I actually got this idea while in the shower. I thought it was such a pity pouring all that hot water down the drain. I thought some absurd stuff like using a stirling engine to charge batteries and whatnot, but it quickly became obvious that this stuff had to be kept simple. So, would it work piping the shower water through a heat exchanger on the cold water inlet on the water heater? How much energy would you save? Why hasn't it been done before? I don't consider myself cheap, but I don't like the idea of pouring perfectly good joules straight down the drain. After all, earth's resources are definitely finite, and the rate of consumption in western countries are most significant.