Anyone who lives in a cold climate and has checked out Ben Falk’s fantastic book, The Resilient Farm and Homestead: An Innovative Permaculture and Whole Systems Design Approach, will be inspired to
think hard about making a good wood stove central in their home design. Ben Falk’s wood stove, a Waterford Stanley (an older version of the one pictured here), burns about 2 cords of wood each season and heats his home, cooks his food, provides hot water, dries his clothes and various food items, and — of course — provides a cozy center (see this very cool video where Ben Falk describes all of these functions and more).
But, before deciding on a wood stove, consider a relative newcomer on the scene — the rocket stove or rocket mass heater. It uses 70-90% less
wood than a wood stove, burns cleaner with 1% of the footprint of an electic oven, and more reliably. How much wood is really required? The picture (right) is the typical amount of wood used by Ernie and Erica Wisner in their rocket mass heater per day. They use about double that for really cold days (2 cubic feet). As a result, a cord of wood will last them about four months.
The rocket stove was invented in 1982 by Dr. Larry Winiarski of the Aprovecho Institute (an holistic, permaculture, education laboratory in Oregon) as an improvement over traditional wood fire cooking. The institute was motivated to provide better cooking solutions for countries suffering from the negative health effects of breathing wood smoke (the equivalent of 3 packs/day of cigarettes for some children; see this post on wood stoves vs. natural gas, a debate that might be made moot by the rocket stove) and the logistical and environmental impacts of burning scarce wood inefficiently (see this excellent video). The institute succeeded in their quest and now work hard to make sure the world has the stoves needed.
Given these potential improvements from rocket stoves, I started on a quest for a rocket-type stove that would meet all the functions of Ben Falk’s wood stove but with all the efficiency of a rocket stove. Silver Fire has many offerings but none that fit the bill. For a beautiful, and wood-stovey feel, Matt Remine uses his riserless
core to create a masonry cook stove and oven that looks great (see picture).
There is also a rocket oven designed by Tim Barker that has risen to the top of the pack in some ways, combining reliability, ease of use, fast heating, ease of building, efficiency, relatively low cost, and flexible use. This
stove is detailed in Tim Barker’s book, The Rocket Powered Oven. Tim Barker and Tyler Morrison show how to build the most recent version of the stove (it has been improved in small ways since publication of the book), in a DVD released in 2018 by Wheaton (with several smaller supplementary videos) as a result of a successful kickstarter. 3D designs of each build in the DVD are posted online with the 2018 plans available here for $20. In addition, here is a short 3D sketchup overview of the oven. This oven uses Walker Stoves J-core plans. This video shows the oven in action, including how little wood it requires.
Add some thermal mass for a Rocket Mass Heater
So, far, we’ve focused on cooking stoves. But, if you use a rocket stove to heat up a lot of thermal mass, such as a cob bench, you have a rocket mass heater. Two of the authors of the book on the subject of rocket stoves and rocket mass heaters, Erica and Ernie Wisner, facilitate the wood burning forum at permies.com and are featured in videos in this excellent article at permies.com. Note that Erica and Ernie Wisner have a new book, The Rocket Mass Heater Builder’s Guide. The article also links to plans you can buy and a course (4-DVD set or streaming version).
Due to popular demand for even better video and sound demonstrations of rocket stoves, Paul Wheaton’s successfully funded Kickstarter campaign (March, 2015) made it possible to produce 4 more DVDs, released in September 2016, covering many aspects of building rocket mass heaters and their real-world outcomes (watch the trailer here and buy the DVDs here). These videos are excellent for understanding to build, light, and maintain different types of rocket mass heaters. And, they also share the results of their experiments.
DVD 1: Cob stoves, including tipi heated well with a cob rocket mass heater.
DVD 2: Pebble stoves. The mass is composed of loose rock and gravel rather than something like cob.
Lessons learned from DVD 2: Pebble style (about 1:35 in video):
- Prefer cob rather than pebble
- Chimney/Exhaust performs better the higher the exit point is. Ceiling is best.
- Chimney should go closer to the barrel — slightly less efficient but it burns clean and well.
- Drying wood over the mass works really well
- Still Experimenting with ceramic fiber cores
- Useful clay? Make a pancake and it dries without cracking at all or very much.
2:10 in DVD 2, Paul reviews lessons from Shop pebble stove:
- Pebble style not as good as cob and holds about 25-30% less heat than cob
- Pebble is more transportable (2 hours), much more flexible to make changes, and better aesthetic for some people
DVD 3: Ship-able Cores.
This would help take rocket mass heaters mainstream b/c it would make it easier to build. Two years of experiments with 3 different design paths:
(1) Pourable Cores. Most failed by cracking though there are some that seem to work.
(2) Wood box ship-able cores. Work well. Heavy to carry (2-4 people to move depending on how dry it is). They use fire brick for the botton, 1″ dura board for insulation, and add a mixture of perlite, clay, and wool that they call “cobbish”. Total material cost is $250. Fire box is 16″ tall minimum (b/c that’s a common length to cut wood). The heat riser needs to be >= 3 x the height of the fire box (48″). Barrel has to be 2″ taller than that. Most difficult part of a rocket mass heater is the manifold. if you are using 8″ round duct work, the square portions of your core (the fire box, the burn tunnel and the riser) must move the same volume of air — in a square that equates to 7.09″ on each side. The riser can be round or square, but if place round riser on square base, a bottle neck gets created. There are limitations on how long the burn tunnel can be. 1:21:30 is a shopping list for version 1.2. Version 1.3 has not been built but may reduce materials by 35% – Plans can be found on permies.com/biggs
(3) Ceramic cores (wood box, fire tunnel and riser). Initial fires result in smoke until rice starch burns off. They cost $700 but might come down to $70 with sufficient quantities. They work “magnificently”. They then experimented with a 4″ system (instead of 8″) – cheaper, easier to ship. Paul Wheaton guesses that a 4″ system behaves like a normal 6″ and 6″ like a normal 8″. Never use steel in burn tunnel or riser (it melts!) and possibly not in the wood feed. Wheaton is moving away from using perlite and vermiculite in core because they get soft and squishy around 1400 degrees F.
(4) Future Cores. Shipped to your door for ~$500. If materials cost was $100 and met UPS qualifications for cheap shipping, that would leave room for profit. 2 designs that might do this: (a) duraboard wrapped in portland cement and perlite mix. Portland cement (needs 28 days to cure) spawls at 600 degrees F so has to be protected by duraboard. (b) Ceramic core in 4″ system.
[UPDATE as of 11/14/18 – Matt Walker has honed things to such a degree that he is now selling plans but not the products themselves.]
DVD 4: Innovator’s Event (2014), featuring Erica and Ernie Wisner, Peter van den Berg, Tim Barker, Matt Walker Remine
The manifold: The place where cooling gases coming down the barrel gets distributed to the chimney or the mass. Shape is key. Also, it’s a place where some ash settles so a space is needed below the pipe to the chimney or mass. Useful to have a clean out port in that area (1-2/year assuming you are not burning paper). Danger is creating a bottleneck when it’s so tempting to create a bottleneck. 1/3 of a barrel cut to fit around the riser and tunnel and with a hole for the exhaust makes a good manifold. On camera, they build a circular (better for not cracking) and then a square manifold out of brick. Space between exhaust duct and side of riser should be 4″.
“The Smoker” by Matt Remine (see below for examples of his stuff also). Emphasis on the stove as cooker and smoker. Fluid dynamics (and some thermal dynamics) relevant here. Imagine heat as water just upside down. Hot air rises and colder air will spill out the bottom. This way of conceptualizing things offers possibility of less ducting and more bench shapes. This is all explained by Matt and Paul about 40 mins into the DVD.
“Ring of Fire” by Matt Remine. Watch the fire and get a warm butt (featured image is a version of this). Glass for the wood feed and the burn tunnel. Wood stove glass can tolerate temps up to 1300 degrees F, a problem when trying to get temps up to 3,000 degrees F. Also, you lose heat out through the glass. Instead of ducts, you have half barrels turned upside down — heats up faster so warms butts faster but holds heat for far less time (an hour instead of a day).
Notes on using and prepping barrels. Don’t use a barrel unless you know what was in it previously! It could have been toxic. Or, if it was old fuel, you could blow up! How to prepare your barrel (1:09:55).
“Shirley Temple” by Ernie Wisner is small but creates multiple fire vortexes to allow the smoke to burn, as if the burn tunnel were longer. The same is true for creating vortexes in the riser — it can appear, to the fire, as if its longer than it is. This is some of the reasoning that has been used by Matt Remine in creating his riserless core (plans are free for personal use). Another example of how important fluid dynamics are: fire acts like water in its movement.
“The Cockit and Lockit Rocket” by Tim Barker. His interests are primarily cooking and heating water. The 6″ system he designed at this event had a temperature-controlled griddle, an oven, and a hot water system. With hot water systems, it’s easy to go squish boom if you do it wrong. So, they don’t get into the details about how to do it. Note, the oven part of this design is featured in a 2018 DVD, summarized above.
The “Batch Box” by Peter van den Berg. Instead of a J style, people like them because you can load wood through a door like you are used to and also view the flame. This 8″ system was the second one built at the time and Peter says it is twice as powerful as the 6″ systems he builds more regularly. Attaching doors is non-trivial or expensive to have it custom made ($300 range). Correctly sized “P-channel” is critical but details not covered in this DVD series (however, see this page from Matt Remine for links to Peter’s designs and Matt’s modifications of them). It heats the space rapidly, holds more wood, and is a much cleaner burn.
Milkwood in Australia has posted plans and pictures for building a very small and portable rocket stove that might be used for camping in 20 minutes. A more traditional non-portable rocket stove can be built from these plans from a leader in rocket stove design. Manufactured rocket stoves are also available here. Small, portable (but weighs about 14 lbs) rocket stoves from Ecozoom (pictured at right) are about $130 and have good reviews. For a solution for maple syrup, see this post on permies.com
Matt Remine (of Walker Stoves) demonstrates a great multi-use, outdoor design with visual flame, slate benches, and cooking (see picture at top). These are his latest versions, including heated benches, pizza oven, and more. Check out his whole series! Note that this is a “batch box” style rocket mass heater where the flame can actually be made more visible (this is a cool example too with plans). He also has a beautiful kitchen stove on his site.
What about building codes? See this post.
Rocketstoves.com is a terrific site for all kinds of articles and details about rocket stoves.
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