Welcome to the Wonderful World of Rocket Mass Heaters!
Rocket Mass Heaters are super-efficient wood burning stoves that, like a masonry heater, use a convoluted flue path running through dense thermal mass (often a combination of brick and cob) to soak up and store the maximum amount of heat, releasing it over time. While masonry stoves can cost tens of thousands of dollars to construct, Rocket Mass Heaters (RMH) can typically be built for much less than the cost of a traditional wood burning stove. We live and works in Canada, and have an intimate knowledge of the need for safe, efficient, and effective heat.
What Makes A Rocket Mass Heater Unique?
While the newest EPA-certified wood stoves are generally efficient, they are not effective! You need 2 E’s – efficiency and effectiveness – to get the most of wood heat. Efficiency means wood is converted to heat with little pollution, while effectiveness comes from being able to store that heat over time. A typical wood stove sends the majority of the heat up the chimney and into the environment, resulting in temperature fluctuations, which move from super-hot to cold in a relatively short period of time. On the other hand, a RMH uses a convoluted (or serpentine) flue path running through thermal mass to extract heat to the fullest extent possible before it exits the building. This ability to store heat produces more even heat which lasts lasts longer, thereby reducing wood use substantially.
How a Rocket Mass Heater Works:
You’ve seen it before: a steel drum propped up on a pile of bricks, with firewood fed vertically into the fire box, and then a big, lazy looking bench connecting it to a far-away chimney. But what’s really going on here?
These systems are based on a J-tube design (see schematic below). Wood is fed into B (the feed tube), it is burned in C (the combustion chamber), re-burned (smoke and all) in E (the heat riser), 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.
The core of the heater – the combustion chamber – is made of refractory-grade bricks and super-insulated to concentrate heat back onto the fire, burning any smoke that is produced. The heat riser, which is the long end of the ‘J’ (E in the diagram above), is a mini-chimney which, when warm, creates a powerful draft (so strong that it often creates the sound of a rocket taking off), effectively acting as a pump that pushes the heat and flue gases through the horizontal section of stove pipe set in the masonry section of the system. The intensity of heat in the combustion chamber and the turbulence caused by strong draft burns the wood and smoke extremely efficiently, often producing little, if any, smoke. You’re simply left with CO2 and water vapour.
The metal drum acts as a radiator, shedding heat quickly so that the room warms to a comfortable level without the entire mass first having to warm. Its top is also a great place to place a kettle to boil water for tea.
A manifold at the base of the drum carries the flue gases down into a system of duct work set in thermal mass (usually a cob bench) to soak up all the heat. This heat is stored and released slowly over time, warming the area with beautiful radiant heat.
Finally, the chimney (identical to that of a standard wood stove) carries away the left overs of the combustion process, often without any visible smoke.
Five Reasons Rocket Mass Heaters Outperform Wood Stoves
In a nutshell, rocket mass heaters:
- heat a home using a fraction of the firewood
- burn the wood so efficiently that there is virtually no smoke, simply steam and CO2 (after all, smoke is simply unburned fuel)
- maintain heat in a building for several days after only one firing
- be built for under $100 and in just a couple of days
- provide the most beautiful radiant heat!
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
Why Radiant Heat is Best
People are most comfortable when they receive radiant heat at a slightly higher temperature than the temperature of the air around them. The two most primitive examples of this situation are: (1) Outdoors, on a spring day when the air is not too hot but the sun is shining. (2) Around an open fire, on a cool evening. Most people will recognize intuitively that these are two unusually comfortable situations. And in view of the fact that we evolved as organisms in the open air, with plenty of sun, it is not surprising that this condition happens to be so comfortable for us. It is built into our systems, biologically.
– A Pattern Language (Alexander et al. 1977)
Unfortunately, it happens that many of the most widely used heating systems ignore this basic fact.
Adaptations of the Rocket Mass Heater
Rocket Mass Heaters are not reserved for home heating but can be adapted to extend the growing season in a greenhouse.