Energy SystemsWaste Systems & Recycling

Rocket Stove – Hot Water!

by Tim Barker

The basic unit, minus the rocket stove and piping

In my last post, I showed a picture of a wood powered water heater, so now we’ll roll up our sleeves and get into how this was designed. But first a warning! Boiling water is easy to do, but boiling water in a closed container and not blowing yourself up is much trickier — in fact I’ve heard it said that there is the equivalent of a stick of dynamite in 500 grams of boiling water! So if you blow yourself up, be it on your own head. Having said that, I have spent a fair bit of time creating a design that is simple to build, safe and efficient.

So, first let’s consider some basic principles. Contrary to what your mothers may have told you your father wasn’t so bad a cook that he could burn water, as you can’t get it over 100°C as long as there is water in the pot. The boiling water effectively dissipates the heat energy as quickly as you can put it in. If however we raise the pressure by putting the water in a container and seal it then we raise the boiling point of the water. Some of you by now are saying "congratulations, you’ve just described a pressure cooker". Nods to you, that’s exactly what I’m getting at. As every schoolchild knows, water boils at 100°C, but remember there is the caveat "at sea level". If we raise the pressure above that then the temperature at which the water boils will also rise. Correspondingly lower the pressure and water boils at a lower temperature. For instance at the top of mount Everest water boils at approximately 69°C. Okay, now hold that thought while we forge on with the water heater design.

A little context: PRI Australia, where I worked for the last two years, had a need to heat water for up to 55 people per night during their PDC courses. Obviously anything we did should showcase appropriate technology while being easy to use and live with. Solar was considered but a unit to heat that much water was both a huge undertaking and very expensive if we were to purchase it. A compost shower had been tried but issues of volume and heat transfer made it inappropriate and, worse, cold. Since my days as an intern at PRI I had been keen to try some ideas I had and so put forward an evil plan to build a wood powered hot water system. The power unit, so to speak, would be a rocket stove and this would In turn heat a drum of water positioned above it.

Now for the details, I arranged the drum (an old 180 litre stainless hot water system tank) on its side above the rocket stove, which in this instance is built of old red house bricks for the feed tube and burn tunnel, while the heat riser was an old stainless flue pipe insulated with perlite. This came up to within 50 mm of the hot water tank and towards one end.

The shell of the unit showing the 200 litre drums and the end
of the hot water tank just visible

Around the hot water tank I arranged a cylinder made of two 200 litre drums welded together with the ends cut off. This basically made a cover around the hot water tank that had a gap of approximately 50 mm all the way. This is where hot combustion gases passing out of the end of the heat riser travel around the hot water tank and give up heat to the tank. As the gases give up their heat they cool and are displaced downwards and eventually pushed out of a hole, low on the end of the outer cover. To stop heat passing out of the outer cover we then wrapped the 200 litre drums in recycled glass wool insulation and then some old corrugated iron around that to keep the rain out and everything tidy.

An end view showing the outer corrugated iron layer, the insulation layer
and the inner 200 litre drum with the end of the hot water tank
visible through the hole

So the gas path is easy. It’s the water part of the system that has to be gotten right or things could get ugly.

So let’s go back to our hot water tank. We plumb in a cold water inlet from a pressurised source (mains or tank) with a tap and we use this to fill the tank. At the top of the tank we have an outlet, so when we open the tap the water fills the tank and overflows out the outlet. We then shut off the tap, and the tank is now full of water. If you want a simple system to fill a bath or for hot water for washing clothes this would be okay, but remember that because we have no tap on the end of the outlet we rely on the overflow for the hot water, so turning the inlet tap off will not immediately shut off the overflow which will continue for a while longer. Also, if the water boils in the tank you can have hot water and steam slugging out the overflow pipe.

To get a system that supplies hot water on demand means we have to do a few more things. We take a coil of copper pipe (13mm x 18 metres long) and put it inside the hot water tank. In my case I cut a small hole in the tank and ‘wound’ the coil in. Each end of the coil then projects out each end of the hot water tank. In the tank I used there were holes already there for the old connections so I passed the pipe through and silver soldered it in place and then welded back in the bit I had cut out to get the coil in. If you don’t have welding skills, don’t despair — all this work can be done at your local welding shop and should be comparatively quick and cheap. Another variation is to use an old clip-top 200 litre drum, sitting upright, and pass all the fittings through the lid and use compression fittings, so no welding — but be warned, the drum will rust out eventually (probably two or three years).

Okay, let’s recap. We have a drum, it has a pressurised pipe leading into it with a tap, we have an outlet or overflow, we open the tap and the drum will fill and overflow out the outlet. We have a copper pipe inside the tank, and where the copper pipe goes through the walls of the tank it is soldered in place so water in the tank will not leak out. The water in the copper pipe does not mix with the water in the tank.

Now we connect the copper pipe to the drum fill pipe, but we do this before the tap. The other end of the copper pipe then goes to your end use, be it shower or sink or whatever, and there is a tap on that end. This means when we open the shower hot tap, hey presto, water automatically flows.

Now is the time to remember our pressure cooker example. The water In the tank is not pressurised or able to be pressurised because it’s open at the overflow. It will boil at 100°C at sea level. The water in the copper pipe comes very near to the temperature of the water in the tank because copper is an excellent conductor and because of the length of the copper pipe, however it’s pressurised because it comes either from a tank or mains. We know because of our earlier discussion that this means it will have a higher boiling point than the water in the tank. This means that you can never boil the water in the copper pipe. So we now have a system that can’t go boom.

Now a couple of tips. You will have to periodically (weekly or so) open the drum fill tap to top up the drum as expansion and evaporation will continually lower the level in the drum. You could set up an external float system to keep the drum topped up but it’s more complexity and I haven’t bothered. How often you fire the system depends entirely on your usage. The PRI system was fired once in the morning to top up heat lost during the night and to cover the small number of people that preferred morning showers and again in the evening, beginning an hour or so before showers with people throwing the odd stick in as they came. The initial fire up of the system brought 180 litres of water to boiling in just over an hour and consumed approximately 4 kilos of scrap timber.

A few words of caution — there are home built wood powered systems around using complete old hot water tanks, but I cannot recommend using these if the system depends on the original safety valve. A wood fire is very hard to precisely regulate so invariably at some stage the water in the tank will boil. Electric or gas powered systems are designed with thermostats that precisely regulate the water temperature. The safety valves on these are there purely for if the thermostat has failed, which would allow the water to boil. The safety valve works two ways, for excessive pressure and for excessive temperature. It is designed to fail (yes fail) if either of these parameters are breached. Once this happens it will leak. In a pressurised system this means it will dribble out water non stop till some smarty pants comes along and blocks it, which then converts it into a bomb. So, if you have one of these systems you’re literally dicing with death.

Contrast this with the PRI system that has no valves to fail and has been designed so that it pretty much would require a failure of the laws of physics for the system to get to an explosive state. If you require even more safety — and, frankly, I haven’t bothered because the laws of physics seem quite stable at the moment — you can arrange things so the overflow outlet is directed towards the rocket stove inlet (not directly above of course) so that if and when the water boils steam blows into the rocket stove, putting the fire out.

On my next post I’ll be talking about how we can reduce the energy needed to heat water by as much as 30% . Hint, what happens to the hot water after we spend so much time and effort getting it up to temperature?


Tim Barker is teaching workshops at Koanga. Learn more about rocket stoves, natural building and appropriate technology and get some practical experience at our Autumn Internship and our Appropriate Technology Workshop. Book now for early bird discount!


  1. Wow thats cool thanks for the explanation. I stayed at PRI Zaytuna Farm in April and loved the rocket stove shower there it is a awesome design.

  2. Hi Tim,
    Great article. I am living and working on a permaculture farm in Utah in the US and I have been looking at various water heater ideas lately. In my case there are a couple of differences, the water supply is not pressurized it is gravity fed from a spring on the property. We also have a much lower demand, currently only myself using it and possibly 3 or 4 people next season.

    The design I was thinking about using to create on demand hot water involves a rocket stove that has the same copper tubing but this time coiled around the inside of the reburn chamber. In theory you simply start the fire and the flow of water and very shortly you should have hot water. Now as you said the fire is going to heat inconsistently so my plan is to place a mixer valve so cold water can be added when necessary.

    Have you had any experience with this kind of system?

    Thanks again,

  3. Tim, that sounds like a fantastic system, it clears up some concerns I had about pressure and plumbing.

    Would you use the same sort of system for a smaller installation – ie. scale down the same design, or use a different design?

  4. Great article I would really love to see more on rocket stoves. I take it that I may become one of many of the courses taught specializing in water heating. I myself are trying to look at using a rocket stove to heat a hydronic system but again want to have all the safety measures in place first as the entire system could be pressurized. Keep up the good work guys.

  5. gday guys I’ve worked with Tim and was at PRI when he built the rocket stove water heater

    Lance, the water coming to the stove was from a header tank about 8 meters higher, the same as your spring,its gravity feeded
    depends how high your spring or a header tank dam/pond ect is to how much preasure you get

    hope this helps

  6. Yeah, waste heat recovery coming up next… I have read about a man called “the Ice Man” (his real name is Wim Hof) who has developed a relationship with the cold-cold is his friend. Perhaps we all should just start taking cold showers…any takers??? On the more serious engineering side-Tim, I was thinking about the water loss through the overflow of the unpressurized heated tank, could you just pass the outlet through a radiator, for example a straight 3/4″ hydronic tube (with Al-fins) placed at an angle to allow condensate to pass back into the tank? Perhaps even further, you could then terminate the outlet with a liquid filled j-trap to form a seal without pressurizing the system any more than what the liquid in the j-trap provides? You could probably even get condensate to keep the j-trap sealed to a certain level if the design was right. If you brought your pressurized line inlet line together with the overflow, you could probably reduce water loss to almost zero, while preheating the water a bit. Or perhaps these ideas are not worth the trouble…at least they are free.

  7. I LOVE this site. Cheers for this amazing post and I will be waiting with baited breath for the next installment :)Serendipity Farm SHALL have a rocket stove :)

  8. Great article.

    You can run a similar system but instead of a rocket stove utilise a wet back (water jacket) on your wood heater. This can allow you to use the wood burning heat for heating your house and/or cooking as well as just heating water.

    The water travels from the wet back in the wood heater via convection up to the open header tank (ie. the insulated open hot water tank as per the system above) which must sit more or less above the wood heater and then returns to the wet back via another pipe to be reheated and sent back. Don’t be tempted with such systems to install a pump because unlike natural convection of hot water, they fail which would be a serious problem for the wet back. Still it wouldn’t heat up anywhere near as fast as the rocket stove, but you do get other uses for the heat.

    PS: You could also add in a drain back solar hot water panel or two too, if you wanted to reduce fuel use even further, but this also adds a bit more complexity to what is a simple and effective system that you’ve setup.

    PPS: I run this system here with the solar hot water panels, so have practical experience.

    Cheers and top article.

  9. Hi all sorry again for the delay but i don’t manage to get on the internet very often . Hi Lance as my mate Dave said the heigh of your spring is fine , in fact even if it were only a couple of meters higher it would be okay although the flow would be poor . Direct heating through a coil in the combustion area can be done but its tricky , as you say it can be uneven and my experience with mixing valves is they dont react well to the fluctuating temps or pressures you get when you get steam formation in the pipes, plus you just dont have the heat exchange area to make it as efficient as it could be . Something i left out of the article, we do in fact run a mixing valve with our system but its not absolutely necessary . We have a plan to install a small heater(50-60 litres) above the rocket oven in the institute so this can be scaled down or up with no problems. Dallas some sort of thermosyphon system would probably work for hydronic heating , ive got an idea i want to try at some stage to use some limited steam formation and a couple of check valves to create a heat powered pump for hydronic systems or even a fluidyne stirling engine (two moving parts ) would do the trick, Sigh! so many interesting things so little time .
    Cheers All

  10. Hi all I’ve replied to some comments but it doesn’t look like they have come through for some reason. In reply to the latest comment cheers Chris yes you can use a wet back and wood stove but as you sat there can be issues . The ones that stick out for me are cost as the rocket system is cheap to build , efficiency as it has a much greater heat exchange area than a wet back . Also there is the fundamental difference where with a rocket you are completing the combustion process before you attempt to use the heat generated. With a wetback it’s in the combustion chamber so it’s removing heat from the combustion process making it less efficient. I have the makings of another post or two on a very simple heat exchanger design for a rocket stove that doubles as a solar hot water panel that should be as efficient as a commercial system but haven’t got round to it as yet .
    Cheers Tim

  11. Hey Shawn how are you mate ? Lots of ways to do the top up of the tank . One of the easiest is a small resiviour with a ballcock on the tank inlet set at the right height. I always work from biggest easiest efficiency gains backwards so the next step I hint at at the end of the article is waste heat recovery from the shower water itself which is up to a 30 % efficiency gain. I worry with things like j traps and condensers that sludge, dead bugs, algae etc could build up, not likely but as you know it’s a elegant dance between complexity and simplicity.

  12. Tim,

    I currently have a swim spa with an encloure that I built in to the roof solar panels that drain back. The solar panels do quite well between April thru November but can’t keep up in the cold months in between. Because I have an abundance of smaller wood, I would like to suppliment the solar with a rocket stove water heater.

    I was comtemplating one similar to the one on your site. I would like to hear your thoughts redarding the variances before proceeding.

    A while back I picked up a stainless steel 25 gallon tank out of a solar batch collector. It is 10″ in diameter and 6′ in length. My plan was to mount it vertically with the lower portion of the tank extenting into the burn chamber and the upper portion surrounded by a 14-16″ diameter steel tube, attached on one end to the burn chamber and the top end capped with a 4″ exaust pipe outlet.

    A circ pump would provide spa water to the batch tank’s inlet and its outlet would eventially drain down into the spa’s filter basket.

    Question 1. As I see it, I do not have to worry about a pressure valve because the outlet is open and sream can escape unimpeded through a 3/4 pipe. I do have a secondary unsed outlet where one could be used.

    Question 2. With the tank being verticle and the exhaust oulet pipe at the top and with the circ pump constantly circulating cooler water when it reaches a set temperature, will I have a problem with drafting due to the cooling of the exhaust gasses. Would I be better off moving the batch tank out of the combustion tank and extending the upper surrounding tube.

    Thank you for any insight,


  13. Hey Tim.
    Great article, many thanks.
    I was wondering, is there any reason you laid the barrel on its side rather than had it standing up?
    Looking forward to seeing the solar heater/heat exchange thingamajig soon!

  14. Hi Crip
    The main reasons for the barrel lying on its side is its easier to construct that way, it’s more stable and in my mind there is better contact and flow between the hot gasses and the barrel due to the curved sides of the barrel.
    Cheers Tim

  15. The emphases of this system is to reduce our carbon footprints and of cause to save on that hefty electric bill we receive every month for a geezer that needs to stay on all day with an electric element and because the electricity companies increase their electric unit price per year more than a bank can give you interest. A friend of our family decided to buy 30 000 South African rands worth of elect and trust me he gained more interest in two years then what a bank can give him in 10. I should tell him about this system. Tim I will definitely build this. I did think about the rocket stove vs. the solar panel and you can’t compare it.
    • You will need 3 solar systems. Tanks and panels included with allot of deep cell batteries to gain the same output comparing to the rocket stove.
    • The maintenance on solar is more expensive then the rocket stove water heater providing you make the rocket stove from stainless steel (will literally last a life time). Will cost to start with but its ones off and you have it forever?
    Regarding this specific system. Bear in mind there can be variable. If you take 3972kwh per tonne of wood chip and 4667kwh per tonne for pellet, assuming 17.5% moisture content. The average of this two is 4319.5kwh. The average needs to be taken because the one product is compressed and the other are not so it will be very close to the consistency of normal timber. Now to heat 1 litre of water by 1 degree you need 1 kilo calorie of energy approx. 4,184 Joule. Water boils at sea-level at 100 degrees C. So let’s say the water is an average of 23 degrees. Now this average will vary from country and region and you are trying to boil 1 litre of water you need 77 kcal.This formulated comes down to 75wph.From one kg of wood providing there are 0% loss of heat you should more or less be able to create 4319.5wph. Times that with the amount of wood that Tim said he used to warm up 180liter of water this system only loose about 300wph.Thats incredible input to output ratio. That’s equivalent to loosing 12.9gram for every kg of wood that you use. That’s an incredible 1.29% of total input. Now that’s something to invest in. The only thing I will change and that’s only personal use, I will connect the heat outflow to my exciting geezer gold inflow and then connect the outflow back to the rocket inflow and to my warm water to the house so then you have a system that can warm the water in the coil and we all know heat rise. That way the water in the geezer tank will slowly circulate through the rocket and heat up. This is only a thought. If someone tried this can let me know it will be great. I will also try and make a coil inside a coil inside a coil so that the maximum amount of water can be heated at one time. That leaves les water to heat in the water tank and more in the coil. The 18meter of pipe that you used Tim probably work well I am just thinking to get more hot water to maybe have a bath instead of a shower. 13mm pipe working on the outside measurements can only hold 132.66ml of water per meter. So 18meter will hold 2.387 litres. But I have check and notice that you can fully coil the drum inside 3 times over and get a respectable 60 litres at one time. I know it will cost a small fortune but it’s just a thought. Looking forward to reading the next post.

  16. If I was still in NZ, this system would make sense, but now I live in Canada where there are below freezing temperatures for 3 months solid and periods of frost in at least another 3 and threats of frost in all 12, and everything has to be thought of in terms of being protected from freezing, collecting wood resources months in advance and stockpiling in warm/dry areas.
    Does anyone have any ideas how this might be adapted for cold climates.
    I saw the comments on wetbacks and drainback systems – that seems more likely in a cold climate since the wood stove is operating anyway.

    Typically, where I am, water pipes must be run about 1.2m below ground level to prevent freezing. This is (one reason) why basements are used and it has its advantages but lighting open fires in a enclosed space is not a good idea.

    Btw, lots of ideas here:

    and in particular, several systems use pex tubing in an insulated epdm lined wooden tank as the heat exchanger. You need more of it (pex tubing) but it’s much cheaper than copper and seems to perform reasonably well

    1. Hi Steve, well for a start wood stove or rocket stove your going to have to collect wood the difference being the rocket stove will use much less wood. The same with running water to your water heating device rocket stove or wetback you would take the usual precautions for you climate. A water heater that uses the waste heat from a rocket oven is easily doable and would be inside the house so less worry with freezing. Also as the water in the outer tank is static you could modify it’s freezing point by adding radiator coolant. Now i know a lot are going to say yuck but if the need is there then it could be done. Besides if you have a car your already using antifreeze so whats the difference ? There is no reason why a well designed and executed rocket stove wouldn’t be safe inside there must be hundreds if not thousands installed in homes as rocket mass heaters by now. Ive loved the Build it solar site for ages , so many great project. Funnily enough it was only last week i was revisiting the article you mention as i am back to NZ next month and one of our projects is a combined wood/solar powered water heater so i was having a cruise around looking for inspiration. Remember though that the tank is simply for storing water so why not recycle an old hot water tank.

  17. Hi ,
    One simple question. Will you only be able to heat the water in the coil, and then wait for the next 2.387 litres (Thanks to Darek) to be hot or do you have instant hot water to fill a hot tub?Best,Lars

  18. Hi
    I would like to heat my Jacuzzi. Would this system work for the heat demand that is required. Is it possible to use just the water mass through thermal siphoning into the jacuzzi instead of the copper coil?

  19. Hi Lars and Pete. This advice applies to both as you are talking about similar needs. Firstly there is no standard answer to how quickly it will heat a set amount of water. It depends on so many variables it’s impossible to know and anything else would be a guess. Just some variables are how hot is the starting water, what temperature do you want to get it to, how much surface area is exposed to the heat ,what is the flow rate of the water, how well insulated is the rocket, what is the thickness of the pipe what is the surface area and volume of the hot tub/jaccuzi ? So i would advise doing those things you know will help. So firstly lots of surface area exposed to heat, the more the better. A smaller rather than larger tub, Insulate the tub a bigger rather than smaller rocket. Pete from what i can work out your talking about using a thermosyphon to heat the water and have it circulate directly into the hot tub. Yes this would work and as it is an non pressurised system it should be safe if done properly. Recently at the Koanga Institute where i run appropriate technology courses we have been looking at using old car radiators as a source of cheap efficient heat exchangers. Lain flat and cobbed into chimneys above rocket stoves they should provide instant hot water but remember they would have to be part of an open system or BOOM. As always don’t build any pressurised system until you have a good understanding of the operating principles of the design.

  20. Thank you very much for sharing your design and process. I’d like to ask what you think about my idea: Instead of storing heat inside the water mass in the burn area, have a separate insulated water tank that contains your potable water and moves via thermosiphon. Inside the rocket heater is simply coiled copper and stone/earth mass. Additionally, have a solar collector plumbed in. The idea would be that solar would take care of most of the heating but then the rocket stove would supplement as needed. The water storage tank could be sized very large to hold a lot of heated water if it were sufficiently insulated

    1. Hi Dan. You are pretty much talking about a wetback system and a rocket mass heater integrated into one and certainly it can be done. You have a hierarchy of heat uses though. So after your rocket you should be heating water and then heating the mass. There are lots of things to consider with such a system though. For starters to make the thermosiphon as efficient as possible have lots of surface area and make sure it is an open loop system for safety. Next thing i would think about is do you want anyway of diverting the hot gasses after the heat exchanger for the thermosiphon because if not, every time you are heating water you will be heating your house warming mass and that might not be a good thing in summer. Water has much better heat storage capacity for house heating and can be pumped which opens up all sorts of interesting possibilities but in the end my gut tells me it will be much more expensive to do and containe properly. Dirt on the other hand doesn’t need expensive containers and can be formed into structures (bench seating). The final big thing i would consider is the water heating loop in the house mass heating water component. I would still strongly consider making this open loop so no steam explosion. I keep going on about it but these system MUST be designed so that they are still safe when operated regardless of the condition of the components in the system or the experience of the operator. If we have leaky safety valves and such eventually someone with no understanding will come along and “fix” it turning it into a bomb.

      1. Super appreciative you took the time to reply, thanks. Hadn’t heard of “wetback” systems (Im in the States) so thats a new thing for me to go research

        1. So Dan i don’t know if it is an purely OZ/NZ term but it’s simply the old style thermosiphon system with a serpentine pipe built into the back wall of the oven. I often forget this site has an international audience and terms may have different meanings in different countries. The back of the stove has water in it hence wetback. It’s like your flip flops, we call them thongs and the Kwi’s call them jandals.
          Cheers Tim

  21. Hi Tim. I know that I am coming late to this discussion but perhaps you could explain why you chose to use the copper coil in the tank rather than adding a 3rd pipe that is used as a safety valve. I have seen these used on “donkey heaters” . The higher the pipe is run the higher the preasure in the system and the use of a ‘shepherds crook” bend at the top should keep the pipe clean.

    1. Hi Alan yes there was a time when across Australia those shepherd’s crooks stuck above many a roof sometimes spitting out steam and hot water. Those systems work for low pressure but remember to balance against the pressure in the system the pipe has to get longer the higher the delivery pressure. Water at PRI was getting delivered in from a head of 20 meters which roughly equates to (someone correct me if im wrong ) 28 psi this means that to have an open vented pressurised system you would have to have a shephards crook 20 meters high. Now just image using domestic pressure water which is typically around 80 psi. You would need a shephards crook 56 meters high. Of course you could use a pressure reducing valve on the inlet(generally pretty expensive) but i kind of like the no specialised valves approach.

      1. Thanks for your reply Tim. I understand the issue of then height required for higher preasures. One way to sidestep this would be to use larger diameter pipes but I take your point about the need for a preasure reducing valve. Could you use a normal tap and turn it down untill water stops comming out of the crook? Maybe a convenient tree might give enough height for more preasure :)
        Your solution is very elegant and excellent for the high preasure, large volumes needed at PRI. I am thinking of a small, low use off the grid sytem so low volume and preasures are more likely.

        1. Hi Alan Having bigger diameter pipes won’t make any difference its all about the height of the water. You could have a open vent pipe a mile wide and if it was 19 meters high and your source water was 20 meters high all the water would pour out . No you couldn’t use a normal tap turned down because all you are doing is reducing flow not pressure, it would still overflow it would just take longer.

          1. Hi Tim, thanks for your reply, I’ve just got back from the bush hence my tardy reply.

            I was thinking larger diameter water pipes to compensate for the low preasure, not the open vent pipe.
            What do you think about a header tank with a ball cock valve rather than a preasure reducing valve?
            All this depends on the avaiable preasure of course. If you have the luxury of high preasure then your design is clearly the one to go with.


            1. Hi Alan. Got you about the larger diameter pipes it’s always good o go as large as you can afford with low pressure. I think using a ball cock and a header is a great way to go if you can get away with a lower pressure system. Ive had some very nice showers with just three meters of head.

  22. Firstly, thank you so much for sharing your knowledge and ideas, it´s a wonderful thing.

    Onto my questions: I have a 2mx2mx1.5m hot tub i want to heat with a rocket stove system (tried solar, didn´t work, too many panels needed and only good during the day).

    The simplest and cheapest solution I can think of is to have a small pump, maybe 1/2hp, take the water out of the hot tub and push it through an old car radiator laid across the top of the heat riser of a rocket stove underneath an insulated cap. The hot gas would then go down the outside of the heat riser and through an exhaust laid inside a bench as in the typical rocket mass heater, giving people somewhere warm to relax beside the hot tub, and the heated water would exit back to the hot tub.

    Since the water in the tub should get steadily hotter as it continually circulates like this, the idea would be that you can simply turn the pump on and off as the hot tub gets hot enough or cools.

    Do you see any dangers or possible problems in this approach? I´d rather not blow up or scald family members, or have to rebuild the thing in a couple of years.

    1. Hi Mark. For a while iv’e been interested in using car radiators as a cheap but highly efficient heat exchanger and often talk about wanting to try it. Recently i met up with an old student Michael who tried it after hearing me talk about it and it was a very successful trial. I’m currently teaching a three week Appropriate technology workshop at the Koanga institute and as i write this we are building a batch solar hot water system with a rocket stove boost. The heat exchanger we hope to use will be a car radiator. If your piping is quite large (19mm +) and the radiator sits so one end is higher than the other then the system should thermosyphon without the need for a pump as long as the top of the radiator is below the top of the water level in the tub. As it is an open tub (open system ) and as long as you don’t put valves in the line you should be fine. Maybe research thermosyphon so you are certain about what establishing a good thermosyphon entails.
      Cheers Tim

  23. Hi Tim – thanks for the prompt reply.

    Unfortunately the tub is in ground, so I can’t use thermosyphon without digging another big hole next to it. I tried thermosyphon with a solar panel, but it didn’t work, either because the panel was several meters away down a hill and the water was losing too much heat on the way to the tub, or the piping was too small, don’t know which, so it will have to be a pump I’m afraid.

    I assume Michael just laid the radiator across the top of the heat riser as you suggested. Did he mention whether that restricted gas flow at all – problems with blow back and so on? I ask because after the gasses heat the water in the radiator I want them to continue on to heat a bench next to the tub. Also, since temperatures supposedly get very high in the heat riser, will a car radiator safely stand up to those temps in that position?

    1. Okay so no thermosyphon. Yes Michael put the radiator across the top of the riser as a crude test but was getting pretty much continuous hot water. If you were setting up as a permanent system you could do it any number of ways but remember that if you don’t have some system to allow the ash to settle out before getting to the radiator it will block it. So maybe a system with a couple of baffles to allow the ash to drop out would work. The radiator should have no trouble handling the heat because being filled with water it can never get above 100c as the water boiling wont allow it. I’m sure you’ve seen the kids science experiment where you boil water in a plastic bag, same thing. So try it and let us all know how it went

  24. Hi Tim – good point, I hadn’t though about the ash problem, that’s very valuable advice. So how about a normal rocket stove-mass heater configuration with a sheet of metal at the top of the heat riser, only instead of using the metal sheet as a stove cooking surface laying the radiator on top of it and then putting an insulated cap on top of that? The gases heat the metal, go on through the system to heat the bench, meanwhile the radiator is completely clear of all ash but heated by conduction by the metal sheet. Any problems there? The only one I can think of is maybe rust – would the sheet have to be stainless steel, or is condensation not a factor in that position?

    1. It kind of defeats the purpose of using a radiator and then not taking advantage of all that lovely surface area. try using some baffles to slow down and drop out the ash before the radiator and or make sure the radiator is easy to access so it can be cleaned. Cheers

  25. Well, the thinking was that all the air between the metal plate and the insulated cap will be really hot, so all the radiator surfaces will be exposed to hot air, while the baffles are extra complication in building and also would presumably disturb the vortex needed inside the heat riser. I really like the idea of a removable cap though, simply take it off once in a while and brush off the radiator and no need for any extra pieces. I think I´ll give that a bash and let you know how it goes, which will be in a couple of months. In the meantime, many many thanks for the help and advice, it´s been invaluable.

  26. Hi Tim, I realise this is a bit late in the day for this thread. However I have a plan of my own to fire my brewery with rocket stoves. I am fairly new to it all so I was wondering if you could shed any light on a few things for me.

    I am planning to try out your design above for the initial part of the brewing process (hot water is mixed with the grains).

    What the system needs to do:
    Raise 200 Litres (42 gal) of water from ambient temperature to 78C (170F) in less than 3 or 4 hours. Water will then be used up in 2 x 100L batches.

    Will the water ever get that hot in this type of design?
    What kind of temperatures did your design get to?

    I imagine this is scaleable, as long as temps are reached – whilst the fire burns, and the tap is running? (Basically can I get 400L of water if i wanted to?)

    Thanks a lot

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