This could be the cleanest and most sustainable way to heat a conventional home. Some people have reported that they heat their home with nothing more than the dead branches that fall off the trees in their yard. And they burn so clean, that a lot of sneaky people are using them illegally, in cities, without detection.
When somebody first told me about rocket mass heaters, none of it made sense. The fire burns sideways? No smoke? If a conventional wood stove is 75% efficient, doesn't that mean the most wood you could possibly save is something like 25%? How do you have a big hole right over the fire and not have the house fill with smoke? I was skeptical.
And then I saw one in action. The fire really does burn sideways. The exhaust is near room temperature - and very clean. The smoke doesn't come back up because a huge amount of air is getting sucked into the wood hole. Neat! I sat on one that had not had a fire in it for 24 hours - it was still hot!
how it works:
The sticks stand straight up. Only the bottom ends of the sticks burn. The fire burns sideways. Since the heat riser is insulated, it gets freaky hot. This causes a strong convective current. When the hot gasses hit the barrel, it gives off a lot of heat, which cools the gasses which get much smaller and easier to push around. The gasses that exit are usually just carbon dioxide and steam.
The real magic happens with the heat riser. The strong convective current is what makes the air get sucked in through the wood feed so that the fire burns sideways and the smoke doesn't come out. It is also the place where it gets so hot that all of the smoke is burned.
Above is a much better image showing the mighty power of the insulated heat riser, reburning the smoke and powering the whole system.
The first picture below represents the most thorough rocket mass heater design I have ever seen. Followed by the rocket mass heater that was created from that design. These are both the products of the leaders in rocket mass heater innovation, Ernie and Erica Wisner, who have built more than 700 rocket mass heaters.
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a conventional wood stove uses a lot of the heat to push the smoke out of the house
a rocket mass heater extracts as much heat as possible before releasing the exhaust
I've now given presentations and taught people how to make these. The question I am most often asked is: "If my current wood stove is 75% efficient, it seems the most room for improvement is about 25%. But you say you can heat a home with a tenth of the wood. Isn't that claiming that a rocket mass heater is 750% efficient? Wouldn't 100% efficient be the maximum?" There are two ways to answer this.
Measure the temperature and volume of the smoke leaving a conventional wood stove (very hot and a large volume) and compare that to the exhaust of a rocket mass heater (a little more than room temperature and a trickle). Far more heat stays inside with a rocket mass heater.
Let's do the math.
A rating of "75% efficient" does not account for some of the heat that goes up the chimney to remove the smoke. The testing labs will use a number of either 14% or 16% for smoke going up the chimney. So the 75% number is actually 64%. Saying 75% is allowed and sells more wood stoves.
The rating of 75% was the most efficient result experienced in a laboratory with experts trying to get the most efficient numbers. So while a wood stove might be able to achieve 75% efficiency in a lab, it rarely does in a home. An experienced wood stove operator will probably experience something more like 35%. Somebody using wet/green wood and shutting the dampers down a lot for a "slow burn" will probably experience something more like 5% efficiency (or less!) with a 75% efficient wood stove. Thus leaving a lot of room for improvement. Rocket mass heaters have no way to reduce the air flow for a slow, inefficient burn. An inexperienced wood burner will probably have a 90% efficient burn every time.
Another question is about creosote. In a conventional wood stove, under inefficient conditions, creosote can build up in the chimney and start a chimney fire. The "chimney" in the rocket stove is the same thing as the heat riser. The rocket stove is designed to have a controlled chimney fire every burn.
Here is Ernie talking about how replacing a conventional wood stove with a rocket mass heater and how he now uses 1/8 of the wood he used to use:
When I first learned about rocket mass heaters, I desperately needed to see a video because what I was told was so .... different. There was nothing! So I now have lots of videos so people can see for themselves, the things that seem so new. This is just a few of the videos.
A short video making the rocket mass heater exhaust aspect really clear:
A general tour of 12 rocket mass heaters:
This second video is a day and a half workshop on building rocket mass heaters compressed into ten minutes:
My first attempt at building a rocket mass heater without somebody to guide me. This is an attempt at a semi-portable rocket mass heater. Plus a nice burn demo.
The latest from Ernie and Erica - complete with some innovations. This video focuses on how efficient a wood burning stove can be.
If these rocket stoves are of interest of you, the smartest next step is probably to get a set of plans that closely resembles what you are thinking of designing. These plans are proven designs that Ernie and Erica Wisner, the leading rocket stove innovators, have developed and tested.
6 Inch Rocket Mass Heater Plans
These have been the all time most popular plans up to date.They are for an L-shaped heated bench. The '6' refers to diameter of heat-exchange ducting. This design provided the sole heat for an older Portland cottage/apartment of about 800 sf.
These plans are for a duct-heated, small heater that warms an 11 x 11 (120 sf) guest cottage in Portland, OR. A bypass from manifold to chimney allows priming the heater more easily for cold starts. Instead of a bench, the mass makes up a platform for a bed!
This is for an 8" duct diameter with room-air circulation channels over reinforced wood flow. This large system currently heats a 3-story, multi-family house in a cool coastal valley. Reinforcement and built-in air channels protect the existing hardwood floor while providing passive circulation of warm air to the rest of the house.
This plan is for a duct-heated compact 8" diameter rocket mass heater, with a 3.5' x 9' footprint on concrete slab floor. The original installation heats a 840-sf cottage in alpine/high-desert climate near the Canadian border (snow Oct to April) on about one to 1.5 cords of wood per winter.
Rocket Mass Heater Operation and Maintenance Manual
This manual includes sample drawings, fill-in-the-blank areas for site-specific details, operation and maintenance logs, and standard instructions for how to light, clean, and maintain the heater. There is also a troubleshooting section that addresses problems that may come up due to design, maintenance, or operational oversights. The total length is about 20 pages (updates will be substituted from time to time).
This 40-page booklet is intended as a general introduction for anyone who wants to learn about fire. It will be entertaining for those who've heated with wood for years, and especially useful for those who are just starting. Offers many insights for campfire cooking, fire safety, and other survival skills.
This is a ROUGH DRAFT of our complete, technical guide to building rocket mass heaters. Our Builder's Guide offers specific, step-by-step building instructions and dimensions for our most popular type of rocket mass heater (an 8" design with J-type firebox), and detailed technical appendices covering various design and materials options.
With this combo you get huge savings in the ol' money column by getting ALL of the plans above
PLUS chapter 4 of Rocket Mass Heater Builder's Guide
PLUS The Art of Fire
PLUS Simple Shelter ideas
PLUS Double-Chamber Earthen Oven plans
image: batch box innovation built at the rocket mass heater innovator's event, discussed in 3 podcasts in the gob
Several people have been able to scrounge parts over several months so that the construction cost ends up near zero. Here are some of the materials you might want to look for, complete with links to be able to purchase them in case you cannot find them otherwise:
And this one shows the rocket mass heater barrel prep. Complete with an excellent demonstration of the rockety stuff - check out that smoke reburn!
Here is Ernie's conventional wood stove / rocket mass heater hybrid, which gives a lot of info on how a rocket mass heater can heat a space with 90% less wood than a conventional wood stove:
This is my grand video on my portable rocket mass heater. This features four huge innovations: a wood frame aesthetic (a bit rough, but the trail is now blazed for better wood workers); portable (can be taken apart and put on truck in less than an hour; can be taken off of a truck and built in a little over an hour); a heat riser that goes all the way to the ground instead of sitting on bricks; the thermal mass is a collection of pea gravel and river rock (that allows air to pass through) instead of cob. This video has a lot of details of construction.
rocket mass heater podcasts
As I was designing my portable rocket mass heater innovations, I called Ernie and Erica several times. After all, they know far more than I do. After the great Earth Day success I recorded a podcast with them. Not only to have a podcast loaded with rocket mass heater / rocket stove info, but to gloat a bit about the success of my innovation. This was my very first podcast about rocket mass heaters!