This is a pretty simple do-it-yourself option for anyone interested in increasing the efficiency of an old (or new!) woodstove. That said, a heater in the home poses serious risks a€” greater than an outdoor oven, and potentially greater than the old stove itself. I’ve been reluctant to publish formal plans or how-to info as I consider this to be an experimental prototype, but rocket goddess (and Sketchup artist) Erica Wisner was inspired to draw up some basic plans, and convinced me that they should be available for other experimenters.
What would be fig 8 (see detail pix) is just a final bit of square tile to make the final connection to the metal chimney.
I live in western Oregon with my family and run Hand Print Press with help from friends Max and Eva.
Well I’m about to embark on a similar adventure to boldly go where no stove has gone before, or so I thought until I found this thread. Q: When the smoke first exits the iron stove can it either circulate around the top of the oven, as well as taking a more direct path into heat-absorbing labyrinth?
A: layer 6 is a thin casting of leftover refractory that I used to add mass and to cover a gap that needed covering. Q: From the added photos it appears that the Layer 1 slab has a hole in it under the oven section, presumably to allow more heat from the iron stove to radiate into the oven.
Q: Have you ever heard of anyone building a masonry heater using flexible or rigid stainless steel smoke pipe instead of fire brick or flue tile to conduct the flue gasses, and then just using ordinary brick to absorb heat, and for the main structure? Q: It seems to me that an inexperienced DIY mason would be better off relying on the SS pipe to not leak smoke, rather then hoping that he had designed the firebrick channels in such a way that they did not crack. GIFT BOOKS & FREE SHIPPING: Buying a book from Hand Print Press helps maintain this site and the people who belong to it.
I just watched the video: The fiberglass he shredded up-that's the crystalized silica I mentioned. I was pondering other casting methods and wondered if you have considered casting non rectangular cores? From a climate change standpoint, the latter of these is surprisingly the least harmful in the long run.
The goal of a rocket stove is to burn a relatively small amount of wood at as high a temperature as possible, resulting in more complete combustion, and to extract as much heat as possible from the exhaust gases. Insulating the chimney ensures a large temperature difference between the exhaust gases inside the chimney and those outside it. Above is a picture of the internal parts of the stove (combustion chamber and chimney) set up for initial testing to make sure it would generate enough draft for vigorous combustion. The above image shows the combustion chamber and chimney being welded into the side of the hot water tank.
Notice that I removed a glass pane from the left door of my fire place and replaced it with cardboard.
I also made a simple metal screen that can be placed over the combustion chamber to prevent sparks from popping out into the room where they could ignite something (or more likely just leave burn marks as they smolder on the carpet). On average the heat energy available from burning wood is around 4.5 kWh per kg (assuming a 20% moisture content). Wood stoves used as a primary heat source for a home will derive significant benefits from thermal mass (imagine replacing my short length of aluminum flex tube with 30 or 40 feet of duct buried inside a couple tons of concrete and you’ll have the right idea). However, for a stove like mine that’s used only for supplemental heating, thermal mass is not of much benefit since an even temperature can be maintained simply by varying the heat input from the primary system (which happens automatically since it is controlled by a thermostat). So in a nut shell, thermal mass is not really that useful for a stove that’s intended for use only as supplemental heat, if the primary heating system is on a thermostat.
Update 2015-02-14: A few readers have emailed me asking if there is any way they could purchase detailed plans so they can build a stove similar to mine or get a welder to build one for them.
If anyone out there decides to build a rocket stove based on this or a similar design, I would be happy to post some pictures of your project, or if you have your own site, let me know and I will post a link below. 2010-12-13: One reader, Brent, built his own rocket stove following a similar design (though he was clever enough to put the cleanout in the side of the combustion chamber rather than the bottom). I had considered building a rocket stove from a natural gas hot water tank, and keeping the tank intact so it could be filled with water, drawing the exhaust through the existing tube through the center of the tank to heat the water, and allowing the tank to continue to radiate that heat for hours after the stove is extinguished.
So, using a repurposed gas water tank, the sticks would go into the central gas exhaust pipe. If your fireplace has air supply from outside the house to burn the wood, it’d be better to use that supply for your stove, instead of using air from within the room. What is the maximum temperature of the radiating surface you noticed when using this stove? One of the downsides of rocket stoves or other high temperature fireplaces is that the very high temperature combustion also produces nitric oxide out of the nitrogen in the air.
Your stove is somewhat similiar in idea to a downdraft wood gassifiers, which use a water bubbler and fiberglass screen of sorts as a scrubber. However, I’m doubtful that the combustion temperature in this stove is much higher than the peak combustion temperature in an ordinary fireplace or wood stove, and given its higher efficiency, I believe the NOx emissions are lower per kWh of useful heat produced. Incidentally, if the combustion temperature were too high, it would be a relatively simple matter to reduce it by recirculating some of the exhaust back through the combustion chamber. If I piped the exhaust all the way up my chimney through a PVC pipe, then I could also draw fresh outside air directly from the fireplace for combustion making the design even more efficient compared to using inside air for combustion. Make sure the tank you use is not galvanized, galvanized tanks will give off toxic fumes when heated. Your warning about burning treated lumber applies equally well to ordinary fireplaces and wood stoves (and especially open fires).
Would it be feasible to have copper lines coiled around inside there to pre-heat water going to an electric water heater? Using coiled copper pipe inside the stove for water heating will work, but you should not simply hook it in line with your existing water heater.
Connecting the exhaust to the house’s chimney improves the draft, but draft is still affected by the height of the stove itself and other factors like the diameter of the internal chimney and the diameter and length of the exhaust tube.
I have been wanting to buy a propane bullet heater for my garage to work on my trucks in the winter. So, if you’re inspired to try something of your own, please feel free to download the drawings (free! Three of them extend from level 2 to level 4, one each at the corners, and one in the middle. We are interested in restoring the arts of living to their rightful, traditional, public role, as cultural medium a€“ and think the web is a poor substitute, but until we can fashion something better, we try to make the most of it.
Hours of staring at the stove pipe and wishing I could reclaim the lost heat, combined with a love of the mysterious masonry heater that filled a Swiss mountain chalet in the days of my youth are compelling me forward. In other words, can the smoke take one of two paths, or must all of it circulate over the top of the oven? However, I may have actually left a passage on the right side of 3a and 3b, to allow hot gasses to flow through the oven chamber. Why don’t you instead use flat sections of the flue tile, such as you use in layers 4, 6, and 7 to cap off vertical runs?
What you’re describing is the ducting approach used in building a rocket mass heater. Plus, the round, smooth SS pipe would create less turbulence of the smoke, and so might draw better then the square-ish masonry smoke channels.
We belong to it because we meet a lot of folks looking for ways to make their lives and the lives of their communities more beautiful and more harmonious.

Portland cures over a 30 day period producing maximum strength-- IF the cement is kept wet. I wonder if sticking small glazed tiles face down on the mold before casting would result in a cast chamber that was lined with a highly reflective, hard layer of material. See the Rocket Stove Plans section at the end of the article and let me know if you’d be interested. Mulching or burying do postpone carbon release to the atmosphere, but the carbon will be released eventually regardless.
Building and operating your own wood stove of any design will almost certainly void any fire insurance you may have on your home and may also pose a serious risk to you and your family.
To generate high combustion temperatures, rocket stoves separate the combustion, heat extraction and exhaust functions.
Note the block of wood between the end of the chimney and the inside surface of the tank to ensure an appropriate gap for the exhaust gases. The aluminum flex tube passes snugly through a hole cut in the cardboard to expel the exhaust gases into the fireplace where, still warm, they rise and exit through the existing chimney.
The difference between burning wood in the fireplace and burning wood in this stove is incredible. Assuming roughly 80% efficiency of the stove (just a rough guess) the heat extracted will be about 3.5 kWh per kg. The thermal mass stores heat and releases it slowly into the home, evening out the temperature and allowing the stove to be operated intermittently. For example, when I operate my rocket stove in the winter, my electric heaters automatically cut back such that the house temperature does not rise significantly. Improved efficiency is often a side effect of adding thermal mass, but the improvement is really just a result of extracting more heat from the exhaust gases.
He’s using a large water reservoir for thermal mass with a coil of copper tubing wrapped around the internal chimney of the stove for heat transfer. I would suggest picking one that interests you the most or inventing something of your own. I considered it but it was not very practical considering the location of the stove in my home. There are a few materials that will store more heat in less volume but the are expensive and relatively exotic (compared to water), storing energy via phase change or chemical reaction rather than a simple temperature rise. The design would be quite a bit more complicated though and I would have needed to use a non-leaking tank, rather than repurposing a leaking one.
We collect it then have a big bonfire in the fall and I’ve often wondered how to capture all that heat.
If there we actually water in the tank, the insulated exhaust chimney and the like would be in a second tank? The insulated chimney is simply inserted through the existing tube through the center of the tank. Sucking the air from the room would cause flow of the cold fresh air to the room, reducing the temperature and decreasing the net heat gain from burning this wood. Comparing natural gas furnaces, an outside air supply seems to raise the efficiency from around 80% to just above 90%.
I’ve noticed that maximum heat output only happens on windy days when the venturi effect of the wind blowing across the top of the chimney develops the strongest draft.
You will enjoy welding a whole lot more when you dont have to deal with smoke, flux, and spatter.
Soot on the other hand is unburned carbon, and the presence of soot in the pipes signals an incomplete burn.
Combined with the steam present in the flue gasses, it forms nitrous acid which will slowly erode your chimney as it condenses on the surfaces.
Though of course, nitrous acid also causes acid rain and nitric oxide destroys the ozone layer, which is why power plants use smokestack scrubbers to get rid of the stuff.
The idea is to combust the wood with too little air, and then burn the resulting mix of hydrogen, carbon monoxide and methane gas in a separate burner. Some experimentation may be in order to determine actual combustion temperature, but I know that NOx emissions are not much of a concern below about 1500 degrees C (around the melting temperature of iron) and as the unmelted nails in my ash will attest, the combustion temperature is lower than that. That would be much more efficient than the more common solution of just drawing more air through the system. I am also wondering if such a preheater would require any modifications to the house’s water system. Does the internal chimney have to be a certain height to create an updraft, or is that mostly taken care of by the draw from the house’s chimney?
Water will need to flow continuously through the coiled copper pipe or it will simply boil in place (if you have a one way valve installed this could cause your hot water tank to explode due to the increased pressure). Alex has been developing bell stove designs, and has links to a Russian heater builder who makes brilliant sense of the theory, which is really just flow a€” think funnels full of water, then turn ’em upside down and re-envision the water as hot gases. See the detail photos for a good view: the hinge is a bit of wire running L to R along the upper side. Since it overhangs the iron stove is there any concern that it might crack and break under the load of the masonry it is supporting? Do you bridge that hole with anything when it comes time to actually bake something in the oven?
I removed the plate to allow more direct heat transfer to the bottom of layer 1, which does also serve as the oven floor.
However, despite all theoretical concerns about flow and turbulence, the square brick channels work!
Beauty and harmony are cooperative endeavors, and books can be good company when a mentor or grandparent isn't there to "show you how." By way of thanks to the mentors, friends, and parents who have given us so many gifts, we send a gift book with every order, and pay shipping for the first book and the gift. The weight of the liquid casting would probably be enough to force it between the cracks like a grouting. What’s worse, mulching or burying the wood will result in some anaerobic decomposition (that is decomposition in an oxygen deprived environment) which will result in the production of methane, a far more harmful greenhouse gas than carbon dioxide. Scrap wood and yard trimmings are burned in backyards across the country each year without any attempt to capture that useful energy. As far as wood stoves go, a rocket stove is probably one of the safest designs since the combustion chamber is tiny, the exhaust volume is low, the draft is strong, and the bulk of the exterior of the stove does not reach very high temperatures.
At least 18″ is recommended by most building codes for ordinary fireplaces and wood stoves.
With an average load of wood my stove burns for about 30 minutes before requiring more fuel. Treated wood, plywood, OSB, etc all contain chemicals that will be released into the exhaust during combustion.
All combustion appliances are capable of producing carbon monoxide which can be deadly if it is released into your home. The hot exhaust gases in the chimney rise, while the cooler exhaust gases outside the chimney fall, and the whole process draws fresh air into the combustion chamber, supporting vigorous combustion.
I know you’re probably thinking I must be crazy to use a combustible material like cardboard for this purpose, but the fact is that the exhaust volume from this stove is so low and the stove is so efficient at removing heat from the exhaust that this aluminum tube reaches a maximum temperature of only about 60 degrees C during operation. It’s useful to quickly extinguish the fire and to prevent air infiltration when the stove is not in use. I just slide it out as shown in the photo and the ash drops into any suitable receptacle (I’m using a plastic tray above).

You really have to experience it to believe you can get so much heat from a small handful of wood. Another enclosure, a steel drum perhaps (or another electric hot water tank) must be inverted over top of the gas hot water tank. It’s a feature I am considering adding, but there is no simple way for me to do so since the fireplace does not have an existing outside air supply. Of course the exhaust inside the tube is hotter than that (the tube temperature should be near the average of the exhaust temperature and the ambient temperature).
In practice, I’ve found there is usually a draft even before lighting the stove since any breeze outside the house draws air out of the chimney via the venturi effect, causing a partial vacuum in the fireplace which in turn draws air through the rocket stove whenever the cover is removed from the combustion chamber.
About 20% of the heat is produced in the gassifier, and 80% is available in the woodgas, and all the ash is left in the gassifier and not blown around the neighborhood. If I did want to address that problem, rather than increase the exhaust temperature to eliminate condensation, I would simply pipe the exhaust all the way outside through a PVC pipe (the melting temperature of PVC is well above the exhaust temperature of the stove and I believe it will not corrode in the presence of nitrous acid). To avoid this problem, you would need a closed loop and a pump to circulate the water through the stove. Incomplete wood burning produces carbon monoxide and that dryer ducting looks awfully thin and easy to puncture. I’ve also been answering questions in the replies below the post, so read through those, too. The horizontal crack is at level 4, from the bottom L up to the bottom of the flat red tile. That said, many modern heater masons will use extruded clay tiles for channels exactly because they’re smoother and easier than laying lotsa little brix. Typical materials are wax, plaster of Paris, or paper mache. I was also thinking about the wear issue you've mentioned. However, if you build a rocket stove for heating a garage or outbuilding, you should probably install a smoke detector there as well. Other combustion appliances in your home such as a furnace or hot water tank are probably a greater carbon monoxide threat that the rocket stove described in this article since they operate continuously, unattended, even while you sleep. What I did have was my parents’ old electric hot water tank that they had just replaced since it was corroded and leaking.
The chimney is insulated with about 1″ of Roxul (a rock wool insulation that will tolerate higher temperatures than fiberglass insulation) and wrapped with adhesive aluminum furnace tape to hold the insulation in place. I’ve found it best to clean the ash out every week or so as once there is more than about a cup full it will fill the bottom of the combustion chamber and start to build up in the tube between the combustion chamber and the internal chimney.
I’ve fed my fireplace for hours with hardly any change in room temperature but it only takes a few minutes for the rocket stove to raise the room temperature noticeably. Therefore, the heating value of my scrap wood is about $0.25 per kg and by burning about 20kg per week I save about $5 per week on heating costs or about 50% of my heating bill for this time of year. For example, I could extract more heat from my exhaust simply by using a longer piece of aluminum duct between the stove and fireplace (I have noted that the exhaust temperature drops about half way to ambient for every 20cm of tube length). Sorry I can’t really help you much without knowing what your particular interests and skills are. I had thought about running 30 or 40 ft of exhaust duct down into my crawlspace to extract even more heat from the exhaust, but that length of tubing would add significant resistance, reducing the natural draft. My main design criteria for combustion chamber size was that I wanted to be able to fit two chunks of 2×4 into it side by side. Thus the hot exhaust gases are directed up the insulated chimney through the tank and then back down around the exterior of the gas hot water tank. Also, the additional tubing would reduce the natural draft, likely requiring addition of a fan.
To start the stove with no smoke released into the room regardless of the presence of an outside breeze, I use a small propane torch aimed into the rocket stove for a few seconds prior to lighting the fuel (regardless of the direction of the draft the propane torch makes it much easier to light fuel that is way down at the bottom of the combustion chamber). Also, the copper pipe in the stove may be prone to corrosion over time due to acidic exhaust products. Finally, depending on how hot the pipe gets, metal and masonry together can generate problems due to the fact that metal expands more with heat than masonry. Scrap wood is loaded directly on top of the existing burning wood inside the combustion chamber.A  The flame is drawn downward by the strong draft rather than rising out of the chamber as one might expect. I did not want to weld it on permanently since I may want to disassemble the stove later for inspection. Then it’s a little harder to get to and it will eventually start to reduce the draft.
Savings will be much greater in winter when the stove can be operated more frequently without the room becoming uncomfortably hot. The best evidence of the lack of combustion products released into the room is olfactory rather than visual. This solution would probably require addition of a small exhaust fan to increase the draft which might be hindered by the longer exhaust path.
That mean build and test your design somewhere outside before you do your final installation! Rocket stove designs are most often used for small cook stoves but larger stoves for home heating are not unheard of. The result is that ALL combustion products pass through the hottest part of the fire resulting in very complete combustion, producing the greatest amount of heat and reducing products of incomplete combustion such as carbon monoxide and smoke.
I also used some flexible aluminum tubing (dryer ducting) to feed the exhaust from the rocket stove into my existing fireplace.
In hindsight, access through the side of the combustion chamber instead of the bottom would have been more useful.
Feeding the stove every evening after work, I have only managed to go through about one bin (perhaps 20kg) of wood per week.
It’s better to suck the exhaust through the system rather than blow it through, or you risk exhaust leaks into your home. When I used to light fires in the fireplace, I could smell it throughout the room even if I couldn’t see any smoke.
Also design and build-in a better clean-out, and send us your news and photos so others benefit from your experience! To capture as much of the heat as possible and radiate it into the room, the exhaust gases are passed through a secondary chamber (much larger than the combustion chamber) that absorbs and radiates the heat. I also considered installing an oxygen sensor (like the ones used in automotive fuel injection systems to determine the proper air fuel ratio) and adjusting fan speed based on the sensor output.
Although you need more volume to store the same heat, solid mass storage can more easily be built into the home in the form of walls or floor for new construction, or the addition of an adobe bench or similar usable fixture. It just doesn’t take much heat to get the house to a comfortable temperature right now in early spring. I may experiment with such things in the future but for now the simplicity of my first design can’t be beat. Lastly there is the concern you mentioned about pressure build up, but as you say, that is easily solved by leaving the storage tank open to atmospheric pressure. Hard brick is for facing volatile environments like wood fire and salts-- salts for the purpose of glazing.

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