[conradpdx]

Don't know if the heat exchanger would work real well with such small amounts of water. Yes, there is a lot of heat lost to used wash water, but really by the time you include the air exposure from use, settling in the typically cold tub, flowing through the drain, heat lost to the pipes I doubt the typical shower would be enough to justify the cost of making the exchanger considering the amount of heat needed to keep your water heater full. Now if you were a laundry or someplace that used 100's of gallons a day the exchanger might be worth while.

However, not to get off topic, for preheating well water or even municipal water supplies a solar water heater is quite easy to build and install for most fairly competent DIY'rs. It's quite easy to google plans for one (I like Mother earth news personally). This is one of my projects for this next spring.