I've been reading and listening to podcasts about rocket stoves and water heating in general. While doing this research, I realised that some cross pollination might be helpful to some of you here...
I've been involved with craft brewing on and off for some years and followed the development of home based all grain brewing technology.
Brewing requires oodles of hot water at various temperatures and the basic approach to providing this is to have a great big pot full of hot water that is maintained on a gas burner. It works but it's cumbersome and not exactly energy efficient.
The next step in the evolution was the concept of steam injection. This represents a big increase in efficiency and control because steam can be used to precisely raise the temperature of the mash without diluting it so that enzymatic activity is not degraded. the steam can be injected into water, wort or even the mash directly.
The early models had a boom squish factor of 12 - basically sealed kegs full of pressurised steam just waiting to take out the neighbourhood.
This was until someone showed how to construct a flash boiler.
In a flash boiler, cold water is fed into tubes that are really hot. The water is rapidly heated and by the time it exits the tubes, it has turned to steam. The system has basically no boom squish factor at all. In fact, they tested them by heating the tubes as hot as they can and then introduce the cold water. Because the steam end is always open, there is no pressure build up.
Because the water is heated on demand (you can just produce hot water instead of steam by increasing the water flow or decreasing the temperature), there is no risk of legionella bacteria contamination.
Here are some links to a brewing forum talking about the construction of a flash boiler: