The Power-Pipe Drain Water Heat Recovery (DWHR) System from RenewABILITY Energy could be able to recapture a significant amount of the heat that currently goes down the drain with the rest of the hot water, and possibly reduce water heating costs by as much as 40%.
The Power-Pipe system uses the outgoing hot water in the drain pipes to preheat the cold water going into the water heater, and can raise the incoming water temperature to as high as 75°F, reducing the energy needed for water heating.
"Water falling down a vertical drain stack does not run down the middle of the stack, but instead clings to the inside wall of the pipe. In a DWHR application, falling drain water forms a thin film that rapidly imparts its heat to the colder pipe wall. The Power-Pipe® captures much of this heat, then "recycles" it to raise the temperature of incoming cold water." - Power-Pipe
The Power-Pipe is a maintenance-free solution, with no moving parts, and is guaranteed for 10 years. Models are available for residential, commercial, and industrial applications, with prices starting at about $500.