter there were more smokeback events that I could not explain. I wasn’t keen on taking apart something that (kinda) worked and embarking on another build project. So I played around with it in my mind for a long time. Eventually I had a design I felt comfortable pursuing, I did a simple cob test to confirm the materials I had … counted and purchased bricks … and decided to go for it.

At the last minute I decided to give it a chance to become a workshop build and so I published an invitation and sent out word to people I knew and thought may know other people who would be interested … this was 3 or 4 days before the planned weekend build … and once person did sign up … making him the first participant in the first workshop I have ever offered at Bhudeva. I had two pairs of helping hands – Annelieke and Horatiu.

The project was born when I did created this layout:

I was able to take real measurements, finalize brick counts … and get confident enough about my vision to move forward. The first thing we had to do was to take apart the existing stove … which was magical … the knowledge that most of the materials can be reused … that the rest are non-toxic and can simply be tossed out anywhere on the land where they will be reassimilated by nature … its one thing to know this and another to experience it:

I was surprised to find the metal heat riser mostly in tact … though it was dry and chipping. Most of the clay-perlite insulation was used in thew new build …  which … began by recreating the layout in place to find the exact position it would be in relation to the existing chimney.

With the position fixed we were able to get to work on building a raised floor:

And then, layer by layer, building up the core of the stove:

… and when we brought in the barrel for a first fitting it started to feel like it just might become a real life rocket stove:

In the following image you can see the experimental part of this build. I discovered these honeycomb bricks and decided to use them to easily create heat channels and storage mass. There are two air passages (barely visible in the image) that allow the hot gasses to flow from the barrel into the two-brick chamber on the left hand side of the image – where they flow up. Then (as can be seen in later images) there is a top chamber that allows the gasses to flow across and down the two-brick chamber at the top of the image (right up against the wall) – where they flow down and then out through the chimney. There were three experiment going on: 1) using honeycomb bricks; 2) introducing a vertical flow both with both bottom-up and then top-down flows; 3) and gaining improved heat storage by having mass outside (the shell of the bricks) and inside (the honeycomb pattern).

This is as far as we got in the two days of work we had available. Horatiu and I agreed that he would come back for another day of work during which we will complete the build and fire it up for the first time. So during the next few days Annelieke and I continued doing some preparatory tasks. The most prominent task was the heat riser. Annelieke started doing a perfect and wrong job. Can you guess what is wrong in this image:

Annelieke is doing fantastic work getting the bricks aligned and leveled … but she is laying them without overlaps … creating a beautifully symmetric and unstable structure. This is something I take so much for granted that I did not spot until a few more layers were built and it became very prominent. So It had to be taken down and rebuilt properly:

While she did that I built some insulation chambers around the core (to extend the insulation that would be placed around the heat riser) and started filling them with the clay-perlite mix from the old core … and as you can see in the bottom-left corner I started playing around with cob … hoping for a better experience (I’ve had very poor experiences in the past):

On the day Horatiu came back we finished building up the honeycomb brick chambers and the top chamber in which gasses could pass from the up-flow chamber to the down-flow chamber:

The top chamber was closed with bricks and we then added on the sheet-metal container for the insulation:

… filled it up with clay-perlite insulation:

… and sealed it:

… and suddenly that was it … everything was ready for a barrel:

… and lighting a fire … the smoothest lighting of a new rocket stove I’ve ever experienced … excellent draft (probably helped by the fact that the core had a few good days to dry):

I was then left on my own to slowly transport cob-worthy material, to mix it up in reasonable one-person batches … and slowly build it and transform the stove from something very mechanical and engineered to something organic and mysterious:

There were a few places where seeds apparently got into the cob mix … and given that there was a lot of moisture inside this happened, in a few places:

It now, though still slowly drying, looks like this:

The second lighting of the stove, in contrast to the first, went very poorly. I am guessing it had to do with the loads of cool moist cob. This is where the experimental part may have also kicked in … the gasses may have had a hard time establishing a complete and continuous flow throughout the stove, resulting in serious backsmoke. During the third lighting I was careful to preheat at both cleanouts, to start very gradually and only when the stove was flowing well to put in a full load … and … to my great relief … it ran perfectly again. The fourth lighting was not so good … I wasn’t as patient. Since then I’ve lit it a few more times and it has been going fine.

I estimate that, aside from the bricks, I put on over half a ton (maybe up to three quarters) of cob. Thats a lot of moisture. At the end of the first lighting (before cob went on) when the full load finished burning the bricks at the back were noticeable warm. Not so during the next few lightings. There are many liters of water in there that need to dry. This is something that should be taken into consideration in a construction schedule. I started the construction early so there would be time to experiment and make corrections. I did not take into considerations how long this would take to dry … it still is drying.

Cob was much more friendly this time … finally. I played around with different finishing techniques … I still am. It is a subtle thing finishing and there seem to be numerous paths to go about it. It is very pleasant work (when it works) to be able to mold shapes, smooth corners, add colors. It felt like a complementary and balancing process to the more structured, measured, aligned process of building the core. It felt free, open, secure,  … embracing and welcoming. It is a pleasant way to finish a build and a much more pleasant result 🙂

The stove has already worked for a few cold nights. It’s still hard to say how good it works because: its not that cold yet, there is still humidity in the mass, the barrel itself is partly wrapped in cob … so a few things still shifting and changing. I am looking forward to experiencing how it works … both the immediate heating and the heat storage for the night. I have a feeling that it is going to be more efficient in terms of wood consumption (then its predecessor) … I am curious how it will compare in terms of heat storage (the previous stove was all storage, slow to heat up but then radiated plenty of warmth throughout the night – sometimes even overheating the room).

When I started playing with rocket stoves the main resource available was the Rocket Stove book. Though I still consider it a must read for anyone heading into Rocket Stove territory, I do not consider it sufficient. It left me with many questions, it has some outdated information and some things that, knowing what I know now, are borderline wrong. Fortunately there is now an additional resource that complements it wonderfully.

Earlier this year we participated in two kickstarter documentation projects on Rocket Stoves. I’ll speak shortly about one and at more length about the other. The shortly: the Paul Wheaton 4 DVD pack is a nice to have thing. It was a huge success on kickstarter which made its production value all the more disappointing. It includes 4 DVD’s none of which felt complete and comprehensive (the Fire Science came close). It has low quality video and audio. It was nice to have a glimpse into a workshop with Erica and Ernie which I do not have access to … but it was an opportunistic production. With the funds it raised I felt there was an opportunity to create something much better … an opportunity that was missed. This kickstarter project felt incomplete and … well … icky.

Which brings me to the second production How to Build Rocket Mass Heaters with Ernie and Erica by Calen Kennett of Village Video. This was the first of the Kickstarter projects to launch and though it got fully funded it did not create the buzz that the second project got. It was delivered late (I just received mine a couple of days ago) but that was because loads of caring production work went into it.

It documents an entire build of one Rocket Mass Heater (the one you see on the cover) – an 8 inch system built over an existing wooden floor. It covers very many details which appeared as questions during my two builds and I have not seen addressed anywhere else, covering tools, materials, design, build and finishing. It has excellent quality video and audio. It is a composite of footage shot on site during the build together with a separetely shot and well thought out interview with Erica and Ernie (with excellent quality audio). Inside the DVD case there is a printed page with a list of tools and materials used in the build … superb attention to detail.

The kickstarter edition included Erica and Ernie’s recently published The Art of Fire (which I have yet to take in). In addition there was a surprise on the Kickstarter DVD – the detailed plans for the RMH in the DVD (which more than makes up for one thing that I believe is not emphasized enough in the DVD – an explanation of the basic structure of feed tube, burn channel and heat riser and their dimensions). It is a production infused with care and quality.

If you want to get started with Rocket Mass Heaters I would recomment the (above mentioned) book and this DVD.

Our first (bedroom) rocket stove worked really well … it literally saved us during our first winter here. We did not have enough wood prepared … and it’s super efficient burn-and-battery kept us warm. However there was one recurring problem. The top surface was made of metal. It was intended to both radiate heat quickly into the room and to provide a potential cooking surface. It was also an easy way to cap the brick tower. This metal surface responded to the intense heat of the rocket by warping which in turn placed pressure on the cob seams that sealed it in place … which in turn leaked poisonous gasses into the room. The temporary solution was to keep fixing it and adding more layers of cob to it (which we did all winter long). The long term solution was to replace the top.

I started by modeling the top and I opted to use rebar to support the bricks that would make up the new top. I didn’t want to mess with or work on the existing brick tower so as not to mess up the room (cutting bricks creates alot of dust). So I added half-height layer of bricks and notched it to accommodate the horizontal rebars.

Opening up the existing top was fairly simple since it was already coming apart. I took this opportunity to replace the insulation around the heat riser. When we built it we used ashes (which is all we had) and they settled quite a bit. I scooped and vacuumed out the ash insulation and replaced it with a perlite & clay slip mix (we managed to find perlite in preparation for the second rocket stove). Then a little bit of refractory (heat resistance) mortar and the rocket was fixed.

Later (this was done back in October, I just now got around to posting about it), when we got to work on the second rocket, Andreea added a layer of finishing.

I still wonder if there was an alternate solution, to somehow prevent the metal from warping …

 

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 🙂