Drain water heat recovery (DWHR) systems can save homes on their energy bills, Forbes reports.
Water heating accounts for approximately 20 to 30 percent of household energy demand. Much of the energy is expended warming the water coming into the shower and other areas of the house. By transferring some of the heat from the drain water to the incoming water through a heat exchanger, reduced energy is required. A significant portion of the heat from the drainwater is efficiently transferred to the incoming water with no energy use. By reducing the demand on the water heater, this also allows for increased capacity of the tank.
This is achieved by a variety of systems called Drain Water Heat Recovery (DWHR). As warm water flows from your shower down the waste drain on its way to sewer, the DWHR device uses heat exchange to transfer the heat to an incoming flow of cold water headed towards the water heater.
These systems generally work on tank water heaters as well as on-demand ones. They are particularly popular in Canada and northern areas of the United States where the ground water is very cold.