Here is a little article I wrote up over at my blog about the awesomeness of rocket mass heaters. I'll share it here for permies to enjoy.
Here we are in the depths of winter and it is cold. How would you like to heat your home with one tenth the wood you would normally burn in your conventional wood stove? How would you like to have the heat from your fire last through the night into the next day so you don't have to wake up to a freezing house to start the fire again? How would you like a heated couch to lounge on during those super cold nights? How about burning so cleanly that only steam and CO2 are produced? Does all this sound like a sweet deal? Welcome to the world of rocket mass heaters, where you burn less wood, generate more heat, and keep that heat for days!
How is this all possible you ask? As it goes with permaculture, it all comes down to excellent design. This stove is designed in such a way that only the end of the wood is burning and the fire is directed to an insulated chimney (heat riser) where it can get even hotter, thus burning more completely and cleaner. The insulated heat riser is capped by a barrel, but leaving enough space for the exhaust to pass by and cool as it falls down the side of the barrel. This exhaust is now routed through ducting encapsulated in a mass. As the exhaust travels through the ducting in the mass, it transfers its heat energy to the mass, warming the mass. Finally the exhaust is sent out the chimney after having transferred most of it's heat into the room.
The secret is in the heat riser. Here is where the magic happens. Instead of trying to capture the heat directly from burning wood, the rocket mass heater channels that fire to a space where it can get even hotter; the insulated heat riser. Here the fire gets crazy hot because it is in an insulated space. This increase in heat leads to the smoke from the initial burning of wood to also burn, making for less smoke and adding to the fuel the fire has available to burn. This is basically creating a chimney fire all the time. Nearly all the available combustible material is burned in this insulated heat riser. As the fire reaches the top of the heat riser, it comes in contact with the encapsulating barrel and rapidly cools as the barrel sheds heat into the room where the stove is located. The remaining gasses, mostly CO2 and steam, fall down the sides of the barrel as they cool and are pumped to the ducting running through the mass by the convective current produced by the crazy hot burn in the heat riser sucking air in through the wood feed and forcing the exhaust gasses through the ducting to heat the mass.
Below is an excellent illustration showing the vertical wood feed, horizontal burn tunnel, insulated heat riser, barrel capping, and gasses being pushed through the system.
Let's examine the sideways burning fire. At first it doesn't seem like this is possible, but upon closer inspection we can see how it works. Again, it's all about the heat riser. As warm air rises up through the heat riser, fresh air must replace the air that is rising and the only place for this air to come from, is through the wood feed. So the hotter the fire burns, the more fresh air it needs and the harder it sucks that air through the wood feed opening. The fire is then pulled along sideways through the burn tunnel with the suction created by the hot air rising in the heat riser, making a fire that burns sideways. This suction also prevents smoke from coming up through the wood feed hole into the house.
Why make the wood feed vertical? A couple things happen when the wood feed is designed vertically. First of all, the wood self feeds. As the fire burns the ends of the wood away, the wood falls down into the fire with the help of gravity. If the wood feed were horizontal you would have to continuously push the wood into the fire to keep it burning. So you can fill up the wood feed with wood and be sure it will all burn without your having to help it along. Secondly, with a vertical wood feed, you have only the ends of the wood burning and fresh, warm air being sucked in past the yet-to-be-burned wood. This allows for a better burn. The wood is warmed prior to reaching the point where the fire is, instead of having cold wood sitting on top of the actual fire and energy from the fire being used to warm the wood. The energy from the fire is going towards burning the gasses released by the wood as it ignites, which are being sucked through the burn tunnel and up the heat riser by the convective current.
So far so good? The wood is standing vertically in the wood feed, while burning just at the tip by a fire that is burning sideways, which is being sucked into an insulated heat riser, by a convective current, where temperatures get crazy hot and all the smoke burns. As the heat and exhaust exit the insulated heat riser, they are captured by a barrel which caps this system. The barrel serves two functions. Firstly it captures the exhaust gasses being pumped out of the heat riser and channels them to the exhaust duct. Secondly, it acts as a heat sink allowing the captured gasses to cool as they exit the heat riser and fall down the inside of the barrel to the exhaust duct. The barrel becomes hot and acts like your conventional wood stove by providing immediate heat to the room. You can also cook, or heat water on the top of the barrel if you so choose.
Now we get to the next awesome part about the rocket mass heater; the mass. Here is where all that excess heat energy which wasn't dispersed by the barrel gets stored. This is the part that stays warm for days after the fire has gone out!
The exhaust that is captured by the barrel is pumped, using the convective current (created by the heat riser) to push now, through thirty plus feet of ducting that is encased in a solid mass. The remaining heat energy in this exhaust is transferred to the mass, where it is stored for an extended period of time, warming your butt long after the fire goes out. In conventional wood stoves, all this extra heat energy is used to pump the exhaust straight out the chimney and is lost to the outside. Conventional stoves have exhaust exit temperatures around 600 degrees F, where the exhaust exit temperatures on the rocket mass heater are more like 150 degrees F. A massive amount of heat energy being captured and stored there for later use.
Here is a picture of a beautifully finished rocket mass heater. You can see the large mass bench where the exhaust runs all the way to one end and all the way back before exiting through the chimney behind the barrel.
On top of that massive decrease in exhaust temperature, there is an even greater reduction in dirty smoke released through the chimney. With a conventional wood stove, all the burning is done in the steel box which directly radiates the heat into the room, making it so the temperatures reached when burning are not hot enough to burn all the smoke. Much of this heat is also needed to pump the smoke out the chimney. Under ideal, super hot burning conditions, with really dry wood, the wood stove would do fairly well and produce relatively little smoke, but that's not how it works in real life. Most people don't run their wood stoves at maximum temperature and don't have super dry wood. Usually semi wet wood is burned with the damper closed to make the fire last through the night and keep the house warm. When burning in this manner, huge amounts of unburned smoke are sent up through the chimney creating unsafe creosote build up on it's way to polluting the out doors.
But with the rocket mass heater, this problem doesn't exist. The rocket mass heater only burns at top speed, which is super hot because of the insulated heat riser, and this super hot burn causes the smoke, which would go up the chimney in a wood stove, to burn and turn to CO2 and steam. This burn is so clean that it produces one one thousandth (1/1000) the amount of smoke of a standard wood stove! That's so clean that people are reporting using these in cities where wood stoves are banned because of the smoke they generate, but the rocket mass heater exhaust just looks like a dryer vent and no one asks any questions. Amazing!
How did rocket mass heaters come about and why haven't I heard of them if they are so awesome you ask?
Let's start with how they came about. Nothing about these wood burning stoves is new technology, only the way the parts have been arranged is new.
Heating a mass to store that heat for extended use has been a documented practice for a long time now. Some bread ovens in the middle ages, for example, were enclosed masses, warmed by an initial fire, with the heat stored in the mass after the fire went out, to actually cook the bread. Masonry heaters are also a good example of storing heat in a mass to keep a room/house warm after the fire has gone out.
Channelling fire through a heat riser is also old technology, used in such as examples as the pit stove. Here the people would dig a pit and dig a tunnel over to the bottom of the pit to feed the wood through and make the fire rise up through the pit. Positioned above the pit is the hot cooking spot. A fire channelled up through a concentrated space burns hotter.
The actual rocket stove J-tube design, which is the combustion unit part,where the wood feed and burn tunnel make up the small vertical and horizontal parts of the "J" and the heat riser the longer vertical part, seems to have been invented mostly by Ianto Evans along, with a group he was working with to provide efficient cook stoves to people in Africa. The original concept of concentrating a fire through a heat riser to make it burn hotter, was taken and innovated upon to make for even better combustion and ease of use. This is the most critical part of the rocket mass heater. It makes the whole thing sing. It seems that it was Ianto again who went and added the barrel cover, over the heat riser, to capture the exhaust gasses and heat and direct them through a mass to store heat for future use.
So the rocket mass heater has only been built for about the last 35 years or so. Being a technology that doesn't make someone a lot of money, it hasn't been heavily marketed. Factors along these lines would probably be why you've never heard of rocket mass heaters, but it's all changing now. The word is really getting out there and further innovations are happening all the time, thanks to folks like Ernie and Erica Wisner, Paul Wheaton, and all the other folks out there on permies.com. Go check out their sites for tonnes more information on these super amazing wood burning stoves.
I have been compiling (more like hoarding) information about RMH's for a while now, and am trying to figure out some things regarding designs, construciton, etc. so that when I build my more permanent abode, I have a good head-start on things.
Context: I want to build a RMH that uses a tamped earth floor as the mass and heats the house via radiant heating through the floors. I figure that since heat rises, and no one likes a cold floor, that I can kill many birds with one stove. Follow Up: Will insulating the barrel prevent the secondary burn of the flue gases that make these stoves efficient and effective heaters? Or, will it allow a much stronger draw and retention of overall heat that can be transferred into my mass and utilized later (ideally what I want)?
My first of two questions is, how does completely insulating the barrel affect performance of the stove?
Follow Up: Will insulating the barrel prevent the secondary burn of the flue gases that make these stoves efficient and effective heaters? Or, will it allow a much stronger draw and retention of overall heat that can be transferred into my mass and utilized later (ideally what I want)?
And my second main question is, why does the exhaust pipe have to come away from the burn chamber instead of going directly underneath it as you pass the hot air into your mass? What I mean is, since fluids flowing in two different directions exchange heat the most efficiently, and because we want a hot chimney downstream (think of the air like a river) of the burn chamber, wouldn't it make sense to have a section of your chimney pass under the burn chamber to guarntee this effect and also more effective heat transfer to your mass? Or, would doing so rob the burn chamber, and ultimately the whole system, of necessary heat that will be used to combust the flue gasses secondarily and thereby generate the efficient burn we all know and love?
Sometimes, not knowing how to do something is the best place to be: enthusiasm with no "box."
It has been reported that adding insulation to the barrel reduces the "push" of the combustion core through the rest of the system. However, others have reported that the downward gas flow in the barrel adds little or nothing to the draft created. I think that if you have a good chimney at the end of your system, you could do without the radiation from the barrel. The secondary burn (really just a continuation of the initial burn unless you actually introduce new air into the system before the riser) happens in the riser and is complete usually before it reaches the barrel. Keeping the barrel hot could only increase the amount of combustion, not decrease it.
The exhaust duct can lead in any direction from the bottom of the barrel as long as there is sufficient space for good airflow. You don't want to take any heat from the combustion zone where the gases are burning, as that will lower the combustion temperature and decrease the efficiency. If the duct passes near the well-insulated combustion zone, that should not be a problem.
Thanks for your response Glenn. I am still unsure about the insulated barrel causing a loss of "push" through the core itself. I would imagine that retaining heat would provide more available resources for the mass farther downstream. I guess on explanation is that the drop in temperature from the burn chamber to the barrel is what provides the first "push" and then having a hot chimney continues the pull with its negative pressure? But the same can be said of having more heat available in the system initially that would heat a chimney more... Any thoughts to help with my spiraling thoughts?
Sometimes, not knowing how to do something is the best place to be: enthusiasm with no "box."
The first and possibly strongest push is the gases heating and expanding in the riser. There might be an assist when the gases cool in the barrel and fall. Obviously retaining heat in the barrel (not radiating it there) leaves more of it for the mass. In any case, this should not affect the chimmney draw, as the same total heat should be taken out of the system by design no matter the location of extraction, leaving the chimney temp the same. Insulating the barrel will not make the system more efficient, it will just change the characteristics of the heat output, keeping more of it for later and less at burn time.
The comments about insulating the barrel come from practical experience, not theory.
Stoves that worked well when the barrel was exposed, started having problems after someone wrapped more than 1/3 of the barrel in cob, perlite-cob, or other insulation (I think one used rock wool).
In the cases where the wrapping was removed, the stoves worked better again.
Another practical aspect is cleaning/inspection access. (If fly ash builds up inside the barrel, or you drive the temperatures up to the point where something holding the insulation around the heat riser starts to fail, you may need to remove the barrel or at least an access port like a removable lid, in order to keep the heater from choking itself inside the barrel.)
There are other masonry stove designs with a masonry bell instead of a barrel, with cleanout access at the bottom, and a shorter heat-sink bench or no heat-sink bench. These designs put a little more heat out the chimney, and the proved-out designs do fine on the chimney draft as their primary draft. Max and Lasse's sidewinder/cabin-cookstove design is one example. There are a lot of European contraflow stoves with similar designs, and some Danish rocket stove experiments that Leslie has photographed.
But the rocket mass heater J-style designs, using the barrel, are proportioned around the thermosiphon "pump" effect of 1500 F heat-riser-upward, 600 F barrel-cooling-downward. If you're basing your expectations on that line of rocket stove experiments, insulating the barrel is a proven trouble-point.
If you create a hybrid design by combining features from two or more lines of masonry heater, in my experience, you can expect to go at least half-a-dozen rounds of testing and fixing before you get results that are comparable in reliability, efficiency, or safety to the parent lines. Sometimes you get lucky in round 2, sometimes it's more like 10 iterations before you work out all the kinks.
If you make one change to a proven design, with the option to remove that change (for example, allowing enough clearance around your barrel that you could safely remove the insulation if it made your draft unreliable), that seems a bit more controlled, as you're building on a proven design not starting fresh.
(Masonry heaters intrinsically have a little more variation than a "model" or "kit" stove, but each style is dozens, hundreds, or thousands of iterations into developing its rules of thumb for predictable performance).
Thank you for the expanded statement about barrel insulating. I had seen much of it in passing, but to have confirmed that multiple heaters experienced the same effect, and that removing the insulation reversed the effect, seems like enough to base advice on without wiggle statements.