This is Gary Klein's turf. He is a lecturer and educator. He pioneered the concept while he was with the California Energy Commission. For more on it, read http://www1.eere.energy.gov/solar/pdfs/sda_saving_water.pdf on the Internet.

But briefly, his idea was to build the plumbing system in such a way that quick hot water would be available to every fixture, with no greater energy consumption than normal, and that no more than one cup of water would run down the drain during a wait for hot water.