A few weeks ago, in October of 2010 I attended a Pyromaniacs workshop in southern Oregon at the Cob Cottage Company – home of natural building guru Ianto Evans. This workshop was a comprehensive lesson on pyro-dynamics, covering everything from the three elements necessary to have fire – oxygen, fuel and heat – to the history of wood burning appliances. We lit many fires in cob ovens, Rumford fireplaces, and rocket stoves, and we also built a number of rocket stoves and a rocket mass heater over a 3 day span.
My long journey to southern Oregon was primarily to gain a better understanding of rocket mass heaters – a super-efficient wood burning stove that a) uses a fraction of the wood used by conventional wood stove, b) can be built for as little as $100, c) can keep a space warm for several days with a single firing, and d) produces virtually no smoke – simply water vapour and CO2. And all you really need to build a rocket mass heater is a stack of fire bricks, a steel drum, some ductwork and a chimney, and a lot of mass – usually thermal cob, which is simply a mixture of sand and clay.
This schematic details the inner dimensions of a Rocket Mass Heater (from the book Rocket Mass Heaters: super efficient woodstoves you can build (and snuggle up to) by Ianto Evans and Leslie Jackson)
These systems incorporate a simple J-tube rocket stove (see photos in gallery below), a metal drum to direct the flue gases down and into a system of ductwork, and finally a great deal of thermal mass (usually a cob bench) to soak up all the heat. Wood is fed into B (the feed tube), it is burned in C (the combustion chamber), re-burned (smoke and all) in E, then the hot gases move into a low pressure area (G, H and J), and travel through K (the duct work) and finally out C (the chimney). All the while, the hot gas moving through the system is being sucked into the thermal mass – usually a cob bench, warming the area with beautiful radiant heat.
Rocket Mass Heaters are a relatively new technology, but are quickly becoming a viral topic. I’m looking forward to building a few demos here in Canada to see what I can offer to making these super-efficient stoves more widely used and understood. If you are interested in learning more, have a look at the Rocket Mass Heater book and keep an eye out for our Upcoming Workshops.
Since we originally published this article (in 2010), we’ve built multiple systems in Western Canada, including ones for use as space heaters in smaller buildings and green houses. We’ve learned what tweeks to make so that the rockets we build hum every time. We’ve also developed a highly informative Rocket Mass Heater Resource page, viewable here.
How We Can Help You:
Dirt Craft offers several Rocket Mass Heater workshops (upcoming dates here) each year, so you can get hands on experience designing and building a system. We also provide advice if you’re thinking about building a rocket mass heater. Or (this is something we do quite often), if you’re trying to trouble-shoot something that’s not quite right with one you’ve already built. Visit our Consulting page for rates, and contact us to get started