Energy consumption for water heating accounts for 17% of residential and 7% of commercial site energy consumed. A study conducted by the U.S. Department of Energy found that approximately 350 billion kWh of energy is sent down the drain in the form of used hot water annually. This month’s column discusses the use of drain water heat recovery (also known as graywater heat exchanger) technologies can reduce the energy consumed for water heating by recovering some of this energy, and recycling it to preheat incoming water.