Rocket Stove!

Last week the 2nd year permaculture class completed the rocket stove in the strawbale house at the college , and fired it up. We were delighted with the results and very excited by the prospects of rocket stove technology. Below is a review of Ianto Evans‘ book which was the inspiration for our project and which guided our design. One adaptation we made was to use 7 8″clay chimney pipes for the flue pipe through the thermal cob bench. These pass through the straw-bale wall (it was surprisingly hard to carve a whole through the tightly packed strawbale!) and we then fitted a steel flexi-pipe as an up-right pipe outside. According to Ianto, this should not be strictly necessary as the stove is supposed to work by building up pressure in the heat-riser which should be able to force the exhaust outside. However, we found there was a lot of “blow-back” until we fitted the steel pipe. Maybe there was another way of avoiding back-drafts; unfortunately, this more than doubled the total cost of the stove which came to about 500euros in total. Also, there appears to be as much smoke emitted as from any other stove, which might raises doubts about its efficiency or whether it is really achieving secondary combustion. One disadvantage is that there is no damper or means of controlling the fire. We will have to use it more before getting a clear idea as to its overall performance, but it certainly seems to warm the room well, boiled a pan of water for tea and retained a lot of heat for a good while after the fire had gone out. There is still a bit of work to do on the cob bench which will be plastered later. Overall it seems to have been a successful project and I will write an update once we have more experience with it.

Rocket Mass Heaters Super efficient Woodstoves you can build (and snuggle up to) Ianto Evans & Leslie Jackson Cob Cottage Publications 2006 100pp 2 colour plates

Ianto Evans, ecologist, architect, writer and inventor, presents a revolutionary stove design of his own invention, the rocket mass heater or “Rocket Stove”. The rocket stove is highly-efficient but easy, fun and cheap to build; most importantly, it represents the kind of innovative democratic technology that could really have the potential to reduce carbon emissions and save fuel in many parts of the world. Ianto developed the design through his work in more than 20 countries over the last 30 years to try to find more efficient wood-burning stoves for heating and cooking for traditional peoples, and now the rocket stove concept is fast becoming popular in North America and Europe amongst DIY natural builders, permaculturalists and others seeking more sustainable energy solutions. In some respects, a rocket stove combines the qualities of masonry stoves with simple thin-walled wood-burners, such as sometimes made from old gas bottles. The main features of a rocket stove consist of a roughly J-shaped combustion chamber with abrupt right-angle turns, usually built of bricks. The “heat riser” is insulated and covered with an oil drum or similar container. The burning gases from the combustion unit are forced upwards and down the space between the oil drum and the outside of the insulation, from where they are pushed under pressure into a horizontal flu pipe which is encased in a

cob-bench. This is the thermal mass which heats up slowly, but retains the heat and emits it slowly into the room over several hours as does a masonry stove. Better than that, the cob bench provides a hot seat which, Ianto claims, is by far the most efficient way to heat oneself – ie by direct contact. “Our goal was to rethink completely the question of how to burn wood inside a house in order to improve human comfort, use less firewood and cut out pollution.” Most solid-fuel stoves are wildly inefficient, as the wood is not fully combusted and much of the fuel’s energy leaves the stove as unburned smoke, particles and unburned gases. The unique design of the rocket stove aims to achieve nearly complete combustion of the wood’s stored energy; Ianto claims that his own stove is at least 6 times as efficient as his neighbours’ more conventional systems, and produces practically no smoke.(Andy Wilson has pointed out to me that this could not in fact be true as most stoves are at least 40% efficient). The drawbacks are that it is still partly experimental – each one built will be slightly different and may need some trouble-shooting – and will certainly involve active participation from the owner-builder in modifying it and feeding it. If you are looking for a heating system that involves simply paying a quarterly bill for fuel delivered with a self-feeding system, such as some wood-pellet boilers, this is not for you. The book is clearly laid out, starting with an explanation of how wood burns and Ianto’s philosophy of heating the body directly rather than the usual strategy of heating the space in the whole room, ie the air, which is much harder to do. The rocket stove gains over even a well-made masonry stove (which typically would cost thousands rather than the couple of hundred for the rocket stove) in that the oil drum heats up almost instantly to provide quick radiant heat and also a spot for boiling a kettle. Step-by-step instructions are easy to follow and well-illustrated with line drawings and diagrams as well as photographs. Colour plates show cob cottages in England and a couple of sumptuous stoves. A section in the back lists adaptations of the design for hot tubs, heating water, the “coffee rocket”, suggestions for further research and case studies. Ianto Evans is also the co-author of “The Hand-Sculpted House – a practical and philosophical guide to building a cob cottage”, founder of the Cob Cottage Company and has run several workshops in Ireland at The Hollies. We have recently completed a rocket stove at the Kinsale Further Education College (see above) and would strongly recommend anyone with an interest in more efficient heating systems – or anyone with a penchant, like Ianto, for pyromania – to have a go.