If you are reading this and interested in the technical aspect of Rocket stoves you may want to scroll down to the last part of the post where there is a description of what we actually built. But first, I do will indulge in some personal reflection about rocket stoves.
What is a Rocket Stove?
If you’ve never heard about rocket stoves, though I’ve mentioned them before, this short video is the place to start. This video drew me out of the theoretical reading and into action – anyone can experience the wonder of rocket stoves by recreating what is demonstrated in it:
A rocket stove mass heater is a more elaborate stove built around this concept … it is simple to build, efficiently burns wood – the rocket part (more then most existing stoves) and equally effectively stores and radiates the generated heat – the mass part.
To learn more you may want to:
- Get an introduction at Paul Wheaton’s summary page on rocket stove mass heaters
- Learn to build one with the one and only rocket stove book
- Visit the rocket stove forum (lots of hands on information and helpful people)
- Of course you can and should search the Internet and find plenty of information about them.
However if you want to build one I strongly suggest you get the book – there are a few core details that you have to get right – once you get those down you can play around with it a lot. Though there is plenty of freely available information you will be hard-pressed to find all these core details without the book – at least that’s been my experience.
On to our personal rocket story
A Screeching Halt
Last year, as we were in the process of purchasing the land we currently live on, we were also working closely with our friend and architect on designing our hemp-built house (much more on that in coming posts). He designed a beautiful (and functional!) house, but the more beautiful it got to be the more I began to question its economic feasibility = I didn’t think we would be able to afford to build it. We were really committed to the process and invested much effort and resources in pursuing it (including a visit I have yet to write about to the UK to consult with an architect experienced in hemp construction).
As we were working on our rocket stove it was Andreea who insightfully recognized the point at which our beautiful house came to a screeching halt. We were doing some financial planning and were inquiring with our architect for some input on the house systems. We were sitting in his office discussing options for a house heating system – seeking a rough budget to plug into our calculations (which were already looking grim). He picked up his phone and called a fellow engineer. The conversation resulted in a mind-blowing figure ouf 15,000 euro (half of our target construction budget). We realized that something was not right – we felt that the architect and the engineer lost touch with us and our wishes. Further inquiry into the subject brought the figures down to 7,000 euro – still a lot of money and out of our budget.
Though the project continued to move forward and is still alive though dormant today, from that point on it was winding down. Our wish to have a comfortable and warm home, it seemed, could not be fulfilled within the budget we had.
Preparing for Winter
By the time we moved out it became painfully (at the time) clear that we would not be building a new house this year and that we would probably be spending a few years in the existing traditional Romanian village (cob) house. So, we began a long and ongoing (though definitely coming to a first end) effort to fix it up.
The house has a small hall, a small pantry (we converted into a bathroom) and two rooms. One room has a traditional Romanian wood-stove (the other had nothing). We had lots of plans to do lots of things. Though we did lots of the things, many were not in the plans and most of the plans did not reach execution. Our plans did not include building a rocket stove this year. They did include buying a second stove and a third wood-burning boiler.
Then, we discovered the (above mentioned) 16 brick video, we purchased some firebricks and tried it out … and it worked fantastically. We used it quite a lot for cooking outside.
At one point, just for the fun of it, I even built up around it and simulated a rough rocket stove . I used the parts of an old wood-boiler for the heat riser:
Seeing the horizontal burn for the first time was a magical experience:
And then when I placed a barrel on top, it got really warm really fast:
Then came the two+ weeks of non-stop cooking during which we made loads of winter preservations. We were practically living next to the small rocket stove outside. Then when we moved indoors in the evenings to escape the cold and complete the jar preservations we would light the wood stove. It was painful to see how much wood this stove consumed (and continues to consume) compared to the 16 brick rocket stove outside. Everyday I would have words of awe about the outside rocket stove and complain about the wood-greedy stove inside.
Slowly I gathered the courage to suggest we try building a rocket stove instead of buying another wood-guzzling metal stove. And so it began … from here on it’s going to get a bit technical.
There were a few constraints shaping this project for us:
- Timing – we had already finished putting in wooden floors including a corner of ceramic tiles in each room to house a stove. We did not feel inclined into removing parts of the floor to install a cob-bench (the mass part of the rocket where heat is stored) and we were not keen into working with cob as it takes time and experimentation … and we already had quite a bit on our plate. So we were going to build a stove without thermal mass … making it’s heating efficiency questionable.
- Barrel – This is going to be a recurring theme here at Bhudeva – but finding used/2nd hand anything is much more difficult here in Romania then in the USA, UK or other western countries. This is especially true in villages where everything is used, reused and used again, often beyond the point of efficiency or even safety. A key element in the rocket stove is a metal barrel – and for many weeks (which delayed the project) we couldn’t find one. We finally decided to build a rocket stove with “firebrick chamber with a top metal plate” instead of a barrel (we have since found and purchased one used barrel and have a few more lined up … we will keep them stored for future needs).
- Insulation We also could not locate (at reasonable prices) any of the suggested insulation materials for the internal combustion chamber – we ended up using wood-ashes – they may not offer the same level of insulation and they may settle over time but this is what we had available to us.
- Size – the actual stove – where all the burning takes place is a systemically and purposefully twisted path which, from a certain point early in the path (the horizontal burn chamber) must not become narrower. This is to prevent smoke and dangerous gases from returning into the room – the stove generates a powerful draft and nothing must impeded that flow. The traditional Romanian stove configuration is for a 12cm chimney flue – which is also the size of the chimneys usually built into the walls. Since we did not want to make a new opening in the wall for a chimney we had to adapt to that limitation. This is when rocket stoves designs are suggested for either 15 cm (6 inch) or 20 cm (8 inch). This meant a smaller scale and tighter design and indeed everything was scaled down accordingly. However, in retrospect, given that we built a spacious “brick barrel” as thermal mass – which slows the flow of gases before they exit, I believe we could have gone with a larger dimension in the stove with the limited 12 cm exit flue.
Like most of the good recommendations in the book, building a model of the stove is an important stage of work – and, as recommended, it should be done on a level surface (I tried to cut this corner and it was a waste of time). To that I would add that it should be built somewhere where you can (a) work on it over a period of time; (b) light it; (c) keep it out of the rain or other wet elements. I built our model in a corner of our barn which answered to all of these criteria – and luckily so because we went through quite a few iterations.
I actually used such images to document my progress of the layers (both as I built them up and took them apart) in building the actual stove in place. There were many small details and the images unburdened my memory and were very useful in actual construction. I spread construction sand on the floor to level it. This is the first floor level model (it changed later in the process) … it is designed to raise the rocket off the floor and create ash collection both under the feed chamber (right) and at the exit flue (left).
On top of that came the “floor” of the rocket itself – intentionally adding more depth to the ash collection chambers.
Then of course the burn tunnel
… and off we go – and you can already see the fire climbing up the feed chamber (the feed tunnel is not yet built up):
The model worked OK. The biggest problem, and one that carried over to the actual stove was the fire crawling up the sticks and out of the feed chamber. I now believe that the cause of this was that the feed-chamber and ash-clean-out beneath it were not sealed properly. So the stove instead of sucking air down from the top of the feed chamber was now also sucking air up from below which both lowered the intensity of the down-draft AND provided an alternate up-draft.
Also, if you decide to build something not quite by the book be prepared to take risks – as not everything can be tested (simply) in the model:
- When I built the model I realized that I could not seal the brick chamber nor the top metal plate to the bricks. This meant that we could not assess how effective the generated heat would be contained and radiated from the stove.
- Though the top metal plate (can be used for cooking) heated up rapidly it had a surprise for us in the actual stove. The heat generated by the rocket is so high that the metal (5mm thick) warps … the corners fold out creating stress on the cob that seals the plate to the brick structure beneath it. The cob has cracked numerous times (and let out poisonous gases) and we have had to reseal it (simply applying another coat of clay-slip)… it looks like we have reached the point where it is properly sealed – though only time will tell – we check it regularly.
The following images depict the actual stove in construction. First a simulation of the base layer to get its position in the corner. Next time I would try to leave more space between the rocket and the walls to make it easier to access and install the chimney parts – it was a struggle.
Then a little messier with the clay-sand mortar to keep things in place:
In the middle layer (top image) I installed a metal grail to support the feed chamber and let the ashes fall – I notched (with a grinder masonry disc) three parts of brick to support the grail itself:
Then a metal heat riser (used pipe cut to size at a metals shop in Cluj) went on:
and over that went a piece of sheet-metal tied into a roll and then filled with wood ash
Starting to look like a rocket stove
Here the heat riser is sealed with the clay-sand mortar mix
Here you can see that the heat riser is positioned away from the center of the brick box. One reason is that I assumed that the brick-barrel would behave like a steel barrel does and that the wider space would heat up more/faster then the narrow space – so in this case the wide space facing into the room. The space from the other (left hand side) wall is determined by the location of the ash-pit and exit flue (in the dark area at bottom left of the image). Though I don’t know if this actual works – I preferred to have the gases “linger” in the box rather then get pushed out by making the space near the exit flue narrow.
The almost finished stove with a completed brick chamber (I decided to use refractory cement which has adhesive function for the brick chamber instead of the sand-clay mortar which has no adhesive function it simply keeps the bricks from moving and when fired solidifies into a brick-like material) and metal top still undecided feed chamber, a temporary chimney leading out the door and to the hall, a small pot of water heating up and Andreea checking something out (and providing you with some sense of scale).
We still had smoke-back and fire climbing up the feed chamber … which frustrated me greatly … until Andreea intervened with a bit of feminine surrender and wisdom and suggested we let go of the vertical feed and go with a front feed … which not only worked but demonstrated that the stove had excellent draft with absolutely no smoke-back. Here you can see a fully loaded feed with fire swirling into the stove and absolutely no smoke coming out:
Then Andreea took over the clay plastering. It failed miserable the first few times – the plaster cracked and fell of in chunks.
The third time she (1) added hemp fibers and some acrylic construction glue; (2) wet the bricks thoroughly before applying the plaster; (3) applied it in thin layers and worked it in thoroughly with a wet sponge; (4) continued to moisten and add clay slip as the stove was heated up gradually over the next two days.
The chimney winds through 3 corner bends (poor planning?) so it was a bitch to install … but we now have a great looking and working kind-of-rocket-stove in the bedroom.
Before I talk about the stoves heating performance I’d like to talk about it’s other values – at least those that are important to us.
- Independence – with no past experience in stoves or any of the specific disciplines involved in creating this stove and with no access to people with past experience we managed to build this by ourselves. It can be built fairly quickly (it takes more time to gather materials then to actually build it).
- Efficient and Ecological – when the stove is first fired up some smoke comes out the chimney … but once the heat riser gets hot (very hot) there is no smoke – the stove performs a full and clean burn of the wood. Not only does it fully utilize the energy embodied in the wood (every time you see smoke coming from a chimney imagine waster dollars – all of that is wasted energy) – but it is also ecological since it releases very little pollution (compared to most regular wood stoves). If you were to buy an industrially made central wood heating stove you would pay a heavy premium for “re-gasification” – which is essentially the burning of the gases release when the wood initially burns. With the rocket stove this is an inherent and simply to achieve function (the insulated heat riser gets too temperatures high enough to burn the gases).
- Beautiful – you can shape and mold it and make it your own … which ends up uniquely individual and special.
- Sustained warmth – the thermal mass (in our case the “brick barrel”) contains all of that efficiently generated heat and slowly releases it into the space instead of letting it escape out the chimney.
It works great. Though we purchased a lot of wood we did not have time or do enough to dry it properly. Fortunately we also have a huge pile of junk wood that we collected from all over the place – that wood, after we cut it to size, is very dry and perfect for the stove. In a regular stove it may be desirable for wood to burn “not too fast” because heat is present only when there are flames (once the flames go out the stove and usually the space, unless it is superbly insulated begin to cool). In a rocket stove it is best to have a fast and efficient burn – the heat is stored in the mass and then released. We are currently lighting it with a few batches of dry wood and then some of the partly dry wood in usually larger chunks of wood. If we were to use only dry wood the stove would probably get really hot (too hot to touch) in 2 or 3 hours. As we are currently running it it takes 4 or 5 hours.
It uses much less wood then the regular wood stove (I can’t say how much as we are not yet setup for measuring and comparing. But more importantly it’s effect on the room is very different. Since we don’t have a radiant barrel (only the small top surface radiates heat immediately into the space) it takes time to warm the space – the other wood stove warms the space rather quickly. But once its warm the space will say warm much longer (again I don’t have measurements for comparison) … but we usually feed the stove one last time at around 10pm and at 7am the room is not cold (though not warm). With the regular stove the room cools drastically in an hour or two – once the fire is out the room begins to cool!
The heat has a different quality in the rocket stove room – it’s hard to put in words. It is a softer, deeper and rounder warmth then the regular wood stove.
It can be used for cooking – doing so requires using either very dry wood (which burns fast and releases a lot of heat) or patience (it is generally slower heating then the wood stove where fire almost directly heats the pots.
Until recently we have had to deal with the metal warping problem. It strains the corners of the cob that join the metal plate to the brick box to the point that cracks appeared and smoke/gas escaped into the room. It has been easy to fix – adding a clay slip – but has required constant attention. Time will tell if this is going to be an ongoing issue or one that we have resolved.
I would have been happy to make it bigger (see above mentioned constraint 4) and to get the vertical feed working properly. But we didn’t have time … it as getting too cold in the room and we still had plenty of work to do to prepare for the winter. The result is a stove that needs to be fed every 20 or so minutes … but that is a small inconvenience we are happy to accommodate.
We are very happy with the stove. It came to life just as the room was becoming unbearably cold. Creating it was a hugely empowering experience. We are very much looking forward to experimenting with it more and eventually incorporating simple rocket stoves we can build and maintain with our hand instead of complicated systems that cost many thousands of euros and place us at the mercy of technicians and engineers and companies.
We had plans to build a second rocket with an integrated baking stove and a thermal mass bench (wood framed!) stove to replace the existing wood-stove, but time did not allow for it. Next year 🙂
It’s been a frozen two days, it’s dark outside and cold is beginning to set – a good time to go and light the rocket so the bedroom will be nice and warm 🙂