In a typical home, a water heater heats water only to be used momentarily.  Then all that heated water is lost down the drain. With Drain Water Heat Recovery, that thermal energy is put to use by preheating the incoming water. 

Hot water flows down the drain pipe which is surrounded with copper tubing.  The new, incoming water travels in the tubing while absorbing the heat from the drain pipe. Now the incoming water has gone from 43 degrees to 72 degress Fahrenheit before it even reaches the water heater.  That means less energy is needed to heat your water.