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Future Directions - Drain-Water Heat Recovery

Hot water that goes down the drain is wasted energy–as much as 80-90% of the energy used for water heating. Drain-water (or greywater) heat recovery systems capture this energy to preheat cold water entering the water heater or going to other water fixtures.

Drain-water heat recovery technology works well with all types of water heaters, especially instantaneous and solar water heaters. Drain-water heat exchangers also can recover heat from the hot water used in showers, bathtubs, sinks, dishwashers and laundry washers. They generally have the ability to store recovered heat for later use. A unit with storage capacity is needed for use with a dishwasher or laundry washer. Without storage capacity, useful energy is only available during the simultaneous flow of cold water and heated drain water.

Some storage-type systems have tanks containing a reservoir of clean water. Drain water flows through a spiral tube at the bottom of the heat storage tank. This warms the tank water which rises to the top. Water heater intake water is preheated by circulation through a coil at the top of the tank.

Non-storage systems usually have a copper heat exchanger that replaces a vertical section of a main waste drain. As warm water flows down the waste drain, incoming cold water flows through a spiral copper tube wrapped tightly around the copper section of the waste drain. This preheats the incoming cold water that goes to the water heater or a fixture, such as a shower.

By preheating cold water, drain-water heat recovery systems help increase water heating capacity.