Future Directions - Drain-Water Heat Recovery
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
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.
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.
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.
preheating cold water, drain-water heat recovery systems help
increase water heating capacity.