Greywater heat exchangers leverage the simultaneous flows of cold water and hot water to fixtures such as lavatories and showers. As hot domestic water is being used, cold domestic water enters the bottom connection of the heat exchanger and passes upward in a counterflow direction to the greywater. Heat from the greywater transfers through the copper tube walls and preheats cold domestic water. The two streams of water are always separated by two copper tube walls. A potential leak in either the inner pipe or outer coil would not cause contamination of domestic water.
In Figure 2-37, some of the preheated water leaving the greywater heat exchanger is piped directly to the “cold” water port of the shower valve. This reduces the required flow of fully heated domestic hot water to the shower valve.
Under typical operating conditions, the entering cold domestic water will be warmed 20º to 25ºF before it exits the heat exchanger. Thus, water entering the building at 50ºF, would be preheated to a temperature of 70º to 75ºF before entering the water heater. This reduces the water heating load by 29% to 36%, assuming a final desired delivery temperature of 120ºF. This is a significant contribution for a simple, unpowered and relatively inexpensive device.
Greywater heat exchangers should only be installed in vertical drainage pipes. They rely on the “film effect” of greywater passing through a vertical drainage pipe. Most of this water clings to the pipe wall, rather than falling through the air space inside the pipe. This is ideal from the standpoint of extracting heat from the inner copper pipe.