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We’re using this awesome rocket stove powered wall oven for all the catering here at Koanga — it gets daily use and we’re loving it! The amount of wood required is minimal, a tiny fraction of what a regular wood fired stove would use.

by Tim Barker

During our last workshop at the Koanga Institute, we built a rocket stove. Our design brief was very specific in that the stove had to be practical, easy to use and long lasting, whereas what is being built by most backyard experimenters like myself, while being fun to make and muck around with, are more along the lines of ‘camp stoves’ built from tin cans that quickly disintegrate with use. Another consideration which I felt was important was that most rocket stoves are just that – stoves, as in place a pot on top and boil something.

What I envisaged was not a rocket stove but a rocket oven. This is a concept I had originally developed nearly three years ago when living in a small two room shack. It had a camping gas burner but no oven. Missing those things that only an oven can provide is a great motivator. Of course, due to my previous work making rocket stove powered water heaters (a subject we’ll get to at a later date), it was always going to be rocket powered. The hard part, or so I thought, was making an oven chamber that was insulated, had racks and a fitting door. Plus, it had to look good. It was then that I realised I had described an electric or gas oven. From there, the rest was easy. A quick trip to the local scrap merchant secured an old bench top unit where the hot plates are beside the oven. Cost: $20.

After discarding the hot plate section, the next step was to build a rocket stove ‘j’ elbow (see photo), cut a hole in the base of the oven and sit it on top of the rocket stove. The next step was to insulate the rocket stove for maximum combustion efficiency and finally the fun part – light her up!

I can honestly say I’ve never done an experiment that went so smoothly and which easily met and surpassed my expectations. With only slight modifications and tweaks, this oven was in near daily use for approximately two years. The stove at the Koanga Institute is the distillation of everything that I’ve learned since in subsequent builds. Between our internship program and various courses, it is in near constant use and has performed flawlessly.

Finally, I can honestly say that as I pass through the institute kitchen during the day, I can’t help but go over to the oven and peer down into the combustion chamber to see if I can hear the soft rumble of the stove as it goes about its business.

Covering the rocket elbow is a 50/50 mixture of sawdust and clay, which acts as insulation and protects against getting burnt by the extremely hot elbow. Note the chicken wire mesh is used to strengthen and hold the mixture in place till it dries.


Our first rocket oven dinner! It roasts meat and vegetables beautifully… and
bakes excellent bread. We’re using very little wood — just a tiny fraction of
what we’d use in a regular wood fired pizza oven.

15 Responses to “Rocket Oven! A Permanent Wall Mounted Kitchen Oven”

  1. Matthias Luthi

    Fantastic, thank you for the post, this is such an excellent idea! I second the request for more photos (and maybe even a sketched up plan?) if you have them. Pretty please?

    Also looking forward to the rocket water heater post(s)…

    Reply
  2. Joao

    Thank you very much for sharing this “time and biomass saving” innovation – should we say? Please consider to update this post by adding the sketch and profile fictures.
    Probably one minus for rocket stoves is the ongoing need (minute by minute?) to surveilance and need to add fuel – how do you deal with that ?
    In the case of the vault-pizza oven we throw a load of wood inside and come back in 10-15 minutes, after doing this 4-5 times the oven is ready.

    Where I live we still use old heavy brick vault ovens similar to pizza ovens.
    Usually we have “too much” biomass in the mountain that we need a way to get rid of – fire the oven and make bread for a week.

    By burning this biomass in bread ovens, we reduce intensity fire risk-fuel load in forest areas in the mountains.

    Shredding this type of biomass, prunings and thinned trees, is not a option because it costs time/energy/money that is lacking in the first place. Even though we already have those machines in the region. To make bread/cook food instead it serves very useful functions.
    Ashes are used as fertilizer and in some special cases as “mulch”, like for onions and garlic patches – to prevent weeds to grow, modulate soil Ph, while allowing light-warmth to reach the soil during the winter.

    Reply
  3. Gordon

    This looks like a fantastic development, but yes, agree that some more information on the design – diagrams / photos and words will help others to emulate it. As it is, I’m left wondering what & where the “combustion chamber” is, and whether the gases from the rocket stove are entering the oven where food is cooked, or whether they merely heat the oven.

    Reply
  4. Fraser Bliss

    Having personally tasted the results of your prototype, I’m excited to learn about your new and improved version. Hope to be able to share a pizza with you again sometime soon!

    Thanks for the article Tim, and take care.

    Reply
  5. Tim Barker

    Hi all and thanks for your comments. I get patchy access to Internet so will try and answer you all in one hit . Yes I agree more details and a follow up are required . I hope to start a steady stream of articles about the stuff I’m mucking round with here at the Koanga institute and our upcoming Appropriate tech workshop. Joao the fuel in the oven gravity feeds down so depending on thickness, type , and length of wood at the shortest the oven is checked every 20 mins and as little as every 40 mins at the other end of the scale . Also because the wood only burns at its end it can combust more efficiently. The other advantage with a non mass oven is that I can be cooking in the oven in as little as 10 mins from start up . I absolutely love the food that comes out of cob and other style mass ovens but the message that these are “Eco friendly ” in the context of having one in someone’s backyard in the suburbs is misleading . Mass ovens store heat and used as you use yours they are efficient because the stored heat only has to be topped up each day. Used in the context of firing one up to cook two or three pizza’s once a week wastes a huge amount of wood which I might add is not combusted efficiently. With any technology I try my hardest not to “love ” it too much , it’s a trap because we find ourselves falling into the pattern of always applying the same solutions . I think rocket stoves are great but I don’t think in every situation they are the answer. As you have rightly pointed out a mass oven works for you in your situation.

    Next steps for our oven will be an insulated container above the stove that houses a hot water resiviour . Because of the nature of a rocket stove the flue gasses are very easy to capture and direct , so using those gasses combined with a hot water system I built for the interns at the PRI in Australia while I worked there we can cook our cake and wash up after it using the same heat source and no additional energy . If we “cascade” our energy usage, then with careful design I can see a rocket stove powering the following processes in order of heat requirements. Hot plate , oven, water heater, TEG, food dryer clothes dryer, room heater and probably a few other uses I haven’t thought of. The TEG I refer to is a thermo electric generator, basically it’s a device that converts heat to electricity so you can recharge batteries, run electric devices . Remember all this without any additional energy that what is used in the oven for cooking. At this point I would like to give a very strong warning to all budding mad scientists out there that there is a huge amount of energy contained in boiling water . If you heat it in a container that is sealed like say a hot water cylinder and it boils and the pressure can’t escape then congratulations you have just created a huge bomb. I’ve heard it said that there is the equivalent of a stick of dynamite in everg half kilo of boiling water. In a future post I will show a solution I came up with that allows you to safely heat water In a way that is simple and relatively cheap. Gordon this type of oven has the somewhat misleading designation of being a “black oven ” basically this means the food is exposed to combustion gasses exactly like a cob oven or a barbecue. The term is misleading because food doesn’t come out black it comes out tasting delicious . You might be tempted to say the flavor is fine for savory stuff and meats and bread but what about sweet stuff , pastries cakes etc. the answer from approx three years of using this type of oven is that it comes out tasting great no undue or conflicting tastes. Of course if your burning painted wood or treated timber then don’t ! Your almost certainly poisoning yourself.

    Experience has show that connected to a normal flue pipe ( the one in the kitchen at PRI NZ is hooked to the old wood heater flue) no combustion products enter the room ever as the rising gasses in the flue will continue to draw even when the oven door is open , meaning air from the room flows into the oven .

    Of course it’s only a little harder to design an oven that is totally sealed from combustion gasses . I have seen examples of such built around old 200 L oil/ fuel drums that are over 80 years old in northern Australia . And more recently in North and South America there are a number of people constructing ovens this way. My own version of this uses a copper hot water tank as the heat transfer is approx 4 times better that steel and is more durable and should look very funky when finished. Oh and one last thing the original stove designed along these lines had no flue as it was outdoors under an eve, all I did was remove a 10cm length of the door seal at the bottom and this was sufficient flow to reach temps of 250 deg C , noting also that the original square stove feed tube was only 90 mm on a side , that’s only 3.9 inches for all those non metric readers that’s tiny!
    Cheers Tim

    Reply
  6. narf7

    I would love to see some detailed descriptions/illustrations of how you built this amazing oven.

    Reply
  7. Paul Murray

    Great post, thanks Tim, seems we’ve all been using WAY too much energy for cooking for WAY too long!

    All the best from Sunny Karamea,

    Paul.

    Reply
  8. maikel carder

    I need something like this…space heating, oven and if possible water heater, but sized for a boat. Has anyone designed or built such a device?

    Reply
    • Tim Barker

      Hi Maikel only just saw your comment. Of course you could build a system based around these principles for a boat but there are important considerations. Having lived on a boat i know you have to be very careful regarding ventilation. Consider the build up of carbonmonixide and other byproducts of combustion and also the build up of flammable gases such as petrol, diesel and battery fumes.
      Cheers Tim

      Reply
    • Tim Barker

      Hi Sheila, not so much talented as just made lots of really interesting mistakes and had fun while doing it but thanks for the kind words.
      Regards Tim

      Reply
  9. eduardo kafka

    Hello there Tim…great work you have done….I have a question, in your oven..the combustion gases go thru the oven itself where the food is or did you do a separate cooking chamber within the old stove and the gases just heat that chamber?….second…could you show some pics of the water heating system please, I am trying to implement these systems in the poor side of town here in southamerica. Thanks…kudos.

    Reply
    • Tim Barker

      Hi Eduardo the gasses go through the oven. Go Here http://www.koanga.org.nz/low-mass-insulated-barrel-oven/ and that will give you some ideas for ovens that separate the gasses from the food. If you email the Koanga institute (where the link is) they have a “how to” booklet that i wrote for them to build ovens from 200l drums. It’s not up on the site i see but i know they have sold a few hundred of them so just enquire or if your handy have a look at the link and i’m sure you will work it out. Email me at timclaire1@gmail.com and i should be able to get some info to you re the hot water systems that should help.
      Cheers Tim

      Reply
  10. eduardo kafka

    Thanks Tim….yes, I found the koanga website, and your blog, but for some reason I am not able to see the pics you posted there….thanks for the info, I will e mail you..keep it up.

    Reply

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