Hi, some days ago I got the idea to design a rocket heater to get very simple hot water. The thing is at the usual barrel construction, that the gases get colder on passing the barrel, so they get heavier and the draft gets stronger. On the one side, the heat riser has to be insulated and has to have a significant height to get good draft. On the other side: If you cool the down going gases better, than you get a better draft.
The first idea was to construct a barrel with water on the outside. But it was to difficult for me to construct. The new idea is to use hot water boilers, fired by wood. In Germany, we call it "Badeofen". Make a google picture search with this word and you know what I mean. In englisch I found the google words: "water heater wood" which showes quite good results.
Usualy these water heaters are fired just under the cold water. Thats crasy. It's nearly that crasy as to make the fire just in the water (
http://snorkel.com/hot-tub-info/snorkel-and-scuba-stoves.php) The fire has no time and space to burn hot and good. So my first Idea was to build a "J" with a insulated heat riser just beside the "Badeofen" and then let the hot gases go down in the waterheater. Then the hottest gases meets the hottest water and make it more hot. A little bit more down, the warm gases meets the cold water and heat it up. Sure, ther will be condensate at the bottom of the water heater, but you only had to let this water flow outside the house.
Than I thought: Just take two of the "Badeofen" and put water only in the second. In the first, just cut of the top and fill in fireproof insulation (vermiculite).
Than you even don't have to build a "J". You just can use the original firebox to make the fire.
If you like you can make a bench out of clay after the water heater to get a Rocket-MASS-Heater. The waterheater is without pressure, so be aware, that the hot water will drop out on getting warmer.
I just believe, that it might me very nice to have such combination of two "Badeofen" just beside a bathtub.
I don't have build it yet, but I want to share the idea.
See the attached picture for better understanding.