BuildingEnergy Systems

How to Build a Rocket Stove Mass Water Heater

With deforestation still moving ahead apace, dealing with the global demand for fuel wood is critical. Life without heat and hot water would be very unpleasant. Life without trees is impossible and to satiate our hunger for heat, richly biodiverse forests are being replaced with ‘sterile’ monoculture tree plantations which give rise to tree diseases and other ecological vulnerabilities. The pressure for fuel wood is on in most parts of the world. Removing tree cover from the upper watershed is particularly damaging, but persistent demand can cause too many to shift the goalposts on how much should be allowed to remain….

Along with passive solar building designs, a rocket stove hot water heater can dramatically reduce the amount of wood used to heat a given amount of space, or a given amount of water — down to as little as a fifth as much as a regular fire. In this video, Geoff Lawton takes you through the design of the rocket stove mass hot water heater that very efficiently provides reliable daily showers for the many students we receive at PRI Zaytuna Farm. This particular stove is specifically for heating water, but other designs can produce thermal mass space heaters as well.


  1. Great looking system. Can you give an approximate heating time. Eg do you start the burn 1/2 an hour before you want a shower or is it longer?
    What I really like about this system is that the thermal mass of the water is going to give you warm water for quite a while after you’ve finished the burn. Am also thinking it could be adapted easily to heat the green house on frosty nights.

    1. Non official answer, because I’m just another visitor ;-)
      Heat storage depends on the amount of mass water you heat and insulation. With a rocket stove mass heater for a house you dimension it so you only have to fire it once a day. You could do the same thing for water.

      There are many examples where people use a rocket stove mass heater to heat green houses. Often they run a long exhaust pipe under the planting beds (at a safe dept of course), using the planting beds themselves as the mass. You need to use a model more like the ones used for heating houses. See the barrel in these designs creates a temperature difference, resulting in an airflow. The barrel acts as a pump that pushes hot air through the pipes (needed to overcome drag), and also gives off some immediate heat.

  2. Thanks so much Craig for organising this – just what I was after. It was very much appreciated. One quick question – do you know roughly what the approx length of the copper coil is and roughly what the volume of the water jacket is?

    Thanks, Chris

  3. Gday guys they are a great system, Eco green, Tim Barker did biuld that stove it was his 2nd design and construction the first one burnt out the burn tunnel, pretty impressive really 6mm steel burnt out in a matter of a few months [thats the power of rockets I guess] and Chris 18m x 13mm of copper pipe in the tank from memory
    hope this helps

  4. You can get a hot shower in 20 minutes, a warm shower in 10.

    Tim and I co-designed the system and interns also helped build it.

    It has 18 meters of copper pipe which is the length they are purchased in here in Australia, and the water jacket is 100 liters.

      1. Hi Anne the copper pipe used is 12.7 mm internal and 15mm external it is a standard size. The length you want is between 15-18 metres.
        Cheers Tim

        1. Many thanks Tim.
          This is very helpful, it is very much appreciated. I am in the process of building my own. I might have more questions as I go along. Feeling so blessed for all the expertise that you guys share. Thank you

  5. Great video. I’m going to build one in a couple of months but I would like to know how you get enough pressure on it for the shower. Is it just gravity?

  6. Dear Lethal Mutha Tim was working for Zaytuna Farm at the time of the construction of the Rocket Stove Mass Water Heater and as his boss at the time I had to work with him on the design to give him the authority to spend time and money on materials to construct it. As a mechanical engineer it is always fun working with another mechanical minded person like Tim.
    The interns and WOOFA’s also really enjoyed working on the construction.

    We fuel the system with sticks and pine cones and have found it does not work very well with tall poppies as the fuel ;).

  7. yes i remember, i was wwoofing there at the time. Its funny, i seem to remember Tim designing it from scratch. oh well. must be slipping in my old age :)

  8. “Lethalmutha” you seem so intent on accuracy yet you don’t sign your real name. How does anyone know you were even on site. I can verify that pine/fur cones work really well and would be worth designing a system around that fuel source specifically.

  9. yea Eric pine/fur cones work great, when I was working for Bill in Tassie he talk a lot about using just pine/fur cones as a feul source. telling a story of meeting a old lady with a dozen mature pine/fur trees in tassie and this was here only fuel source no cutting or spliting just collect and use

  10. In the 70 hour course with Geoff Lawton and Bill Mollison they both mentioned a way to get get oils out of the stove. I think the word they used was distill. I’m surprised that Geoff Lawton didn’t bring that up this time. Is it not practical in this situation? Does the Rocket Mass heater consume all of the oils? It would be nice to use the oils with that hessian sausage that Geoff Lawton mentioned in that other video.

    I might play around with the passive solar heating stuff, but I think I’m going to stick with this thing when it comes to heating water and air.

  11. Hi Thomas,
    In order to collect the oils from the fuel wood you have to design the stove for a specific wood/fuel source, as this rocket stove is for the campgound shower and is maintained by students/volunteers there are just too many variations in fuel sources used at the moment to make that a practical addition.

  12. Thanks for the video Geoff and Craig.

    Would it be wise and or possible to perhaps take the exhausted hot air/steam and pipe it up to about 20 metres to heat either an shower room or to some how heat a near by green house/poly tunnel?

    Has any one done this kind of thing with success or would up to 20m be too far?

    Thanks again.

  13. Hello all i’m pretty slow so hadn’t seen this till lately. I’m somewhat confused as to peoples fascination with who originated the idea. All i’m interested in is that these ideas get out there and so are freely given. Having said that without the support of Geoff and the Institute i wouldnt have had the time nor resources to do the work. Thomas in order to “crack” oils and volatiles out of the wood you have to heat the wood in a low oxygen environment (pyrolize it). The whole idea of the rocket stove is complete combustion so typically the oils and other volatiles are consumed in the process . Yes you can do it but it is a fairly complex process and more importantly optimizing something like that for a home built system would take a considerable amount of time and effort. As an interesting side note there are some very clever people cracking plastics back into their constituent oil products using only a fraction of the energy gained to complete the process. Im hearing yields of roughly 1 kg of plastic to make 1 litre of high grade oil to run a diesel engine on.
    Check out this link at
    Cheers Tim
    PS ill be putting up some new posts shortly about a new style of very efficient barrel oven that ive built for the Koanga Institute in NZ while i was there.

    1. Hi Tim, I was wondering if the hot water that accumulates in the spiral hose and which is used for the shower does not come under pressure? Doesn’t this closed loop need an outlet valve like the external water bin?

  14. Great solution. Just checking some details – You need 4 holes in the stainless steel (SS) drum (1 cold water inlet, one hot water outlet, 1 pipe to fill/top-up and 1 pressure (steam) release.
    Then you need to remove one end of the 44g drum, place the stainless steel drum on some bricks (or similar) inside the 44g drum and then re-seal the 44g drum. You need 6 holes in the 44g drum: one for the heat inlet, one for the chimney, one for water in (to the SS drum), one for the hot water out (from the SS drum), one for the top-up water (to the SS drum) and one for the steam release (from the SS drum).
    Am I on the right track?

  15. Hi again,

    I know I’ve already posted one this thread but…

    My lovely wife and I watched this again on Sunday PM here in the UK…

    We both said at the same time when we saw the steam outlet and the water top up process being shown Mr L…

    Why couldn’t there be some kind of a coiled copper pipe attached that uses a loads of coils of copper, similar to the inside of the tank, to cool the water and recirculate it back up to the top of the top up valve. This way it becomes a sealed self servicing system. :)

    I imagine the coils would kind of like what you see on moonshine stills.. Not that we know anything about illegal booze or as I call it… Rocket Fuel…

    It would need to be about quite long I guess..?

    I am not an engineer… Just a simple pastor of a church and business goals trainer, who was an electrician in a “past life”…

    It might also need some kind of pressure relief valve added too, but at least the water could be recirculated and save someone’s time topping it… We all know what happens when “someone forgot”…

  16. Hi Richard yes you’ve got it right although because the donor hot water tank was longer than a 200l drum i actually put two together and cut to the correct size) to get the correct length. Hi David a much simpler solution and one i have already employed in other similar systems i have built is to simply add a small ballcock set to the right height to maintain the correct level in the tank much, simpler, cheaper and safer than condensing the steam. Allowing the water in the tank to get low will simply make it less efficient it will not create a dangerous situation. See my earlier post on hot water heaters for my reasonings for steering away from pressure valve systems. However I need to reiterate here that boiling water in closed containers is dangerous buisness and so don’t attempt to construct anything like this till you are comfortable with your understanding of how it all works.I have deadline to finish a booklet for the Koanga Institute on all things rocket stove by this time next month so stay tuned.

  17. Thanks for the suggestion Tim. Living in the UK we have experienced faulty ballcocks valves and know they must be well maintained. Coming home from holidays to find our house flooded due to the valve going was not nice. I thought of a BC valve but thought the extreme temp of the water jacket, not the shower water, would melt the float at the end.

    Is there one in Oz that’s temperature resistant perhaps?

    Any clues on your books contents other than Rocket Mass Stoves? :)

  18. Hello David The BC valve is external you put it in a small separate tank, you just make sure the tank is at the right height so that the hot water system is nearly but not quite full. Having a small air gap allows steam formed to vent. If you set it up so the main tank is totally full you will get water slugging (gouts of hot water shooting out) with the steam. The Booklet will be about wood fired hot water systems, Black ovens (combustion gasses in the cooking chamber) and white ovens (no combustion gasses in the cooking chamber). The main difference from all other DIY home wood stove books is that i concentrate on direct use of the heat rather than relying on charging up thermal mass to cook with which makes for much more efficient use of resources (wood). As usual recycled materials are the order of the day so the instructions will include how to’s for stoves constructed from 200l barrels old ovens (gas and electric) also ideas for ovens heating water with waste heat as they cook and a combined solar/ wood fired system where the solar panel doubles as the heat exchanger for the wood heater. Im also working on a rocket waste oil stove at the moment that will hopefully make it into the book if i get it sorted in time
    Cheers Tim

  19. Hey Geoff,
    how wide / long is the firebox / burn chamber / flute / chimney? Is the flute an insulated type or plain and same question for the chimney? I want too build one of these for my brother who currently is living on a diesel genset off grid. This and a space heater type stove would greatly diminish the amount of work it takes too heat, 12 cords of wood each winter….

    Thanks again for posting ; )


  20. Hello,
    Wonderful video, thank you so much for sharing.
    I have a question about the boiling/steam water. If the water mass is boiling, wont that eventually transfer to the copper coil water and make the shower water in the coper coil boil? and since the copper coil is under pressure do we need to worry about explosions if we dont use the shower water after a while?


  21. Hi Randy and Luke. Firstly Randy, The dimensions of the stove depend on a number of things one is the materials used so for an old brick lined stove the maximum you can get a brick to span is 150 – 175mm (6-7″). This is a good size for a water heater or oven. With steel or other materials you can go as big as practical and or the limits of what the material will span reliably at those temperatures. There is a rough relationship (1:3) between diameter of the burn tunnel and height of the heat riser(flue) so if the diameter of the burn tunnel is 150mm then the heat riser should be at least 450mm and typically for this sort of application i would be going longer say between 700 and 900mm for greater draw which in turn will allow the burning of slightly larger wood. Cross or burn tunnel should be as short as possible and as written about previously try and keep cross sectional area constant through the stove for good flow. Remember keep the feed tube short as possible while providing adequate support for the burning timber but remember that the length of the feed tunnel directly cancels the same length of heat riser so short. Also makes the stove hard to light.Luke As i have written about in previous articles water boils at 100c at sea level so the water in the drum will be boiling at approximately 100c .However the water in the coil is slightly pressurised by gravity or a pump so has a higher boiling point and can never boil as the steam boiling off the main drum removes all the energy as the water boils. Good old physics. By the way Randy your brother could route the exhaust of the diesel genset through a heat exchanger and heat water its reasonably easy to do and is call CHP or combined heat and power. There are a number of simple units available that use the cooling system in cars to heat water for remote area camping. These use the radiator water. If your brothers diesel is air cooled you could purchase or make one of these systems. You could do the same even if it was air cooled but it would be a little more difficult but it is doable(i’ve done it).
    Cheers Tim

    1. Hi All

      Great idea here, so imple and efficient!
      In practice, how long after the fire has gone will the water remain hot/warm in winter/summer?
      What’s the typical fuel consumption in winter/summer?

      thanks a lot

  22. Would it make sense to use a similar system but run the hot water through radiators in a home to heat the home? I want to put a RMH in my unfinished basement, but the thermal mass heating would not work. I’m thinking this system, piped into baseboard radiators would heat the rooms. I guess a circulating pump would be in order as well.

    1. Hi Jon what setup are you contemplating ? Are you talking about having the heater in the basement and piping the hot water upstairs ? If so you may be able to get away without a pump simply by utilizing the thermosyphon principle.
      Cheers Tim

  23. To : Tim Barker – Great looking hot water system !! How did you get the coil in the SS tank? Did you cut it open then weld.
    Could regular steel be used ( old propane tank ) I realize there is a rust factor involved.. What type of SS tank did you use and its’ size ? Also mentioned, the burn tunnel was compromised on the first build !!! I think it was 1/4″ steel. How did you remedy that on the second build. And have you had the same problem on the burn tunnel for the scrap electric oven/rocket fired. Thank you

  24. Hi Carroll and Tineke Check this earlier article i wrote explaining the inner workings of the stove for some of your answers. Carroll i pretty much use plain old house bricks for my rocket stoves now as anyone can get hold of them and all you need is the bricks and some mud mixed with sawdust to insulate. Not perfect but very practical and easy. The Oven rocket ran for two years without rusting out and that was with two to three times a week use. The water heater saw a lot more use and generally was run much hotter plus it was totally exposed to the elements which didn’t help

  25. Hi Graig,

    Thanks for sharing this video..I am totally a newbie into the rocket stove world. I have a few questions I hope you can help answering. Will I be able to connect your design directly to the faucet, and still output hot water or do I need a tank with cold water “up hill”. Do you need to insert the cobber coil into the water tank, or can you place it into the 200 L tank, so it absorbers all the heat directly?… I have a wooden hot tub, and are looking for a solution to warm it up as fast a possible from 4 C to 38 C…I live in Denmark, so the winter is quite cold…best, Lars

  26. I worked in a refinery when I was younger and we called these “heat exchangers” but adding the rocket heater onto it as a heat source is novel. Is the steam outlet on the backend of a pressure relief valve? Also, could you install a float and valve on the intake of the main tank to keep it full rather than having to watch it? and finally, how did you get the copper coil inside the old tank? Thanks, i really enjoyed this build.

  27. Have any of you guys yet designed a system that would incorporate a rocket fired hot water system with a mass heater for use inside the house? We’re about to build our place and I’m dreaming of a single mass in the kitchen with a rocket stove heating the mass for heating while also providing an oven and hot water (combined with solar panels on the roof for when the sun is shinning). I’m super keen to find someone who’s already done it so I can learn from them!

    1. Hi All
      As usual sorry for the long delay but i’ve been away on various projects. So firstly Lars you can connect directly to the faucet no problems at all. Check here for an article i did which will answer you question re copper coil placement but the short answer is the coil is within the water tank you are heating . Mr r there is no pressure relief valve on the heated water tank , remember the tank is open to the atmosphere so it can’t pressurise, and the copper coil has the pressurised water supply in it. In actual fact another system i have constructed for The Koanga institute in NZ has a float valve attaced in a separate small bucket which is linked by hose to the main tank, so by setting the height of the bucket we can set the height of the water in the main tank and keep it topped up automatically. Andrew yes you could run a rocket mass heater and a hot water system but you would want to set it up so the heat could be run solely to the mass heater at times and vice versa. This wouldn’t be too hard to do but would require careful consideration of wether it would be better to just have a separate water heater as if yo had a combine system once the water in the tank was boiling it would have the effect of stripping a lot of heat out of the flue gasses and dumping the outside as steam as you wouldnt want it venting inside. And yes you could also run an oven but again you may very well end up with something that performs all three functions but not particularly well.
      Cheers Tim

    2. I don’t know where in the world you are, but in Europe I would say that you are looking for Sjang van Daal from
      Vuur et leem. I’m just on my way home from a very enlightening weekend’s course with him, helping build what is apparently the UK’s first batch rocket stove mass heater in a house – and it heats a bench, a black oven and a heat exchanger for hot water and radiators. It also incorporates a clever design tweak for secondary air, as well as having a door (no smoke when it’s gusty). Check out his website for more info; he’s a great teacher as well as a very experienced stove builder so I’d highly recommend his courses. He puts a lot of info online open source too if you’re not into courses.

        1. Ah! My comment was aimed at Andrew, not you Tim…think you’ve got a lot of experience! The stove we were constructing at the weekend is designed to just run the water in summer, with the option to block off the oven as well. In normal winter mode, it will distribute heat to all – sjang does build stoves that allow for variable control of this, but in this instance, because he’s too far away to pop back and trouble shoot, he’s planning on physically impeding the flue gas so that a set quantity goes to the heat exchanger and bench. This will be done with bricks in the channel (the bench is a Russian bell design due to limited space). Hope that’s of interest and I’d like to add my thanks for all the interesting work and help that you are sharing here. It’s really inspiring!

          1. Hi Tania thanks. The minute you think you know it all is the minute you
            don’t learn anymore. I’m just happy there is so much i don’t know, it’s
            what makes life interesting. I Haven’t had much to do with the mass heater
            side of things simply because here in the Sub tropics it would be totally

  28. I am very impressed with this project! So impressed that I am seriously considering an implementation! I’ve been fascinated with Rocket Stoves ever since my first build. You are so right… Rocket Stoves can be built to look really nice. I think the combination of water heating with a rocket stove is the missing link we have all been searching for. Great work!! Please feel free to stop by and check out my RMH project.

  29. Since you don’t have an internal buoy to automaticlly open the influx of mass water after the mass water boils out through the mass water outlet, you have to do it manually, loosing time and water. Wouldn’t it be simpler to not use mass water, wrapping the copper tubes inside the heat exchanger, and transfering the hot water to a water heating cylinder that would keep the water hot, and at the same time heat the house? This way you would need less wood, less water, have an autonomous system, and much more efficiency.
    Thanks for the generosity of sharing the experience and the know how. Together we shall get there.

  30. Hi Roy. The system can be autonomous for topping up the mass water with an external ball cock(float valve). I have built such a system in NZ and the water never needs to be topped up. I reality the topping up is probably 30 seconds once a week at the most so its up to the individual if they want the extra cost and complexity of the float system . Also you don’t tend to boil the water in the system unless your new to it and very excited or a pyromaniac. By your description your talking about a thermosyphon system but ii’m not sure how you are topping up autonomously and pressure relieving. If the system is a closed loop with some sort of pressure relief valve the you would have to be very cautious because the potential for a massive explosion is real. The Clever bit about this design is not that it gets water hot but that it does so in a manner that is simple and near fail safe.

    1. Dear Tim,
      As far as I see it, there is no point on having excuses or assumptions about how good the system will be used by its users to not make things simpler and cheaper.
      If I mentioned the usage of a water heating cylinder to store the hot water, it shoud be obvious that it has its own pressure relieve valve, like they all do.
      This is one of the many examples I have meanwhile found:

      I thank again for the comments and generosity of sharing your experiences and keeping such a forum, so useful for all interested on having a simpler living and making a better world.
      Best regards.

      1. Hi Roy The system you are showing is a simple flue heater type but it would most likely be connected to a open loop system with ball cock (float valve) and an in tank heat exchanger as when using wood fired hot water heating its hard to regulate the heat input so the water in the thermosyphon system could and often does boil. Using a standard hot water cylinder with a TP (temperature pressure) valve and directly coupled to a thermosyphon is not a good idea as TP’s are not designed to constantly regulate the pressure in the cylider and could fail. In fact they are designed to fail in the open position (they have a burst disc). Far better to have a system that requires no valve and cannot blow up because of how it is designed than rely on a valve that is not designed for the job.

  31. I have to admit I like the idea.
    Being a plumber with over 35 years experience in Australia, I have a couple of concerns about the system that I feel I should point out.
    1/ As the system is an uncontrolled heat source, the water at the tap, has the potential to reach up to 100 Celsius quite easily.
    Human skin subjected to
    49c will take 5 minutes to have second and third degree burns
    60c will take 5 seconds to have second and third degree burns
    71c will take ½ a second to have second and third degree burns
    The above is for an adults skin, babies and the elderly burn at lower temps. Also this information only goes to 71c, the system can reach 100c easily so the burns become deep third degree in a flash.
    2/ If, at some stage in the life of the hot water system, somebody sees the water coming out of the overflow and decides to either put a tap on it even cap it off, you have the biggest bomb you have ever seen. Search you tube for “hot water heater explosion” and see what damage can be done with one.
    3/ No Plumber in his right mind would instal this system, as its not Australian standards approved. This mean that should you ever have a problem, somebody burnt or the roof gets blown off your house, you have no insurance and a big problem with the authorities.

    Don’t get me wrong, I applaud what you are doing, just this one scares the crap out of me.

    1. Hi Bob Yes your concerns are valid that is why the systems are designed the way they are .The System in the video has a tempering valve fitted so the hot water temperature is limited to what you set the tempering valve to. To stop people blocking the steam outlet pipe it is deliberately cut off so short that you can only push a piece of garden hose oven and have no way of clamping it so it cant go boom. I am greatly concerned by some systems i have seen built which use a standard hot water service and use the PT valve to act as a pressure relief valve for the system. Being a plumber you would understand how big a no no this is as it will pressureize every time the hard to regulate fire boils the water and eventually make the PT valve fail and then some smarty will block it with the resultant boom. Of course it’s not something the authorities would like and i’m not suggesting we all run out and do this but in certain situations we all have to take some responsibility for our own actions and explore alternatives to the current system.

      1. The comments where only made because I didnt notice them elsewhere. The tempering valve is great, but as long as the tempering valve is a orange cap NOT a blue cap as that will not work properly at such high temps :)

  32. Hi guys,

    I love the video – what a fantastic concept. I’m contemplating building one myself, but have a question.

    Would it be possible to replace the steam outlet on the hot water tank with a vertical stainless steel pipe, insulated to retain heat, allowing the steam to rise say 30 feet, and then re-condense in a non-insulated (thus rapidly cooling) water tank? This water tank could have a pressure release valve as a safety precaution if the steam were to build up.

    The reason I ask, is because I live in a flat temperate region, and collect rain water, which amply supplies my water needs. The problem is, I don’t have sufficient solar energy to run an electric pressure pump, so there’s insufficient pressure in the system to pump water where I need it. Obviously when it comes to water, increased elevation equates to increased pressure, and so my thinking is that I could use convection to drive the steam to a water tower, from which I could draw it down again once cooled, using gravitational energy to create pressure for my water system.

    One additional benefit, if this worked, is that by boiling the water it would also be purified of organic contamination.

    Would this work, or am I missing something?


    1. Hi Daniel. I wouldn’t advise it as it would require pressurising the system and as you can see from the most recent comments that is fraught with danger if done incorrectly. Far better to use a solar pump or even a good foot operated pump. How do you currently pump water if you have no electricity?

      1. Hi Tim,

        Thanks for your reply – with a few small scale tests I realised the water transfer would be painfully slow anyway. I’d have to have the fire going for 5 hours a day to convect a high enough volume.

        I’m going to have to look into solar pumps as you suggested – we don’t use any kind of pump right now (apart from the one in my arm). I’ve always avoided electric pumps because of the huge wattage they require, as we have a very small solar array and relatively low solar input (living in northern Europe). If I can find a low energy solar pump to pump my rainwater for irrigation and outbuildings that’d be ideal.

        Thanks again.

  33. Hey Geoff and Tim,
    This is one of my favorite rocket hot-water designs – the video goes over a set of simple failsafes, but doesn’t really get into all the problems you’ve ducked by a simple, clever design (like conserving potability of the hot water regardless of the gluck in the tank). Thanks for adding maintenance and construction details in response to comments.
    I’d like to mention this system, with a brief description and links, in a hot-water appendix to an upcoming rocket mass heater builder’s guide. Could you please contact me to discuss permissions and citation? If you’re OK with my using a screenshot diagram or images, or have a reference to that book or preferred resources for the complete design, I’d love to include them.
    Thanks, Erica W

    1. Hi Erica thanks so much for your kind comments. Of course i’d be happy for you to reference the design. I’m actually building a water heater and a barrel oven this weekend. I have a few design tweaks i will be incorporating into the HW design from those things learnt since iv’e started building the systems. I would be more than happy to take some nice hi res photos this weekend if you would like. I am also putting together the Info for a little ebook and would love if it could be referenced. Cheers Tim

  34. Hello Tim,

    This is a beautiful system, that works so well. Was very impressed by how long the water stayed hot for (was still slightly warm the next morning (during my PDC in January))
    Currently thinking/ wanting to combine this with Ed Revill’s Biochar Rocket Hybrid. ( )
    Do you think there would be any reasons why these two designs couldn’t be combined, and would it be as simple as just taking the flue from ed’s design and continuing on with your design?



    1. Hi Liam thanks for the kind comments. Thanks for the link i hadnt seen Ed’s design before but will be having a close look at it for sure. Recently we built a TLUD burner from a 200l drum cut down to 1/3 rd height and double skinned. We kept the overall primary and secondary air volumes low and got 2 hr + burns while making char. The idea with the lower height was so the unit could fit under a number of devices and use the heat rather than waste it which has always been somewhat of a sore point for me when watching 200 L drum burners. As a trial we cut a hole into the back of the rocket hot water system i build for the Koanga institute and fitted the TLUD and it performed beautifully so Ed’s design should work but let me have a good look and i will have some comments.
      Cheers Tim

  35. This build is particularly interesting for me because the “failsafe” built into it appears to be workable. However, the question I raise is with that water coming from the mass jacket which is so hot. In his lecture, Geoff shows the steam and hot water pouring from the pressure relief part of the system but shows no interest in capturing the energy contained in that hot water or the hot water that it produces. Seems a bit wasteful to me.
    While I realize that you do not want to allow the pressure to build for what are obvious reasons, hasn’t anyone thought about capturing that overflow water/steam and using it for its valuable energy?
    Where I live that would be difficult to ignore. We had -20*F for much of this winter. Geoff’s outdoor sink makes this place seem even colder.
    So, Geoff…what to do with all that hot water coming from the 40 g. mass drum?

    1. Hi DrDave. The thing with wood fired systems is that the heating is ultimately controlled by the operator as it is problematic to regulate the heat output of wood fired systems. It can be done but we introduce complexity which is not really needed. The water in the system never really needs to be heated to boiling and typically isn’t. I’m guessing in the Video it is to demonstrate capability of the system to do so. Due to the reducing temperature differential and the high Latent heat of evaporation of water (2257 kj/kg) it is wasteful to heat to so high a temp although when showering lots of people like the heater at PRI does the higher stored energy allows more showers. And yes the fail safe works well as it is not technically a pressure relief but a vent with no method of closure. In normal use the system will waste very little energy out the vent maybe 4-5 litres of boiled water a week to top up. As to what is done with the hot water as previously mentioned it is used for showers for students and interns (up to 50 showers a night at times) try doing that with an electric or solar heater! Cheers

  36. OK Tim, I get that. Now for my next question. Is the inner chamber SS to prevent corrosion of the tank or is there some other reason? SS is quite expensive and a bit hard to weld. Most of the hot water tanks here in the USA are not SS but I imagine some are. The last rocket heater I built had a bell made from a hot water heater tank rather than a steel drum. It gets pretty hot but the tank itself was NOT corroded on the inside and the water here is very alkaline which leaves huge amounts of corrosion on water fixtures.
    This is why I asked about the requirement for SS. I can get hot water tanks rather easily as folks tend to dispose of them if they spring a tiny leak (which is easy to weld shut.)
    I like this build but have difficulty getting SS and welding it…what is your opinion on that issue?

  37. Hi Dave The tank is SS simply because that is what if found. It is out of a solar hot water system. I have used old copper hot water tanks as well and even a 200 l steel drum. SS is very durable but it’s thermal conductivity is lower(16 W/(m.K) than copper(400 W/(m.K) also because SS is less ductile the tanks are a little thicker. Having said that i haven’t really noticed any real world difference. When you say corrosion on fittings i’m assuming mineral deposits. Stainless is very easy to weld with an arc welder and stainless rods but as you say copper is easier as all you have to do is silver solder with a MAP, propane of oxy acetlyene torch. One thing when using copper tanks they are less robust you need to make sure they are well supported as the can sag a little and this can end up closing the gap between the top of the heat riser and the copper cylinder chocking the stove somewhat. This can also happen with settling of the unit from new as the stove and the stand are separate. I have also used 200 L (55 gallon) oil drums in the past as that is all i had. Now we both know these will eventually rust out but one unit ran for over a year with no problems till it was replaced with a more permanent heater. Iv’e mentioned before but some corrosion inhibitor in the main tank could make it last for a very long time. Not that thats a great idea but we seem to be happy enough to drive around in our cars with corrosion inhibitor in the water. There are also water soluble oils (machining oil) that could be added to the water in the tank. Now before anyone reading this gets a smart idea, filling the tank with All oil is a very bad one. Sure no corrosion issues just uncontrolled combustion of oil vapour issues DONT DO IT. So in answer Dave use whatever you’ve got. Half the joy in this for me is not getting the perfect materials list together but improvising from what you’ve got.
    Cheers Tim

  38. Hi there,

    I am interested in using this concept on my farm, but for the purposes of heating a hot tub. Would it be feasible to use a thermo-siphon as opposed to a straight cold water source? i.e. tub sits a little higher than heater/rocket assembly, cold water comes in from tub, cycles through heater assembly and flows back up to the tub.

  39. Also, I have a 48KG LP Gas cylinder lying around on my farm in addition to a drum (no water geyser tank). How well suited to this purpose might the gas cylinder be? I realise that I would have to make holes for pipes and insulate them etc. I think the cylinder has the advantage of being narrower in diameter and therefore not needing as much water, resulting in quicker heating up times?

    1. Hi Tarn. It is quite common to use a thermo-siphon to heat hot tubs so yes it would work fine its just a matter of setting it up. Your main problem will be you want lots of surface area for the thermo-siphon and fairly large diameter tubes going in and out as the siphon action is quite weak . I would recommend at least 3/4 (19mm). Remember as always to run this as an open system no sealed tank so no boom if you boil the water. if you are serious don’t hesitate to ask if your not sure about something. Cheers Tim

      1. Thank you for the informative reply Tim. I will be sure to make sure there are safety precautions in place. I will be combining this with a solar heating system which I already have.

  40. Hi Darren The idea of hot tub pizza does sound good. If you have a look here you will see a low mass efficient rocket oven that can get over 400 degrees C (750 F). An oven like this could easily be configured to heat water. In fact when i started building it that was the intention with a small open circuit water unit on the back, It wasn’t finished before i had to head back home and when i returned the idea hadn’t been incorporated but certainly would work well. Imagine the unit like a rocket mass heater where the radiant heating barrel is now our oven barrel and the water in the hot tub the heated mass. It would be a hoot to build.

    1. Hi Debra yes it can be done upright although you would have change a few things like baffeling of the gas flow to slow it down to transfer heat as it will try to rise more rapidly and will give up less heat to the water tank. Also the mass is higher so the stand would have to be nice and stable as you don’t want it falling over. I’m currently in the US at a rocket stove innovators event and we are working on some (hopefully) exciting developments in water heating so stay tuned.
      Cheers Tim

  41. Tim,
    I recently had a stroke of incredible luck. A hospital where I do some work had an autoclave that they dismantled and were going to discard. The SS tank was sitting on the loading dock awaiting the scrap metal guy but I beat him to it. It is a double jacketed, SS tank used as an autoclave jacket for sterilizing surgical instruments. I do NOT have the door but figure I can weld the SS with a TIG welder and SS rods. The question is now that I have this 300 lb behemoth, is there any benefit to having something that will withstand significant pressures such as this one? Would there need to be any modification to this in order to comply with the issues of safety?

    Getting this bugger on a stand will be fairly easy for me because my tractor has a pallet fork attachment that allows me to lift rather heavy things without difficulty.

    I am also in the process of building a rocket stove out of fire clay, perlite and refactory cement. I got the plans from Matt Walker on the rocket stove forum. His plans are quite simple and easy to construct. Since it will NOT be made of metal, it should serve for a very long time, assuming no cracks that make it “unserviceable”.

    Any suggestions as to the employment of this tank with this rocket stove configuration?

    1. Hi Dave i actually cast a stove body today with Matt . Would you believe that i’m actually hanging out with Matt , Erica and Ernie Wisner and Peter Van Den Berg at Paul Wheaton’s place near Missoula for a rocket stove innovators event. As for the tank if you can configure it as iv’e explained here and in other places then it should be fine . But you really really really need to understand the dangers of getting it wrong and be honest with yourself about your ability to do it right or the result could be catastrophic in which case the stronger vessel could actually make it worse as you could build a higher pressure before it blew up. Sorry if i’m making it sound scary but thats exactly what i’m trying to do as you have to be sure you know what you are doing and not take any short cuts. Also the mass of the tank is mass you have to heat at the same time as your water which means more fuel consumed, still i envy you it sounds like a beauty.

  42. Hi, I am planning on using this system to heat a 8mx50m valipini. Thats 400 square meters or 4305 square feet. The water would be transported form the heater into PVC pipes layed on the ground and the circulated back with a pump.
    Do you have any suggestions?

    1. Hi Steve as always i’m about simplicity so the first question would be do you need the extra heating as i thought the solar passive aspects of the walapini meant you didn’t need extra heat. I’m not sure of your context so can only guess but if you do need the extra heat boost have you considered a rocket mass heater piped to heat the soil below ? This would be the cheapest and simplest approach. Otherwise yes you could certainly have a rocket hot water heater and pump the water around a loop but it will cost and add possibly needless complication, certainly would be cool but then we always need to be on guard for doing things simply because we think they are cool.

  43. I’m not sure if you’ve already addressed this but would this system work as a boiler for a radiant floor heating system??? I have dreams of having my own wood shop one day and it would be spectacular to be able to heat the shop with waste material.

    1. Hi Daniel Yes you could certainly use the system for radiant floor heating but you would need to incorporate a pump to circulate the water. This would mean that you could get away with an open vented unpressurised system which is simpler and easier to build. Think of the system but with the heat exchange coil taken out of the water tank and installed in the floor. Have the inlet and outlet of the heat exchange coil back to the tank with a pump somewhere in the loop to pump the fluid around.

  44. …. and a filter, yes? I would want a good filter on the floor loop to prevent it gunking up, especially if you plan to pour the pipes into a slab or tile over them. Open systems have more oxygen in the water so they do corrode and grow gunk better than sealed ones, from what I hear. You could also keep the heat-exchange coil and run either a more sterile closed-system with treated and pre-filtered water, or any other heat-exchange fluid of your choice as long as its boiling point is higher than 100 C.

  45. Hi. I had a thought while watching this video. You are not only running a hot water unit, but also a distillation unit. You could run some good hooch, or desalinate water, or…

  46. Great project. I’ve started building myself one and wondering on what type of insulation was used and how it’s done over time.
    Thank you

  47. Hi there – we are building one of these at the moment! Thanks for the design :D

    We have 16 meters of copper pipe – but we are having difficulty coiling it… Does anyone have advice on this? We tried filling it with sand but since its such a long piece of pipe its hard to know if the sand is going all the way down.. Then actually coiling it tightly is difficult. If there is any advice of DIY copper coiling we would be immensely grateful!! Thanks!

  48. Nice project! First actual implementation of the concept I’ve seen so far. My question to you, wouldn’t it be better to add a secondary burn to it? Preferably an air pipe coming from outside that gets heated from the fire and burns the wood gases up the chimney to create more heat and a more complete burn that produces even less smoke. This way the stove also draws less warm air from inside the house that you’re trying to heat. Cheers!

  49. Thanks for sharing!
    We are needing to heat water to supply a radiant floor system. (The old “Kentucky wood gobbler” type wood boiler is too pitted to repair anymore.)
    When we retrofit an old fisher stove with riser and bell it serves to heat the zone we had used a radiator to heat. Took that zone out of the system and so have a spare radiator.
    Our plan is to use that radiator inside a non-pressurized 50 gallon barrel of water, heated by a small batch box rocket stove, outside, essentially the same system as yours except oriented vertically instead of horizontally, for ease of plumbing.
    The water heated in this manner would be pumped using a circulating pump into an insulated second tank (250 gallons) to mix to under 140 degrees F so as not to melt the PEX tubing in the floor zones. (this second tank could later also be fed from a solar water heater, to add heat during the day) Tank will be outside, never used for anything but supplying the radiant floor so not worried about Legionella.
    Prefer to use that second tank and not to rely on a mixing valve.
    Also, how large a PVB style batch box might be necessary for heating the 50 gal barrel? I’m thinking a 4″ system would be sufficient for heating and keep the tank from being so far in the air, but don’t want the extra work in shortening wood to fit such a small box. Thinking the box style so I don’t have to feed a J tube so often, and still keep things warm through the night.
    We’re in Kentucky, so it will get down to 0 F occasionally, sometimes 10’s or 20’s, but usually down to 20 to 40 deg F.
    Asking for a last minute input if this is a really stupid idea.

  50. Wait, I’m over thinking this – since the water in the 50 gallon drum is not pressurized, and the 250 gallon tank is also not pressurized, I can just heat the water in the smaller drum and circulate it to the larger tank to mix – then circulate from there into the house.

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