I’ve been looking at lots of applications for Rocket Stoves. It is a beautiful and simple DIY technology I want to use as much as possible in our new house. This includes heating water. We currently have a simple (purchased) wood-based boiler that does an OK job and I suspect even has an inherent “rocket” effect … and mostly prooves that it can be done. So I’ve been looking around for hot water solutions based on Rocket Stoves.
Before I go into the details of the one I found I would like to point out one piece of advice about Rocket Stoves that I came across, stuck with me and is exemplified by it. Rockets work best when they are designed with one primary purpose in mind. This means that a rocket designated for hot water will probably be more feasible and work better then a rocket that is used to heat a space AND heat water. I can testify that it can be very tempting to build a supreme-do-it-all rocket … but it just doesn’ work.
It is a rocket-stove dedicated to heating water and does nothing else. Because it is a single task rocket it is actually simpler then the basic rocket-stove since there is no heat exchange barrel and the water tank itself is the thermal mass. The key element is a heat-exchanger that sits on top of the heat-riser. It is a metal box within a box – where the heat from the rocket is transferred into the water. Depending on the position of the water tank flow is either achieved either passively via thermo-siphoning or with a pump.
The heart of this solution is the heat exchanger and at the heart of the heat-exchanger are small plates of metal welded into it, which increase the contact surface between the hot exhaust and the metal itself. With these plates a 1 meter tall heat exchanger can be designed to have 6+ meters of contact surface (as if the exchanger itself was 6 meters tall!). The trick is to size the internals in such a way that the surface area of air-flow will not become smaller then that of the heat-riser so that the exhaust can flow smoothly out. If designed optimally then, as with most good Rocket Stoves, there should be very little exhaust heat left in the chimney pipe.
We don’t have (yet) the skills to make this kind of heat exchanger but I am confident we can find a metal-worker who can create one for us. My thoughts are to connect a 6 inch rocket to a 300 liter tank of water and that should provide a simple and efficient and backup for days where the sun does not provide enough heat for the solar hot water panels to kick in.