Drain Water Heat Recovery
Any hot water that goes down your drain carries away energy with it. Drain water heat recovery systems save energy by using the heat in drains to preheat water coming into the water heater. Estimates based on the DOE test procedure for water heaters indicate that the equivalent of 350 billion kWh worth of hot water is discarded annually through drains, and a large portion of this energy is, in fact, recoverable. To capture and put to use heat from wastewater produced by all sources in a dwelling would require a regenerator-type, double-walled heat exchanger, one that can capture heat from wastewater generated by one fixture or appliance (e.g., a clothes washer) and apply this heat to assist another hot water demand that may occur at a later time. If wastewater generation is concurrent with the need for hot water (e.g., a shower), a nonregenerative, straightforward heat exchanger can be used. Heat exchanger systems of each type are available for use in buildings.
Figure 41: Drain Water Heat Recovery: This system captures waste heat through heat exchange from the wastewater drains.