Our rocket stove water heater: 2.5 years on

| Appropriate Technology, Permaculture, Rocket Stoves | comments | Author :

*Update* – Since writing this article, we’ve deconstructed, improved, reconstructed and cobbed this rocket stove. Have a look here.

Way back in the summer of 2009, we built a rocket stove water heater so we could have hot showers at Milkwood. What a revolution. And 2.5 years later, our rocket powered shower is, surprisingly, still going strong.

Since we get a lot of questions (and comments from warm, clean farm visitors) about this home-made hot water rig, I thought I’d do a little appraisal of the system: how it’s fared and what we’ve learned from such a simple, effective system for happily heated water.

Our rocket stove water heating system

First of all, you should probably have a look at the original post dealing with building this system, The Rocket-powered shower. It explains the basic setup. I like to think I’ve got a bit better at explaining things since i wrote that post, but that’s life. I’ll leave it un-trammeled.

Our post Rocket Stove Roundup touches on the why, what and how of rocket stoves, and hints at their glory, if you’re not up to speed.

To summarize, our rocket hot water system has a rocket-stove style feed barrel, with a J-shaped feed tunnel. The hot air moves up through the heat riser and through a heat exchanger, which is the black box thingy, where the water gets heated. This is the only tricky bit of the system.

After the (not so hot now) air moves through and beyond the heat exchanger, it goes up a chimney, out the top and that’s the end of that. It’s a very simple way of heating a very respectable quantity of hot water with minimal fuss, fuel, and footprint.

I’ll go through each bit of the system and explain how it’s faring, and whether we’d like to improve it:

The feed barrel in action, shortly after it’s construction
The feed barrel nowadays – worn, but working fine

Feed barrel:

Still works fine, though it could do with a re-cobbing. This is partly due to in sufficient roofing in the first year, where the water came in and the cob partly dissolved. Happy with the design and construction otherwise, though.

Ash pit:

Works well, we remove the ash daily when this system is in heavy use. The ash is removed via the little front door (a firebrick with a handle) with a trowel and placed in a metal bucket. From there, it goes wherever ash is needed on the farm. A simple task, easy to do before starting the first fire of the day.

The heat riser filled with vermiculite, shown during construction of the system

Heat riser:

The internal insulation for the heat riser (vermiculite) has settled slightly and there’s a 5cm part at the top of the heat riser that now gets hot, indicating to us that it’s not as insulated as it was.

Not a big deal in our context, but if this system was inside your house it would be best not to have a hot part in the heat riser that your kids could burn their fingers on. Otherwise, this part of they system is fine and dandy.

Water jacket:

Slight rusting occurring (it is made out of iron), but otherwise performing perfectly. Best 20 bucks we ever spent at a garage sale.

Water tank:

Working well, no problems at all. Pressure release valve working fine (though this system rarely gets a chance to overheat, due to the enthusiasm to use the hot water a.s.a.p.)

The completed system, circa January 2009
The system circa June 2011. Well worn in with slight adjustments only, mostly to the water inlet and outlet pipes.

Water inlets and outlets:

We modified these slightly, so that the angle wasn’t so sharp for the water to pass from the tank to the jacket and back again. We also insulated the pipes with rubber, to make the most of the heat energy. Otherwise, working fine.

Fuel efficiency:

Reasonable. Or great, i’m not certain – this is the only rocket powered shower we have! The amount of sticks you need to get a truly hot shower does depend on the ambient temperature, but as a general rule in say mid-spring (18-25ºc daytime temp), you’re looking at 10-15 minutes of feeding the rocket shower sticks for the first hot shower, then 3-5 mins of stick feeding for each hot shower thereafter.

In mid summer, that goes down to 5-10 mins of startup stick-feeding for the first hot shower and a couple of minutes of stick-feeding per hot shower thereafter.

Stick storage! Sometimes this is full. Today it is not.
Marco and Derek, master stick collectors.

Fuel storage:

We learned this one pretty quickly. It is a sad, sad and soggy day when you need a shower and there are no dry sticks for miles around. So we built a stick storage facility! It’s right next to the rocket shower. Where else would you put it.

Doing ‘the sticks’ has become one of the daily chores when Milkwood Farm is buzzing with people. Each day, the appointed choree fills up the stick compartment. This means that even if it rains tomorrow, there will be enough dry sticks.

By having two levels, you could (in theory) separate a collection of wet sticks from dry ones, and hope the wet sticks dried by the next shower time.

The view from the rocket at Milkwood. If you’re not entranced by the sunset, consider that pile of woodchips, or that strange yellow pole.

Siting of the system:

As the above image only sort of demonstrates, siting your rocket stove shower system somewhere with a pleasant view is a good idea. You will be sitting there for 10-15 minutes each day, after all (assuming you are the first showerer). Ours faces west and there’s some lovely sunsets to be had while firing the rocket. Hot chocolate recommended.

And that’s about it, really. All in all, we’re super happy with how this system has performed.

The whole system in june 2011, with showerblock (a converted sheep drenching station) behind

During our twice-yearly on-farm PDCs we have over 30 people at our farm for 2 weeks straight, all using this system exclusively for their showers. While I’m not sure everyone chooses to shower every day, many do and the system holds up under the enthusiastic strain. A pretty good recommendation, i think.

One of the things i still like most about this system is this: the energy in for energy out, and the simplicity and honesty of the transaction.

You want a hot shower? Go get ye some sticks. And then poke a fire a bit. Now you can have a hot shower. Now you may be cleansed.

A big, big thanks to all the thinkers and tinkerers who collectively brought rocket stoves into being, and have caused us at Milkwood Farm to be warm and clean these 2.5 years past.

Any comments or suggested improvement to the above system are welcome as always – bring it on.

>> Original photo set that outlines the construction process of our rocket powered shower.

*Update* – Since writing this article, we’ve deconstructed, improved, reconstructed and cobbed this rocket stove. Have a look here.

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28 responses to “Our rocket stove water heater: 2.5 years on

  1. What an impressive use of a rocket stove. You have proven it to be more than just a stove. I wonder if you could incorporate a single stove to power the cooking, water, room heating?

  2. Good to see it’s going well and you are still happy with it. We have recently built a similarish system using your basic design. Had a few teething problems, but all in all happy with it.

  3. Great water heater, if you didn’t have a water jacket do you think a coil of copper pipe (expensive I know) which wraps around the full length of the heat riser back filled with the insulation would work? or does the heat exchange mechanism need to be on top of the heat riser to work?

    1. Simo
      I keep seeing this same question on other blogs too, allow me to warn you. copper pipe has too much surface area in relation to its volume. you will make steam instead of hot water. I have welded up a boiler type heat exchanger, and if you don’t want rusty water you will need a stainless one, don’t let that put you off though. I have a boiler 250mm dia. 500mm long with 8 flue tubes dia 38mm. works a treat just to be safe add a pressure, temp relief just like a hot water cylinder. A fabrication shop can help you might even be cheaper than you think. I can supply you with a drawing.
      Kind regards Joe blow

  4. Have been filing off-grid technologies “just in case” Thank you for this one. Gold at $1840.00 /oz. not normal at all? Just what is next? China liquidating U.S. assets? buying resources around the world with U.S. money, dumping it as fast as they can without crashing the dollar?

  5. At home We’ve got a solar hot water panel connected to a tank very similar to yours. The bottom of the tank has been converted to a fire pit and the chimney runs straight up inside the centre of the tank, so if it’s been a cloudy and wet day, we boost by fire. I’d love to modify my fire pit to work more like a rocket mass heater for those cold days. Perhaps the addition of a solar water panel to your system would also be of benefit? 4-6 hours of sunlight and we’ve got hot water until about 7pm in winter. The panels are very basic, pipework under black plastic in a glass or perspex housing. Nice and low tech. Although you will need your fire when you’ve got lots of people using up all your hot water 🙂

    1. to modify your firepit, just build the lower part of the j type rocket stove using firebricks, it will be self stoking to some degree too kind regards joe blow

  6. We are very happy with all your information. We live in Holland and are planning to build an ecovillage inspired by the earthshipsconcept. And we want to arrange our heating system (warmth, water and cooking) with this wonderful rocket stove principal. We already made a small rocket stove and are happily cooking with it for months now. We have a few questions:
    – in your rocket stove shower you made a small opening in front (air inlet). In a lot of other designs of rocket stove mass heaters I don’t see that opening. Do you know if it is a necesarry aspect of the design?
    – you say you need 15 minutes for heating the water, in spring and summer. What about wintertime? No showers?
    – we want to make the rocket stove shower inside our house. Do you know anybody who did this already? Any adjustments necesarry?

  7. the heat leaving your chimney should be a pretty good indicator of efficiency. If it’s not coming out the vent, it’s either escaped somewhere else (heat riser?) or it’s in the water and stove parts. So, you could feel around for heat loss. Maybe some insulation on the heat exchanger? otherwise there is just the insulation sufficiency of the storage tank. I’m curious, just how warm is the chimney? Can you put your hand over it indefinitely while it’s burning? It looks pretty great. I think solar would be an awesome addition.

  8. To whom it may concern at milkwood
    I had a little question about the rocket stove shower system.
    I think I’t great!
    is there any reason why the burn tunnel is so much higher than it is wide?
    Thanks Joe Blow

  9. hi i have three questions: 1) where does the water come from and go to after the shower, 2) are you using electricity to pump the water to the shower, 3) what’s the temperature of the air coming out of the exhaust vent, if you know?

    k thx. beautiful job, i like your creativity and courage to try something new.

  10. Awesome !! I built a small rocket stove out of cans just to try it out & it works great !! I wanna build one of these next !! Need some land first !!

  11. Do you have a ratio of size for the feed barrel compared to the burn tube and also to the burn tube to its house outside diameter? I would also love to know the ratio for the height of the burn tube to it’s size? I want to work on something very small and test ratios by making mini rocket stoves. Say I have a 4 inch round cylinder by 7 inch tall. I could guess at the tubing used for the heat riser and fee barrel but want a ratio that can be adapted to a larger model?

  12. Dear Milkwood people
    Brilliant!! thanks so much for sharing this, well done.
    Many thanks because it has helped me build a system which I have just tested, and I would like to share the details:
    Rocket stove j type 150mm square section, feed tube 500mm high, no heat riser.
    Boiler 200mm dia. 305mm high, 8x 38mm flue tubes (6.6l water capacity). positioned straight on burn tunnel.
    firebox with slide gate for control to 100mm dia flue 3meters tall.
    for test connected to 80l hot water cylinder.
    9:30am water temp at start 10.8degrees celcius
    10.00am 42 degrees
    10:30 65 degrees
    11:00 68 degrees
    During test the smokebox surface temp reached a max of 146 degrees.

    Very happy with results, just need to work on making the burn tunnel from firebrick now. Kind regards joe blow

  13. ROTFL! Comment 13 and your reply just made my day…I can feel a melding of “The Star Spangled Banner” with “Tie Me Kangaroo Down Sport” in my immediate future 😉 Hugs from sunny Tassie and cheers for the excellent share. I LERVES me a good rocket stove! (And one day I am gonna build one! 🙂 )

  14. Great work!! I’ve used a rocket mass heater before for space heating. I am planning on building something very similar to this in the very near future. I like the thought of heating large batches of water to be used for space heating as well as household hot water. So many applications and possibilities here! Check out my Rocket stove at streetjesus.blogspot.com

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