Heat Your Home with 1/10 of the Wood Used in Conventional Wood Burning Stove

There is an incredible opportunity to heat your home without using any fossil fuels.  The answer lies in “Rocket Technology” and “Thermal Mass / Heat Retention/Release”!!!

As those of you know who have used conventional wood burning stoves, a few to several cords of wood are burned each winter, depending on the size of the home or space.  Those that have used conventional wood burning stoves, and then transitioned to using a ‘rocket mass heater’ (also sometimes referred to as ‘rocket stoves’) have observed a decrease in wood consumption – rocket mass heaters used 1/10 the amount of wood that a conventional wood stove uses.

I don’t know how the efficiency of a rocket mass heater would compare with the efficiency of the clay plastered masonry ‘bread oven’, and the smaller oven made of clay tiles in the kitchen in this sustainable home in Latvia, but I know that his ‘oven’ is incredibly efficient.  Who knows, perhaps the Latvian builder utilized rocket technology in his masonry ‘bread oven’?  But from looking at the pictures, I don’t see that he did.

In the builder Jacob’s words… “Exterior measurements of the house is [21.33 feet] x [42.65 feet]. Living space in both floors are [1,292sq/feet]. The house is being heated with clay plastered brick bread oven and smaller oven made of clay tiles in the kitchen (the picture above is of the larger of the two – the bread oven).  To heat up both floors of the house, when outside it is [?] only [the] small oven is heated once a day.  When freeze gets below [?], we heat up the bread oven.  Once it is heated, because of it’s thermal mass of 5 tons, it keeps the warmth 2-3 days. To heat up all the house in the winter time we use not more than [1.1 cords] of dry firewood.” (5 cubic meters (which is 100 euros for 6 months).

There must be something really saavy about his design, if he only needs to fire it up every 2-3 days!  It sounds like it is just as efficient as a rocket mass heater.  Rocket mass heaters are at least 9 times more efficient than the most efficient conventional wood stoves, as explained in this video.

Another large masonry stove built by the author of this Mother Earth News post, is almost 3 feet x 6.33 feet by 7.5 feet high, and the firebox is 40 inches long. I’m not sure where this author lives, but I’d be surprised if his winter temps are lower than -5, and he says they have to fire it twice per day (morning and evening).  The Mother Earth news blogger didn’t say how many tons his bricks weighed, but the bricks in Jacob’s Latvian design weighs 5 tons, as stated on his website.  Granted, the Mother Earth News builder used a masonry stove design from a library book, but the size of his stove sounds like it is at least somewhat comparable to Jacob’s Latvian masonry stove, doesn’t it?  As the two designs appear to be somewhat similar in size, the amount of bricks (thermal mass) used in the design seems to be only partially responsible for the efficiency of Jacob’s masonry stove.  I wish I could get a look at the inside design of Jacob’s Latvian ‘oven’/heater!

Many people’s floors may not be able to safely support 5 tons of bricks, and some people find the expense of that amount of brick to be too expensive, however, I know that lots of bricks can be found for free, especially if you have the ability to haul the bricks yourself.

After reading the www.permies.com forum and other online forums, I believe that rocket mass heaters are probably more efficient than straightforward masonry stoves (unless you have the capability to build one like the Latvian design above), but as the rocket stove ‘experts’ say (rocket stove builders who share their knowledge on the http://www.permies.com and donkey pro board forums), efficiency probably depends on

  1. the type of rocket core
  2. the heat harvesting solution (thermal mass?)
  3. and on the chimney

Testing conducted by Peter Van Den Berg has proven the “j-tube” rocket mass heater design to  be a very capable contender on the efficiency side, especially when built with ‘bells’ and ‘batch box’ features: http://donkey32.proboards.com/thread/355/small-scale-development?page=6#page=2.”

You may already be asking, what is the difference between a rocket mass heater and a rocket stove?  The Green Energy Experimenter wrote a great blog post on this which states: “It seems that people confuse the term rocket mass heater with any type of rocket stove heating system. While all rocket stove heaters have a rocket stove at their core, how they handle the heat output is where they all differ.

In this post, I’m focusing mainly on rocket mass heaters, the goal of which is to heat a home, rather than cook food, as is the case with rockets stoves.

If you want to find out more about rocket technology in use as a stove to cook food, check out these videos:


A major bonus to rocket mass heaters and masonry mass heaters is that the heat is released radiantly, meaning, there is no ‘forced air’ system!!!  This means less dust, and an incredibly warmer and more relaxing environment.  Once you experience radiant heat, you will never go back to anything else!!

Rocket mass heaters designed to be part of the indoor home environment may cost more than one designed to for outside the home, because you will want the indoor heater to look nicer, and you will want to pay more attention to detail to ensure it is built correctly.  Paul Wheaton says that one can expect to spend $400 to $800 in materials for a nice indoor rocket mass heater; however, he also says that some people have built rocket mass heaters in their shops for less than $20 – using a lot of scavenged materials.

Here’s an example of a build using no materials other than ‘cob’ made of clay, water, and straw, I believe!!!  https://permies.com/t/52509/Clay-Rocket-core-Bell-RMH

So, are you interested yet? 😉

If so, I’m sure you’re asking,

‘but what IIIIIIIS a rocket stove?’ 

Get a crash course by visiting Paul Wheaton’s www.richsoil.com site.  It doesn’t take that long to go through, and you will need this basic knowledge to understand the rest of this post.

The following is my attempt to explain rocket technology in the context of a heater.

Think of a cabin in the woods which uses a wood stove – you know that big, black steel box with a chimney.   Now, take that out of the cabin, and replace it with a rocket mass heater.

Rocket stoves were invented by founders of the Aprovecho Institute.  I’m not 100% sure, but I believe these people are Dr. Larry Winiarski, and Ianto Evans.  Dr. Larry Winiarski began developing the Rocket Stove in 1980, standing on the shoulders of two previous technologies:

  1. the VITA stove, which was designed by Sam Baldwin
  2. the principles of the systems developed by the Romans in hypocaust heating and cooking systems.

One of the simplest examples of rocket technology is a simple cooking stove.  Here are three pictures of rocket technology used for cooking.


insulated cook stove

cooking aprovecho
taken from the Aprovecho Institute

How is a rocket stove different than a Camp Fire or Wood Stove?

Camp Fire (or even a Fireplace)

When wood is heated/burned, it releases its volatile compounds (which make up approx. 50% of the energy contained in wood).  Most of these compounds require at least 1,300 degrees F to combust.

In a campfire, wood combusts, burns and emanates heat in small area around and above the fire.  I don’t know how hot everything gets, but apparently not hot enough to burn all of the gases produced (hydrogen, carbon monoxide, etc), which rise into the atmosphere, and the solids (ash, embers, etc) which fall under the wood and are leftover after the fire has gone out.

Why doesn’t it get hotter in the combustion area of the fire, so more stuff in the wood is combusted?  Because there is too much cold air introduced around the combustion area, as the fire is ‘open’ to the surrounding air.  Thus, the energy/volatile compounds in the wood that is lost/not combusted/not burned/not utilized inlcudes:

  • Gases (lost to the atmosphere)
    • Hydrogen
    • Carbon Monoxide
    • Methane
    • Carbon Dioxide
  • Solids (remains in the fire pit)
    • Carbon Particulates
    • Black Ashes
    • Embers

The Aprovecho Institute estimates the efficiency of an open fire is 3-7%.

3 stone fire
taken from The Aprovecho Institute

Even though a design like the 3 stone fire pictured above can increase efficiency of an open fire to 11.7 – 17.8% (depending on the expertise of the fire tender), especially when used inside of a cabin or other structure, like I saw in Guatemala, it is not healthy, as proven by the blackened walls and ceiling in every kitchen building I visited in Guatemala in which an open fire was used.  Many people suffer serious disease from inhaling the cooking smoke.  Also, can we make a cooking fire even more efficient?

felicia making tamales in guatemala 2013
The kitchen building where the Guatemalan family and I prepared tamales which were then cooked in an enormous clay pot over an open ‘three stone fire.’

Wood Stove

In a wood stove, as there is a cast iron structure around the burning wood (combustion area) limiting the amount of cool air that can reach the fire, and because there is a chimney creating a draft (increasing combustion), some percentage of the gases normally lost to the atmosphere in a campfire, are burned, but probably more than half of the gases are still lost as they exit in the form of smoke through the chimney (with its accompanying air pollution and creosote danger).  The solids remain, unburned.  — a lot of energy in the wood is still not being utilized.  I’ve talked with people who insist that their wood stove, like a Harman, is as efficient as a rocket stove, but ask Peter Van Den Berg, who measures everything under the sun with fancy meters and calculations, and it just doesn’t seem to be true.  Harman wood stoves are more efficient than other wood stoves, but it doesn’t seem like it can really compete to a properly designed rocket mass heater.

Why doesn’t the combustion area in wood stoves get as hot as the combustion area in rocket stoves?

The material used to envelope the combustion area in a wood stove is cast iron, which is a heat-conducting material.   While it seems like a good idea to harvest heat from the combustion process so it can radiate into the room to heat the occupants, this is actually counter-productive, because it steals heat from the combustion zone, or, to put it another way, it cools the combustion zone.  When the combustion zone is cooler, you get less combustion of the gases and solids.  The gasses and solids which are not combusted, are then lost, as is shown by the chimney smoke as well as the solids which remain (ashes, creosote).  In conclusion, we don’t get as much heat for our ‘buck’, or, our wood.

wood stove

Rocket Stove

In a rocket stove, the combustion area is enveloped with non-conductive materials that are also insulative.  This allows the temperature of the combustion area to greatly increase….and almost everything in the wood is burned (depending on the efficiency of the rocket stove).   The gasses and solids have a dance party in the combustion chamber at hight speeds, swirling and twirling around as they combust.  In a properly designed rocket stove, you will not see any smoke coming out of the chimney, and there will be a lot less ash and solids left after the fire dies down.

So, who cares?

The hotter the combustion area, the more complete combustion we have, meaning, the more energy we are able to extract from the wood fuel, and that means, we can get more heat from less fuel!!!!  That means less fuel is needed!!!   That’s less wood chopping, less work, less money spent, and more trees left in the ground, or left for other purposes!  

Some Basic Designs of Pre-Heating & Combustion

How air reaches the fire is important in a rocket design, because how it reaches the fire determines how hot or how cold the air is when it does reach the fire.  Hot air assists in complete combustion, and cold air decreases combustion.

In the picture below, air is reaching the fire in what is called a “downdraft” or “downfeed.”  The air is coming in through the top, and traveling down.  This  means that ambient air is sucked down towards the fire and is heated while on its way (pre-heated).  If the air coming into the combustion chamber is cold or colder, it can reduce the temperature and decrease combustion…..so, to prevent too much cold air from coming in too fast, a brick or a stone is placed over part of the opening on the top, as is shown in the picture below (some people make more permanent ‘lids’ or vents to achieve this function).

This design is known as a “j-bend” or “j-tube.”

taken from The Aprovecho Institute

The pictures below compare three different wood feed and preheating of air designs.  I think the downfeed design is convenient as the wood advances or ‘feeds’ itself as the bottom of the sticks burn and disintegrate, whereas you’d have to move the wood along as it burns in a sidefeed design.

The air is coming in through the top, and traveling down, in the “downfeed/downdraft” design.  The air is coming in through the sides (at the bottom) of the second design, and the air is coming in through the side (toward the bottom, where the wood is being fed), in the third design.  Where the air is being sucked in, is what determines if it is a “downfeed/downdraft” design, or if it is a “sidefeed” design.

3 ex
taken from The Aprovecho Institute

Here are some fun examples of side feed systems:

concrete block sidefeed
other sidefeeds
non insulated sidefeed two pot system
This design is made using cob, which is non insulative. However, the design is a wonderful improvement over an open fire, because it takes the smoke out of the house. Any vapor that is left after combustion is taken out through a chimney. This may not be the most efficient design, but it has improved the health situation in this home.
 justa update
justa update 2
Pic taken from the Aprovecho Institute. These stoves can be built with low-mass ceramic material. A co-op in Honduras called Nueva Esperanza makes durable material from sand, clay, horse manure, and tree gum. Cultures around the world have developed refractory clay mixtures that stand up to the heat of flames. There are cheap commercial pourable or moldable refractory cements available as well, like Spurlite 60. (Matt Walker uses Spurlite 60 in his designs).

Components of a Rocket Mass Heater

The most common rocket design for a permanent home installation is a “j-bend” or “j-tube,” which is shown in the pictures below.

diagram 1
taken from http://www.RichSoil.com (Paul Wheaton), and adapted by me.
  1. To start the fire, place some paper or kindling or tinder or something that quickly catches fire at the bottom of the ‘wood feed‘.  The wood feed is where the sticks are inserted vertically, in the picture above.
  2. As the fire-starting material burns, the fire travels sideways, further into the rocket design, through a space which is called the ‘burn tunnel.”  The fire burns sideways due to the draft created by the whole design (there is air flowing through a path from beginning to end – follow the arrows in the picture.
  3. Then the fuel (sticks or other biomass) is inserted into the wood feed.  The bottoms of those sticks begin to burn, and the fire increases and continues to burn sideways.  As the bottoms of the sticks burn, they disintegrate, and the sticks fall, in a ‘self-feeding’ motion.
  4. The compounds in the fire travel upwards, swirling and dancing at high speeds, combusting inside of the ‘combustion chamber‘ (also known as the ‘riser’).  The gases are VERY HOT at this point!
  5. If you keep following the arrows, you can see that the after the compounds combust in the combustion chamber, the resulting heat from this process continues to travel up over the sides of the combustion chamber and down in between the outside of the combustion chamber, and the inside walls of the structure surrounding the combustion chamber.  Often, a 50-gallon steel barrel is used to create this enveloping structure.  If people want some ‘quick heat’ to radiate into the room and warm the occupants quickly after starting the fire, they leave part the top and sides of the steel barrel exposed, but if people want to store more of the heat so that it radiates for a longer period of time, they cover the steel barrel with cob and other materials that retain heat (thermal mass).  Some people want both, so they cover PART of the barrel with cob, etc.
  6. If some of the barrel is left exposed, some of the heat is released through the sides of the barrel, as the barrel is made of steel, which is non-insulating, and doesn’t retain heat for a very long period of time (steel is not considered a thermal mass material).  Even though some heat is released in this scenario, the gases inside the combustion chamber remain extremely hot (above 1,100 F), and sometimes much much hotter, which burns EVERYTHING, including the ‘smoke’ you would typically see in a wood fire……full combustion.  As the fuel burns in the combustion chamber, convection draws new air into the combustion chamber, ensuring that any smoke from smoldering wood near the fire is also drawn into the fire and up through the system.
  7. Next, the exhaust travels through the manifold, and through a tunnel (the tunnel is often called a ‘flu’ and if often made out of metal ducting), which is often surrounded by cob or other thermal mass materials such as stones or bricks filled in with cob, to be used as a bench for snuggly warm sitting — this ‘thermal mass’ of cob (surrounding the flu) is what absorbs, and then slowly releases, a majority of the heat, which is part of the reason these systems are called rocket MASS heaters (mass, as in thermal mass).

Finally, the exhaust exits the system.  This design is shown with the exhaust exiting out of the flu, rather than a vertical chimney we normally think of.

Why is it called a “J-tube?”  In this second picture below, I’ve circled the components which make a “J” shape in the dotted green line, thus, the “J-tube” name.

taken from http://www.RichSoil.com (Paul Wheaton), and adapted by me.

In this third picture below, you can see that if the steel barrel was left exposed on the top you could use it as a cooking surface.

j-bend system

And here is a diagram, followed by a picture of what a J-tube + Flu Rocket Mass Heater looks like when finished.

RMH with bench diagram

finished of diagram.jpg

Here is still another design – how it looks when installed and finished!  Do you see the steel drum?  They have left the top half of the drum exposed to the air to get some ‘quick heat,’ and the bottom half of the barrel is covered with thermal mass material (cob).

finished 2.jpg

I had to include this design below too, because the most exterior layer (about 6cm thickness) of the cob on this rocket mass heater was made of 2 parts sand, 1 part donkey dung, and 0.5 parts clay.   The dung makes the outside very hard and resistant to cracking, but some people are too squeamish to use dung, although many people in the world handle and use dung every day as a fuel source.  The lady who built this rocket mass heater has some style!  She found a very unique fancy barrel!

Screen Shot 2018-03-20 at 8.10.08 PM

Safety / Longevity

You might ask, as I did, how long a steel barrel will last, since the temperatures hitting the inside of the top of the barrel are so high.  I would think it would disintegrate over time.

In the words of wonderful participants at http://www.permies.com: “Nobody really knows how long a metal barrel used in a RMH [rocket mass heater] in a dwelling or dry shop will last, because the oldest ones installed (20 to 30 years ago) are still reportedly working.  For the core (burn tunnel and combustion chamber), hard firebrick as the Wisners use will probably last numerous decades, though the first brick in the burn tunnel roof sees exceptional thermal stresses and may fail sooner. That brick also happens to be the most accessible for replacement, which is good. Other more insulating but softer components will give higher performance but may have shorter lifespans.”  — I’m wondering if ceramic fiber board would fall in the category of “more insulting but softer”?

The builders and permies participants also say: “It would be wise practice to build your RMH with the core (burn tunnel and combustion chamber) accessible for inspection and possible future repair. This can often be done without tearing up the casing if planned right.”

Matt Walker says: “there are examples of heaters using barrels that have been in use for longer than 20 years at this point. Your concern is valid as any metal in the core is doomed to fail in short order. The reason the barrels can hold up is not due to a difference in metal, but rather that the thin metal exposed to the space can shed heat quickly. Only the top is exposed to the hottest gasses, which are quickly cooled, and the remainder of the barrel acts to conduct the high heat away from the hot spot on top. Add to this low oxygen and no more combustibles left in the interior gas stream, and you have a situation that does not deteriorate the metal in the same way the core area deteriorate. The core areas are insulated, increasing the heat, and as well are actively combusting which helps to strip carbon from the steel in the combustion zone, causing spalling and failure. It is due to these factors that the radiator barrels and thin steel can hold up indefinitely.”

Breaking it Down

Up until this point, we’ve been looking at the rocket mass heater system as a whole, but now, I’d like to break the system into two parts:

part 1 part 2

Regarding Part 1….what I call The Rocket Part

part 1

Up until a few years ago, the rocket mass heater community built heaters with this “j-tube” design.  However, as the rocket mass heater community and brilliant innovator Peter Van Den Berg continued to experiment and refine the “j-tube” rocket mass heater designs, a second design evolved, which is called the ‘batch box’.  Instead of periodically inserting fuel into a vertical wood feed as occurs in the “j-tube” design, you place a load of fuel in the ‘batch box’ – all at once.  Putting a load  of fuel in will seem more familiar to most people, as this is what we do when we create an outdoor fire, or when we fire up a cast iron wood stove…we gather a load of wood, put it in, and light it up.  This results in more of an “L” shape.

batch box real pic.PNG

“It appears that for bell sizing, an 8″ J-tube is fairly similar in capacity to a 6” batch box designed according to Peter van den Berg’s formulas at batchrocket.eu.” – wonderful contributor at http://www.permies.com.

You can find more information about batch boxes in the DVDs I mention below in the “How-To” section, or you can find information about them at the following links.

batch box components

Regarding Part 2…or what I call Using the Heat / Heat Storage (and Release) Part

part 2

Once the combustion process is complete and the exhaust gases leave the combustion chamber/riser, the heat extraction can begin, via thermal mass materials.

There is a difference between (thermal) mass and insulation.  Good insulation is made up of little pockets of air separated from other tiny pockets of air by a lightweight, relatively non-conductive material.  Earth, especially rammed earth, doesn’t contain many pockets of air.  Good insulation resists the passage of heat, whereas thermal mass does the opposite, absorbing heat.  Insulative, low-conductive materials (wood ash, ceramic fiber, refractory cement, or heat tolerant firebrick, or clay mixed with lots of straw or fiber) are used around the burn tunnel and combustion chamber, whereas thermal ‘mass’ materials (clay, sand, stone, brick, water) are used to store heat in part 2 of a rocket system.

If heating a home is the goal, it is important to capture heat from the combustion process into a thermal mass, which will then radiate the heat slowly into space over the course of the next several hours or even days, depending on the system design and materials.  Heat is ‘captured’ or absorbed by placing a significant amount of thermal mass around the exhaust after combustion — the thermal mass absorbs the heat from that exhaust.  There are different ways to achieve this, and how much heat you can store is dependent on the materials used and how the heat from the exhaust is transferred to those materials.

There are two common designs utilized to absorb the heat from the exhaust into the thermal mass.

  1. flues
  2. bells (aka stratification chambers)


The most common way to capture heat in the western rocket mass heater community up to the present time has been flues.  The hot exhaust from combustion typically travels through a serpentine path through thermal mass.  In the picture below, the serpentine path is created using ducting (pipe).  The path must be long enough to allow sufficient time for the hot exhaust to transfer heat to the surrounding thermal mass, but not so long that velocity is lost, which would cause the stove to ‘stall.’

flue serpentine
taken from Dragon Heaters

Often metal ducting (pipe) with a diameter the same size as the chimney is used, because ducting is easy to work with.  The ducting is then surrounded by thermal mass like stones or bricks, and cob.  The exhaust heats the ducting which then transfers the heat to the surrounding thermal mass, but you don’t have to use ducting.  Flues can be made with other materials – in masonry heaters, a path is created using bricks.  Often these brick flue diameters? are larger than the chimney to allow additional time for capture of their heat, but they are still considered flu designs since all the gases travel along the serpentine path ‘together.’

These thermal masses are often designed in such a way that they can be used as a sitting bench – how lovely it would be to relax on one of these benches and feel the radiant heat emanating gently!



With the bell, also referred to as a stratification chamber, serpentine flues are no longer necessarily necessary, as you can see in this video made by Paul Wheaton.  I captured some pictures and explanation from the video below…

This is an example of a rocket mass heater where Part 2 is a serpentine flu system. Normally, the half of the ducting which leads to the chimney is slightly lower than the first half of the tube coming out of the manifold but this is a low-tech drawing….:) This design forces the heat to travel to the end of the sitting bench and around to the very end of the ducting, which is next to the steel barrel. When the exhaust is at this point, the radiant heat from the combustion chamber and enveloping structure (barrel) warms the exhaust to a higher temperature, creating a thermal siphon. The thermal siphon pulls the exhaust even more through the system and up and out of the chimney, and outside.

Bells rely on stratification rather than forcing heat through a serpentine path, as occurs in the flu system.

The following pictures show early predecessors of the bell/stratification chamber.

A Kang bedstove relies on stratification rather than forcing the heat through.

roman hypocaust

When one is employing bells in Part 2 of a rocket mass heater, the first change made to Part 1 of the rocket mass heater design is raising the heigh of the manifold, as you can see in the picture below.


The second change made to Part 2 of the rocket mass heater when employing bells, is to simply make the sitting bench hollow, instead of filling it with serpentine ducting and thermal mass (the materials used to create the enveloping structure of the sitting bench should still be thermal mass materials).

As the hot gases fill the bench, the hottest gases rise to the top, and the cooler gases fall to the bottom (this is the practical definition of stratification).  Stratification ensures the bench is evenly warm all the way across.

As the vertical exhaust/chimney starts to warm from the radiant heat of the combustion chamber/steel barrel, a thermal siphon is created.  The exit point of the hollow sitting bench is at the BOTTOM of the hollow bench, so that the chimney easily pulls the cooler gases out which are already hanging out at at the bottom of the hollow bench (stratification).  This design is wonderful because less materials are needed, decreasing the cost and labor.

Bells have been utilized for a long time in Slavic masonry heaters.  On the Dragon Heaters blog I learned that “a lot of the basic research was done by V. E. Grum-Grzhimailo (1864-1928) in Russia in the early 20th century.  Subsequently, Igor Kuznetsov has been developing and implementing masonry heaters using bell chambers in Russia.  He has also written about the physics of gas movement to maximize heat extraction [absorption] and put much of his findings in the public domain.”

If two or more bell chambers are put in a series, the hottest and coldest gases in each chamber will be successively cooler.

bell sytsem with labels
taken from Dragon Heaters and adapted by me

I was confused at first regarding the entry and exit placement in the bell chambers.  If one is trying to maximize heat extraction in a bell system, ideally both entry and exit would be as low as possible.  The exit does not need to be lower than the entry if both are low and there is room above to stratify.  The key is to have the exhaust (exit) low in the bell, so gasses must cool and descend from the upper level in order to find the exhaust (exit).  Ideally the entry is low as well, to encourage stratification, but in practice it will work either way. The exit does not need to be lower than the entrance, but it should be low in the bell. Check out this build: https://youtu.be/ELt-41Majsk.

Here is a list of benefits that bells offer that flues do not, taken from Dragon Heaters blog.  I’m not a rocket scientist, and Dragon Heaters articulates these aspects better than I ever could:

“Hot Gases are not swept out with cold gases

In a flue based system, both the hot and cold gases are intermixed and carried at equal speed to the exit. By allowing the gases to stratify, only the colder gases are being evacuated and the hotter ones are trapped and remain in contact with the thermal mass until they have cooled.

Prevents damper induced rapid stove cool off 

Because flues sweep all the gases together, if the damper is not closed “in time” the remaining hot gases are swept away along with in residue heat in the flue. With bells, the hot wood gases collect and cannot escape until they have cooled, preventing rapid stove cool off from a damper left open too long.

Gas velocity losses reduced

As gases move through flues, they develop drag. Each turn creates even more resistance reducing the chimney’s ability to pull the gases out. Too many turns or flue runs which are too long can result in a stalled and failed heater.  Conditions are not always uniform, so when designing a flue system a “draft reserve” is needed to insure proper stove operation. The problem is that providing for additional draft margin, often means compromising on heat extraction capacity.

When heat extraction is done via bells, the travel distances and directional re-routing of gases is minimized, allowing heat extraction to take place without large frictional losses. Gravity separates the hot and cold gases without introducing any form of drag on the chimney’s draw.

Improved heat retention with less friction and less drag

In a flue approach, the longer the stove is run the hotter the flue walls become, decreasing their ability to absorb heat. However, a second chamber (or bell) will always be cooler (than the first one) and thus allow better heat extraction.  in a flue, the pipes nearest will combustion will heat up first, and the flue farthest will still be cool, and able to absorb more heat. The goal would be to burn until the flue farthest from the combustion has reached it’s optimal heat storage capacity, there is minimal drag introduced with additional bells compared with extra 20-40 feet of flue. 

Faster removal of ballast gases

Exhaust gases from burning wood are comprised of those gases which were part of the combustion process and those that were merely heated by proximity to the combustion. Gases that do not directly participate in the combustion are called “ballast gases”. For example, nitrogen, which comprises approximately 80% of atmospheric air, is a ballast gas. Ballast gases are not as hot and cool off quicker. In a bell system where gravity naturally separates the temperatures this allows the ballast gases to be removed 1st, providing more time for the higher temperature gases to transfer their heat to the thermal mass while not slowing down the overall gas velocity. If all gases are expelled at an equal rate, as in a flue system, this is not possible.”

More information on bells can be found:

The rocket stove community continues to innovate as we speak!!!

Matt Walker even developed a design WITHOUT a riser! — http://walkerstoves.com/walker-riser-less-combustion-core.html

Here is another “riser-less” rocket heater with cooktop developed by Peter Van Den BErg, called the Double Shoebox Rocket.  As I understand, it is the latest evolution of the batch box rocket mass heater style.

People are getting more creative with the rocket technology and its related concepts every week!  Here is a post detailing a design for a tiny home that incorporates several really innovative design concepts listed below.  Watch a video of the tiny home build and its innovator here.  The video begins with a barrel-style rocket heater designed for a tiny home, then moves onto the masonry rocket mass heater I’m discussing now, and finishes with a fascinating new experiment, where the stratification rocket heater is heating a hugulkultur bed, ultimately intended for a greenhouse setting!!!  I laughed out loud several times while watching this video.

Here is a person who took inspiration from the batch boxes to build his own boiler, which I found in this thread on permies.  It’s not a rocket, but it is pretty innovative.

“I‘ve done a lot of modifications to the boiler since I posted in this thread and have documented it all on video. Here’s my youtube playlist from start to finish: https://www.youtube.com/playlist?list=PLq-ETrB2scGH8-PKQOL0MPwEeEro-gC_T

Video #7 shows the old insulation after I removed it from the boiler. It held up well except for the very top, the constant abrasion of putting in firewood was too much for it.

This year, I switched to a more proper material. I used a ceramic fiber blanket, coated with a hard liner to protect it. So far, it is holding up so much better. For those who don’t have broadband to watch videos, I’ve got a lot of pictures on my blog: http://greenenergyexperimenter.com/wp/?cat=8

You’ll notice the heavy inspiration from batch burning rocket stoves in the videos and pictures. This setup creates more of a side/down draft combustion than a down draft found in commercial smokeless wood boilers. The firebrick in the back and secondary chamber all glow a brilliant orange to yellow when it is up to full temperature. The secondary injectors bring air back to that chamber whether or not the induction blower is going.”


What it boils down to are these three factors which will affects performance of the heater, but there are many other factors that will make or break your rocket mass heater.

  1. Efficiency of the combustion system
  2. Material used in the thermal mass for storing/releasing heat
  3. Design of exhaust path for absorbing heat


Designs & Ideas

Pre-made Cores (ready to ship)


Matt Walker’s recommendations for sourcing Ceramic Fiber, Refractory, Masonry, Glass, etc. – https://www.youtube.com/watch?v=6dC2fHGR3S8
ceramic heat risers – https://permies.com/t/53413/Inexpensive-vacuum-formed-ceramic-fiber

INSULATIVE CORE (burn tunnel + riser) materials in order of best performing to worst.
  • fire clay – Lincoln 60 for fire mortar, $10-$15 for 50# bag, masonry stores have
  • sharp sand/mason sand/silica sand, masonry store, or hardware store may have traction sand that may be masonry sand
  • try to find salvaged brick, put an ad on craigslist, Home Depot has $0.44 clay bricks
  • stone – nice flat stones for tops
PIPES (flu, if applicable, and chimneys)
  • flue pipe or half barrel – it can be ratty because it’s really just a form.  – try to salvage, put an ad on Craigslist


  • 15 gallon grease drums – ask at oil change stations, or fuel company yards.
  • i wondered if using a recycled barrel would be a potential source of toxins and found this thread:  https://permies.com/t/72034/gallon-drum-gas-heated.  I think using all brick and no barrel would be ideal, but brick will be more expensive.
  • bricks + mortar
  •  old ranges or stoves, old metal recycling places and junkyard, or habitat for humanity usually salvages the metal, and they don’t want the glass.  you should be able to cut the seal with a razor and carefully pry it off, sometimes the junk yard will want $5.


  • building stove doors, dampers, and hardware, as suggested by Matt Walker
    • 1″ ceramic fiber board
    • windows: 6″x6″ ceramic glass called Robax, manufactured by Schott, can be found on ebay.  you want rating of 1300 -1400 F at least.  it should ceramic glass, not tempered glass.  or you can find small pieces on ebay.
    • steel
    • optional seal made of fiberglass rope, pinned to fiberboard using small finish nails or tacks.
    • latch – roadcase hardware butterfly latch
    • USSC barrel stove kit doors are really easy to apply to 18g steel and use as a rocket stove firebox door. – These aren’t hard to modify to fit flat sheet metal
    • There is a side loading barrel door manufactured for Ugly Drum Smokers, which can be used and its easy to add wood stove door glass for a window
    • maybe a cast iron lid would work

Build Services

  • Kirk “Donkey” Mobert – I don’t know to what extant he provides design and build services, but you could post a request in his forum and ask. 
  • Erica and Ernie Wisner 
  • Max Edleson and Eva Edleson – ask them to create a cooktop mass heater for you, they also sell design plans –here are their latest builds
  • Work with Uncle Mud, info@unclemud.com.  As of March 2018, these were the goals Uncle Mud has and is currently offering opportunity #1.
    1. Build one with me. As a a $200 add-on to the ATC in Montana in July Paul’s deal is the best and the product will be more refined then. Plus the ATC is going to be great fun. If you want to build one with me at my place in Ohio I’m doing it about monthly. The next one in Ohio is March 16-17. The workshop is $300. Parts (if you don’t bring your own based on a parts list I provide you) are $300. You will take home a functional rocket mass heater. A prototype heats my doublewide in Cleveland for less than $100 per year. Other Cottage Rockets have been installed in Montana, Denver, Portland OR, Columbus OH and Cleveland OH (6 in Cleveland now). Email me at info@unclemud.com if you want to come.
    2. A ship-able rocket mass heater. These are actually easiest to produce because proper DIY instructions take time. These will come in Short or Tall (1 barrel or 2), and Full Mass (Cottage) or Hollow Quick Radiator (Workshop) configurations.
    3. A ship-able core (burn chamber and heat riser), preassembled around a 3d burn-out template, ready for you to put in whatever enclosure you desire to build your custom RMH your way.
    4. A DIY core kit consisting of the 5 special carved bricks, a 3d burn-out template, Ceramic Wool Riser Parts, and complete instructions to build your own Cottage Rocket from off-the-shelf hardware store components.
    5. A ship-able IKEA type kit, preassembled and packed in a cut barrel, with instructions, ready for you to assemble and fill.
    6. An 8″ model, a Mobert-style water heater, an Oven, a batch-box-in-a-barrel, and all sorts of fun. These will take a while because see above.

Rocket Mass Heater – Design Function

Building a rocket mass heater can be approached from many angles.  After you watch the 8-DVD set from Paul Wheaton linked above, you might begin thinking about what components you want in your ‘rocket’…is it just for heating your home?  or do you want to heat your workshop or community space?  do you want to incorporate a cooktop?  an oven?  or perhaps you’d like to design it to do double duty as a warming bench to sit on, or perhaps you’d like your system to heat water?   Is your home or workshop a small space to heat, or is it intended for an event space or workshop that is very large, and in which you need to heat very quickly?  Do you mind tending the fire for a period of time (‘j-tube design), or do you want the convenience of loading the wood all at once (batch box design)?

All of the questions will steer you toward a design that will best meet the needs of your space and lifestyle.   You can see many different functions which the rockets perform in the 8th DVD mentioned above, including an outdoor smoker with butt warmer, pizza oven, water heater, grill, and more.   Here’s a video of a “three-in-one” saute, oven, and water heater.

I think it is important to note that one rocket mass heater system may not meet all your heating needs…think about it – there are a lot of heating needs:

  • heating the home
  • cooking
  • hot water for showering or bath
  • hot water for washing dishes
  • hot water for laundry

Rocket mass heaters tend to actively burn fuel (wood, etc) for less than an hour a day or every other day in the winter, so you will need something else to cook on three times a day!   Also, you may consider installing a rocket mass heater on which you can also cook or in which you can also bake…but are you going to fire that thing up 3 times a day in teh summer?  There are a lot of things to consider….  Here is an excerpt from the http://www.RichSoil.com website which discusses designs for an additional rocket mass heater  – indoor and outdoor that can meet your needs.  Check the site often, as they keep it updated more often than I update this page.

DVD 4 shows a design that does very well for outdoor cooking and another design that could be used for indoor cooking. We are seeing a lot of new designs lately for indoor cooking. Here is one from our 2015 innovators event that is primarily for indoor cooking, but can also warm a small bench:

Example of Design, Features and Functions of a Rocket Mass Heater for a Tiny Home

Here you can see the build almost finished (on the left), and finished with earthen plasters (on the right).

Here is a list of design and building concepts used for a ‘tiny house’ rocket mass heater, and if you scroll down, you can find Erica Wisner’s follow-up comments regarding this design that I copied and pasted.  I wanted to include all of these details to give a taste of how many different ways one can go about building a rocket mass heater.

  • batch box used vs. J-tube
  • no barrel used
  • “casserole door”
  • the mass is upright vs. a horizontal ‘bench’
  • the mass is a stratification chamber
  • according to Kirk Mobert, this has a “single vortex “Cyclone” because the bell (stratification) chamber was a bit tight and [they] needed to push the riser off to one side. Otherwise, it’s a 4 inch Batch Box RMH, as described by Peter Van den Berg here: http://batchrocket.eu/en/
  • “the mortar is clay/sand, again using locally sourced clay soil for the clay and locally sourced sand too. Cement based mixes can’t handle the heat and should NEVER be used in stoves.”
  • Arch.  they used a plywood form to make the arches.  “The bricks are laid over the form and then it’s taken down and moved for the next row. The front and back wall of the arch –  filled it with bricks, brick bits and filled with cob.” Did they use anything to compensate the outward force at the bottom of the arch? Or is it so steep that it doesn’t matter?  They used a catenary arch to reduce thrust. The Bottom course comes just not quite flat, so thrust is minimal. We did put in some stainless steel straps just in case and we watched them carefully as we added arches. They never tightened or moved, hopefully they stay that way.”
  • If the builder would do anything differently, [he’d] “make a longer bell. It was too short and didn’t leave room for the riser to go in straight and allow flow to the chimney. It forced us to offset the riser to one side, which made the double rams horn pattern impossible. I think we made up for it pretty well with the single sided cyclone, but I wanted to look down and see those horns!!”
  • it is built a few inches away from the wall, and also a few inches above the floor.  “The stove never gets hotter than you can touch”.
  • “the chimney is attached at the bottom side and goes pretty much straight up through the roof.”
  • “Both primary and secondary enter in the same opening. There is a metal channel that moves secondary air to the throat at the back and an air slot below the door that feeds primary to the front.  That metal secondary air channel is the only part of the stove that can not be easily replaced with some kind of neo-lithic mud mixture.  I’m working on an idea…”
  • The firebox is 18 inches deep. The longest wood can be around 16 inches. The regulation box length, according to the numbers for a 4 inch batchrocket is just too short to be conveniently useful.
  • “I think that because my plaster mix is really light (wood ash and all) it insulates well, which helps get the temps up”
  • the heat riser is made from clay and perlite mix.  The inner form was the same piece of 4 inch  chimney pipe that was used for the chimney. They packed in the mix and slipped up the pipe as we worked.
  • The firebox mix is inended to be able to make anyplace in the world, by the poorest people. The door and also the metal P-Channel are the stumbling blocks to that. This door solution removes it from the list and I wish I can say it was my idea. Chris McClellan came up with it and kudos to him! Still need to find a reasonable replacement for the metal P-Channel.

I love that they used a casserole dish lid as a ‘door’ for the wood box; however, these lids are not recommended for many burns, because they can explode violently and be very dangerous.  Propery masonry doors are recommended, as well as ceramic glass – see list prior in this post for suggestions.

Erica Wisner’s follow-up comments:

Standard building brick usually works on an 8″ modular pattern once you add mortar… 3 courses is 8 inches tall, 1 brick is just under 8″ long (with mortar, they are 8″ apart), and 1 brick is just under 4″ wide. 

Our bricks were the 3-hole kind, I’d guess they were 3.5 x 7.5 x 2.5 or so (I’d go with “Standard Modular” from the chart above, but they could have been Standard or Series 70, I suppose). 

Because of the small size of the fire, and the chimney, the heat extraction balance is critical.  
This heater performed really well in that respect – once dry, it put enough heat out the chimney to get reliable draft, and the chimney was touchably warm but not excessively hot. 

Donkey talked about making it a little longer, to have room to center the heat riser – but if you did that, you’d also have more extraction surface. 
I might suggest also raising the firebox if you did lengthen the body, to balance out the surface areas, and to make it more convenient to load and operate.  
Ernie had trouble getting the fire started properly (people commented on the smoke on two different occasions) because he is tall, and unable to get down on his knees due to old injuries. 

Rough guess as to internal SA based on the pictures: 

Main Heat Extraction Surfaces of Bell:
The firebox moulding fills up the first few courses, up to the height of the ‘arms’ in front by the door, back to the heat riser. 

Above that, it’s 9 horizontal courses before the bell, and internal dimension is 3 bricks (omitting the half-brick wall thickness front and back), by 1.5 bricks. 
So the inside of that chamber would be about 24″ by 24″ by 12″.  SA = 2(24×24)+2(12×24)=1728 sq. inches.  

The inside of the dome is 6 brick-edges tall on each side, by 24″ long, so roughly (16″+16″) x 24″ = 768 sq. inches 
The end caps of the dome look to be maybe a square foot each, rough guess, or 300 sq. inches total. 

Total of the inside bell surface areas: 2796 square inches, or 19.4 square feet, or 1.83 square meters.

Lesser Extraction Surfaces: 
Not sure whether to count the firebox top; that would be an additional 12×24=288 sq. inches. 

The heat riser is in there (8″ cylinder, extending from the floor up to the rim below the arch); exposed SA about pi*8*24=602 sq. in.   
Not sure whether it counts for surface area; the mix of that and the firebox looked dense enough to store some heat, but they also function as insulation. 

Beside the heat riser, the path going down to the chimney is also about 8″ long by 4 or 5″ wide by about 12″ or 13″ tall. If we count it, it’s roughly 12″ by 24″ total, another 288 sq. inches 
I think Peter sometimes doesn’t count the bottom surfaces due to lower temperatures, so we may be able to omit this too. 

If we did count all those surfaces too, then we’d have another 1178 square inches, or 8 square feet, or 0.77 square meters. 
Total would then be 2.6 square meters / 27.5 square feet / 3974 sq. in.

Plus a bit for the firebox itself, if we like.  That was 18″ deep by about 8″ tall by 5″ wide, so 380 sq. inches or so. 
I believe this is usually not counted – however in this build, I think that block of refractory at the bottom was a significant factor in holding heat through the night. 

Another calc we could do might be total mass of the system. 
A cubic foot of this dense building brick weighs about 125-150 lbs.  A cubic foot of brick would be about 20 bricks. I’ll go with 135 lbs/cu ft for estimating. 

13 courses x 13 bricks per course + 6 bands x 8 bricks per band = 10 or 11 cu ft of brick. 
+ 1 cu ft arch ends and wedges 
+ 2 cu ft firebox and heat riser (I’m counting a 5-gal bucket of mix as 1 cu ft. for estimation purposes) 
= total of about 14 cu ft. of masonry in this project.
Thermal Mass (without footing)= 1850 lbs = 0.9 Imperial tons or 0.8 metric tons / 840 kilograms)

Incidentally, for US code purposes, this would definitely qualify as a masonry heater (over 1500 lbs per the US standard description).  
However if someone wanted to build a pre-fab model, it could also squeak in under the definition of a wood burning stove (1980 lbs max).  
If you can pin all that masonry together for shipping, and add a door, and replace the footing brickwork with legs, using not more than 130 lbs of additional steel. 

Again, the performance of this stove was lovely – the surfaces got very hot, but not dangerous to touch (a little hotter than I’d want my bath water, however the brick and plaster are less conductive so it was not painful to touch).  Heat storage is figured on mass x heat capacity x difference in temperature, so the hotter you can get the mass (within safety limits), the more heat it stores.  
Donkey really nailed this one, I don’t think it would be improved with either more mass, or less mass.  It’s probably within 2  cu ft of absolutely optimal, and may be optimal as-is. 

I will be interested to see how it does with its final coat of high-fiber plaster… I anticipate that will give it slightly more mass, insulate the existing mass for a slightly higher internal temperature without causing the external temps to become uncomfortable, and may slightly extend the heating curve later into the mornings. 


Build Materials

The wood feed, burn chamber, riser (called a ‘core’), and encasement can be made with fire bricks meant for high temps, refractory cement, ceramic fiber (for the riser?),  firebrick (wood feed?) or potentially, some type of cob mixture (could this be ceramic fiber?), as seen in minute 5:52 of this video, or cob mixed with wood ash, and the encasement can be either a combination of cob and a metal barrel, or bricks, or straightforward cob.

Metal barrel and ‘mud’ (cob) are easy to source (barrels are considered a ‘waste’ stream in many places) and cheap – making rocket mass heaters built with these materials accessible to almost anyone, but if you want something that ‘looks nicer,” you don’t have to use a barrel at all!  You can use masonry.

In an effort to streamline the building of rocket mass heaters, Paul Wheaton and team tried various methods to develop pre-made cores that could be shipped to people to decrease the overall build time of rocket mass heater systems.  As you can see in this video, the first attempts to build these shippable cores out of refractory cement (Kerneos ‘Ciment Fondu, and other brands) failed, but then Matt Walker discovered a way to build them so that they will not crack (using Sparlite 60 brand).   You can see a quick example of how the Sparlite 60 is poured into molds to make the cores at minute 4:50 in this video.  You can purchase instructions to make these cores yourself with refractory cement on Matt Walker’s website for a very cheap price.  You can also purchase plans for the entire build of various designs on the same site.  This site is amazing!!!

Matt makes fire clay for mortar to hold fire bricks in place for a batch box combustion chamber or anywhere where fire clay is needed.  He uses store bought Lincoln 60 Fire Clay and sharp mason’s sand at 1:3 by dry volume.  Some say clay from the bottom of creek bed would work fine instead of the Lincoln 60 Fire clay.

Finally, here’s a discussion and example of a build using only ‘cob’ made of clay, water, and straw, I believe!!!  https://permies.com/t/52509/Clay-Rocket-core-Bell-RMH.  Here’s an excerpt from that discussion:

“as far as i know, cob without sand will very often develop cracks when drying. it totally depends on the mixture of your clay. the clay will need the sand particles to form a structure.

often it s 1 part of clay to 1-2 parts of sand along with some short fibres for tensile strenght (chopped straw, animal hair …). your mixture will need testing at different ratios.Using tiny pebbles of pre-fired clay that I mentioned in the original post in this thread would probably eliminate or replace the need for sand in the cob mix (while adding insulation, in relation to sand). I would think that this, as well as grass fibres and sawdust (both saturated) would create a fairly insulated structure when dried and burned out. I was thinking that, if I was wanting it to really have an insulated burn tunnel and riser, I could make a double system, and fill the gap with bone dry pre fired clay pebbles. But I don’t know if that would be necessary, or if the saturation would be necessary. I would just like to maximize the insulative qualities that can be incorporated in the cob in a more primitive way.”

The builder of this RMH described the process as follows:

“I didn’t do much special with the cob mix; most if not all of it was my native clay with all stones larger than small gravel picked out, and dried grass or straw worked in. (It has a high percentage of sand and gravel as is and makes fine cob with just straw added.)

Because I had to bring in the clay from my house 75 miles away, I was very frugal with its use and built in as many honeycombed air cavities as I could around the core, which was a couple of inches thick.”

Do they Meet Code?  Can I insure my home if it has a Rocket Mass Heater?

I don’t know the code rules that might hinder someone in building a rocket mass heater, but I found this on the http://www.RichSoil.com website in the FAQ…I encourage you to visit the site and scroll down to the FAQs for the most up to date information.

They are currently legal in all states for outdoor use. For indoor use it varies. Portland, Oregon building codes now allow rocket mass heaters indoors and many other places are adding them to their codes. In my area, they are legal in the northern half of missoula county.

Due to the benefits far above and beyond electric heat, natural gas heat and conventional wood heat, many people are building now, and are confident that their local codes will catch up soon and have elected to blaze a trail in their community.

Here is a discussion thread about when you need permitting, when you don’t, etc.

Dispelling Fears & Myths

Many people don’t understand rocket technology and its application for heating, and there are many myths which Paul Wheaton and rocket heater innovators debunk in the following videos

  1. for Tinkering in Garages and Shops Only
  2. they violate the Laws of Physics and Thermodynamics
  3. they are too ugly; they don’t go with my couch/drapes/spouse
  4. You need more wood than just twigs
  5. The Hot Barrel is Too Dangerous
  6. you have to use small wood; a conventional wood stove has more options
  7. heating with a big hole in the wall; ziplock bag effect; tipi results
  8. carbon monoxide poisoning – everybody is going to die!
  9. There are better ways to heat homes than rocket mass heaters
  10. i’m too old or frail or handicapped or lacking in skill to figure out how to build one of these
  11. i’m not handy enough to build one

Other Cool Stuff

Here are images from what is created in DVD4. The 3-in-1 griddle, oven and water heater by Tim Barker and an outdoor cooker/smoker/bench by Matt Walker:

After the event, Matt Walker went on to create a really nice rocket cook stove:

Just for fun, here’s a video showing how to make an outdoor ‘oven’ for meat and naan bread!  https://www.youtube.com/watch?v=mfXXamj8lV4

Also for fun, maybe we could start burning our poop in these rocket mass heaters: https://youtu.be/XPdAm4vjzLE.  Some have suggested kitty litter as well.

*Many people have used pea gravel and sand as thermal mass materials in a flue system, but they have been proven to not be a good material for thermal mass…rather these are insulative.  Thermal mass materials used as thermal mass surrounding a flue system should be cob or stone or brick filled in with cob.

One thought on “Heat Your Home with 1/10 of the Wood Used in Conventional Wood Burning Stove

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