Rocket Mass Heaters

Introduction —By Leslie Jackson

Stand around a bonfire in any backyard USA, a cold beer in one hand, and the guests turning like pigs on a spit to keep one side of the body or the other warm, and the notion of the “rocket stove” often comes up. It’s remarkable how many people from all walks of life have heard of them, whereas ten years ago, they were limited to natural builders and cookstove improvement nerds. Rocket Stoves were invented as a solution to the problems of third world communities, cooking in enclosed spaces with scarce fuel resources on three-stone hearths (three equally-sized rocks supporting a cook-pot over a wood fire). Fuel scarcity, poverty, and indoor pollution, not to mention health issues like asthma showing up in the kids, and greenhouse gasses from all kinds of inefficient combustion, drove aid organizations to look into the kitchens for the problems and the solutions. In the 1970s, a research group of fire scientists in an organization called Aprovecho Research Institute were challenged to design a wood-burning device that was easy to reproduce out of inexpensive materials, that would burn the wood so well as to emit very little smoke. The Lorena Stove that Ianto Evans developed in Guatemala is one such stove. Ianto, a Welshman, a recovering Professor of Landscape Architecture, and likely the best Ecological Designer there is, was one of the developers. He traveled to kitchens in Africa and Central America; spent time with (usually) women in their kitchens, and cooked with them. The stove he and his colleagues developed is still in development by organizations like Aprovecho, and Ashok Gadgill’s Stove Lab at the Lawrence Berkeley Lab in California, among others.

What Makes a Wood Burning Stove a Rocket Stove?

The rocket stove burns cleanly and almost completely, and it does so by means of a combination of materials that is it built with, and geometry. The chimney is insulated, maximizing its interior temperature, thus helping the fuel burn completely. The wood is small, dry and straight. The chamber in which the wood burns is of a size to reflect back on itself, also maximizing the burn by helping it stay hot. The opening in the stove in which the fuel is fed, the chimney on which the cook pot sits and the gaps–around which the gases flow past and around the cookpot–are very carefully sized to control the amount of oxygen mixing with the burning fuel. The stove’s “L” shape or “J” shape contributes to an ideal mixing of the combustion gases for clean burning. It’s all about the proper conditions for complete combustion. Knowing what we know about indoor cooking health, if we can warm a pot of soup with minimal fuel and minimal smoke, can we also create indoor comfort, warming bodies?

Today, Ianto Evans teaches people to get off the treadmill by living simply, in hand-built homes with renewable energy, and food growing. Working with the problems of human comfort back at home in rain forest Oregon, Evans started playing with storage of the heat produced by these fuel-efficient wood burners. The Finnish Masonry Heaters used in very cold climates like Russia and Northern Europe have a lot to teach us about efficient space heating, but are very complex in their construction, requiring a highly skilled designer to size the heater, and a skilled mason to stack the bricks, which form the smoke’s path through a complex labyrinth of channels, as the gases cool down and give their heat to the bricks. A masonry heater can be installed for 10 or 20 thousand dollars and will last for generations, providing a pleasant kind of heat from its mass; a wetter, more comfortable form of heat than a metal box stove, which burns to the touch, and cools down as the fire dies. If we can mass-produce a five dollar heater that cooks food safely, does indoor heating comfort need to be so out-of-reach? The challenge of dwelling in health and warmth with plenty of fuel isn’t limited to the third world. Ianto writes:

A gigantic cultural deception: We have been persuaded that houses need to be heated. Let’s be clear. A house doesn’t give a hoot, in fact, it will last longer if it isn’t heated. So why bother with any heating at all? Because in cold weather we humans have inadequate heat generation. We tend to get uncomfortable when we’re cold.

New wood burning stoves can be pretty efficient at turning most of the wood into CO2, water vapor, ash and heat, getting that heat from Stove to Chilly Willy is still wildly inefficient. We rely upon radiant heat, which effectively toasts one side of the human (while the other side continues to freeze), and advection, that is, blowing heated air into the whole room. Two problems: Chilly Willy can’t make use of most of this heat. It’s mostly wasted because he can’t occupy the whole volume of the room, and air is very thin so it can’t contain much heat, plus it’s arguably the most effective insulator, so what heat it contains doesn’t easily get to his body.

So what’s this got to do with Rocket Mass Heaters? Well, you can sit or lie on your Rocket so the heat goes right into your body, helped by your weight squeezing body parts onto a warm surface. Conduction more than convection or radiation. There’s a bonus: 90-odd percent of the heat stays in your building, you’re not heating the sky. When my office stove is burning full-blast, I can lick the single-wall stack where it exits the wall, it’s lost so much heat (it’s given it to the mass bench). And no smoke, just steam, so we’re using every little BIT of potential heat that was buried in that firewood.”

What Happens When you Plug this Improved Cookstove into a Cob Bench?

A Rocket Mass Heater is a wood-burning stove that uses Rocket Stove technology to burn wood very efficiently and clean, and to then store the heat it produces in thermally massive furniture–the bed or couch you sit or lie down on. By understanding enough about how wood burns and how heat moves; and by understanding the role of the materials in the parts of the stove, you can build one of these heaters for yourself, using the materials available and inexpensive to you–often from your own waste stream.
In its basic form–the Ianto-Style stove, as people call it–there is a combustion unit (the rocket stove) with an overturned 55-gallon oil drum surrounding the insulated chimney, which traps the gases and feeds them down and around the insulated chimney into a manifold from which exhaust pipes–enshrouded in thermal mass (cob in this case)—run through the bed or bench. The 55-gallon drum ain’t pretty, but it’s amazing to see the creative things people do in order to live with an oil drum in their space. Some, like Art Ludwig wrap the drum with fancy decorative metalwork, which also detracts the uninitiated from leaning against its hot surface. In Denmark, there are rocket mass heaters, with masonry in place of the oil drum. I have seen the cob bench or bed evolve from a blobby mass with cushions, to a sculpted day bed with designs and relief sculpture, creating a cozy work of art on which to snuggle. The sky is the limit with cob, since you sculpt it by hand.

Is this heater right for you?

• The first thing I ask people is what kind of home they live in and whether it is insulated. I always encourage people to maximize the house as-is before sizing or installing any appliance. For this advice I send folks to my old employer Home Energy Magazine a building science/home performance publication. It caters to building professionals, and is the unbiased straight poop on dwelling efficiency.
• Do you know what your heating needs are? This two-part article at Green Building Advisor helps you arrive at the numbers.
• The mass bench is heavy and foundations may need to be reinforced to accommodate the weight.
• The overturned 55-gallon drum can be a deal breaker with folks, it is quite a presence in a room.
• Another consideration is time: The small diameter, very dry wood needs processed and stored before use. There’s a tinkering factor, as stoves require maintenance and troubleshooting in the case of obstructions in the pipe, creosote build-up, or burn-back.
• The biggest downside that I hear from people is that they need too much attention when they are actually burning. In only 45 minutes to an hour, you can charge your bench to give back for a 12-24-hour period. But in those 45 minutes you are checking the fuel, stoking and adding fuel. If you prefer to build a fire, adjust the fresh air intake and walk away, this may not be your stove. This may change with the innovation of a “batch-box” style rocket mass heater that has been in discussion experimentation and in use for a few years with early adapters like Lasse Holmes, in Homer, Alaska.

What people love about it is the immediate warm-up from the barrel, and the lasting warmth from the thermal bench as they do paperwork, read, craft, and snuggle. It can be difficult to remove people from it, in fact…which may or may not be a downside. As Lasse Holmes says of his mass heater in Alaska, “a human spatula may need to be invented.”

The culture of DIY, teaching (rather than contracting for others), and open source sharing on forums (instead of writing patents–which can stop the technology short of its full evolutionary potential) is hand-in-hand with the revolutionary philosophies of living in simplicity, and taking our housing and energy needs into our own hands. Evans and I wrote a little booklet called Rocket Stoves to Heat Cob Buildings in 2005. It tells the story of the rocket mass heater and instructs a basic model with illustrations. Now in 2015, it is a perfect-bound paperback and downloadable .pdf called Rocket Mass Heaters, reflecting the fact that today, these stoves are in wooden cabins, yurts, conventional homes, and strawbale homes, earth bag homes, rammed earth, adobe, earthships, etc.) In the time since its first publication, it has been translated into Russian, French, Japanese and Spanish, and workshops are taught on every continent. Two major contributors to the Second Edition in 2007, Kirk Mobert and Ernie Wisner both started online forums to save time as their inboxes were flooded with the same questions. Now with several forums in several languages worldwide, people are finding workshops, sharing regionally-specific materials resources, and building together.

The Anatomy of a Water Heater Workshop

It’s fun to watch people begin to understand this simple ancient art and craft of fire. Once the lights go on, many people say, “what about heating water?” This is dangerous territory because under the high temperatures of the stove, water will quickly expand, and if the system is closed, now you have a potential bomb in your space. Lots of discussion has ensued on Kirk’s forum, and he began drawing, experimenting and asking of the plumbing professionals.

Now he has a way to do it that doesn’t require welding skills, though some sweating of pipe is necessary. At the workshops, perform a live fire show, telling the evolution of the stove through fire and story. He’ll chalk-talk the fundamentals of rocket stoves: sizing the system, how to safely construct (often from waste materials like used water heaters and parts) a wood fired water heater, some plumbing basics; cobbing and masonry basics; how to build safely on foundations that will support the weight, and also how to build with safe clearances away from flammable surfaces.

Laytonville Ecovillage Workshop
Kirk’s most recent water heater workshop was at the end of summer, 2015, at Laytonville Ecovillage. There, a dozen people gathered to camp out, eat great food provided by Dan Antonioli and prepared by Laytonville Ecovillage’s culinary wizards Rain and Chi.
The LEV site has had a bath-house for its workshop participants and a small community of full-time dwellers. There are two wonderful outdoor showers, powered up to now by a solar-thermal hot water system in one shower, and an on-demand water heater in the other.

Owner Dan Antonioli wanted a wood-fired rocket stove water heater to supplement the system, so that during a busy workshop—or just a shower-house traffic jam!—could accommodate back-to-back showers for up to ten people. He also wanted an opportunity to further remove the Laytonville Ecovillage from dependence on fossil fuels, take advantage of an overgrown forest with tons of small diameter wood, and have fun geeking-out on rocket stove technology.

Over the course of two days, the class learned:

• Basics of cob building: sourcing clay, testing for clay content, mixing by foot and sculpting by hand
• Pipe sweating
• Site preparation
• Brick stacking
• Rocket stoves and wood combustion
• Water heating
  …and by Sunday afternoon had a functional, working rocket stove hot water heater. Mission accomplished.

There is a good write-up with photos on Kirk’s forum, here.

In addition to running the first online forum for rocket mass heaters, Kirk runs Sundog School of Natural Building in Point Arena California. See you at a workshop!