Hi,
I guess the first thing to think about is safety. Water heater explosions are extremely violent -- they can literally destroy homes. One of the reasons that solar water heating became popular for a while years and years ago was that the early commercial gas water heaters did not have any thermostat, and had to be monitored by hand -- there were a lot of explosions. The "new" solar water heaters were advertised as having a big safety advantage because they would not blow up :)
You might take a look at this scheme:
http://www.builditsolar.com/Projects/Conservation/PilotLights.htm
Don basically just runs the pilot light all the time on his water heater, and has it set up so the main burner never turns on. The pilot alone provides enough heat for two people. So, I guess, you might be able to use this, or some similar sort of fixed flame that is set to just meet your daily demand. Just be certain that you have a good, working pressure/temperature relief valve hooked up to the tank in such a way that it cannot be isolated from the tank with any of your shutoff valves -- maybe two of them.
I like the suggestion for making it into a solar batch heater -- getting a good tank is the hardest part about building one of these, and you have that already:
http://www.builditsolar.com/Projects/WaterHeating/water_heating.htm#Batch
Gary
Gary
gary@BuildItSolar.com
www.BuildItSolar.com
