How to make a rocket stove water heater

Eco Films
Thursday, 22nd September 2011

It looks like a white camel with a fat hump on a pillar but in actual fact, its a hot water system. Rocket Stove technology has been around for many years, dating back to ancient China as a method of central heating, but this rendered mud brick construction is used out in the field near the student camping area at Zaytuna farm.

We're outdoors filming the last sequences for The Urban Permaculture DVD at the Permaculture Research Institute in Australia and managing director Geoff Lawton points out his new toy.

Washing his hands in the outdoor sink he demonstrates how well it works. Upright timber that looks like thin fence palings or tomato stakes are standing upright and a warm fiery glow. A great idea I think. Only the last few inches are burnt and the stick burns down like a candle, falling down under its own weight.

In the cold winter air, its comforting to stand next to this quiet system as it burns very little fuel. The strange hump contains a 44 gallon drum that is the engine of the water heater system. We're impressed. Geoff always has something interesting to show us.

Geoff explains the benefits of using a rocket stove to heat your hot water. Very little timber is used he says, in fact only one fifth the timber is needed to harness the energy used in traditional stove hot water systems.

Rocket stove design

The rocket stove is an ingenious system that radiates hot air around the thermal mass of a water filled drum. Inside the water filled drum is about 18 metres of tightly coiled copper pipe that will turn cold water into hot. Its the water in the coiled pipe that will be used in the house. The water in the drum transfers its thermal mass energy to the coiled tubing. Low tech solutions like this are terrific to see. There is very little smoke out of the flue. We think of our own increasing power bills as Geoff points out the timber drying and storage cabinet built into it.

Rocket stove pizza oven?

By now we're getting hungry. I mention to Geoff that I have an improvement on his design. Could he also turn this into a pizza oven as well? Now that would be an even better idea – suited to good permaculture design. One system but with many alternative functions. The essence of good design ;)

There are many different rocket stove designs. For some reason they appeal to permaculture people who always hanker for good innovative design that is also energy efficient.

You can visit Milkwood Permaculture for even more innovative uses including a hot water shower system.

Rocket stove used in aquaponics

Rocket Stove technology like this could also be used in many unexpected ways. If you're into aquaponics – imagine a system like this to heat the thermal mass of your greenhouse to keep the water at an ideal temperature and the fish happy?

Bacteria slow down activity in colder weather so a cheap system like this could keep your system at optimal condition and also be very cheap to run.

Detailed rocket stove plans

If you decide to build a rocket stove, two things you may need to know. You will need add some kind of blow-off valve as pressure will build inside the drum and the water temperature will need to be regulated so as to not scald people with boiling water – or indeed explode!

The other thing to be aware of is that you will need to cut an access hole in your structure to reach the screw-on lid on your drum and top up water lost through evaporation or condensation. 

Eco Films is an independent production house based in Queensland's Sunshine Coast, Australia. Frank Gapinski produces, shoots and co-edits a lot of the titles with his wife Jane who also researches, edits and fills in as an occasional sound assistant when needed.

Permaculture DVDs by Eco Films


Establishing a food forest

Introduction to permaculture design

Harvesting water

Andrew Drummond |
Sun, 23/12/2012 - 17:38

If you want to get as much heat from the fire exhaust into the water as possible (thermal efficiency), in order to save fuel, then you need to maximise the surface area that the hot exhaust gases come into contact with, and having that drum there minimises that surface area while adding some thermal inertia and convection losses to the system.

You would be better off either having that cold-water-carrying copper piping directly exposed to the hot gases by removing the drum, or running the exhaust through a twisting/winding flue/pipe through a heat transfer tank. The latter may be difficult to clean though if your burnt fuel deposits creosote in the chimney.

Also, any twisting pipe should ideally be coiled around the vertical axis, much like a tornado (they generally don't go sideways for good reasons - natural laws of thermodynamics), instead of coiling along the horizontal axis as the one shown above does, since in every one of those coils shown the heated water has to fight against its tendency to rise above cold water, in addition to the internal friction of the pipe, in order to pass into the next coil. Without a barrel there this alignment change would be crucial, otherwise you would have to worry about steam bubbles collecting at the top of those horizontal coils.

Possibly the simplest improvement that could be made to that water heater design would be to align the barrel on its end with the same old piping fitted, so that heated water is constantly moving upwards in its winding path instead of repeatedly up and down. That wouldn't be easy to do now though since they bricked it in.

Have a look at heat exchangers such as those typically used in biomass, gas or coal-fired power plants for more ideas.

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