The Department of Energy estimates that 80%-90% of the heat energy that is produced by water heaters is utilized once and sent down the drain. A drain-water heat recovery system draws energy from warm waste water and uses it to preheat incoming cold fresh water. The process involves copper drainpipe wrapped with copper coils to allow warm water to transfer heat through the pipe to the copper coils carrying cold fresh water. By preheating incoming fresh water a large amount of energy is saved, saving water in the process.
Learn more about drain-water heat recovery systems here.