Waste heat recovery is the collection of heat created as an undesired by-product of the operation of a piece of equipment or machinery to fill a desired purpose elsewhere. Waste heat recouping methods range from the simple to the complex. A common simple example is household water drain heat recovery. In this method, the heat going down a sink or shower drain is recovered by a copper pipe coiling around the drain pipe. The coil is then used to heat water as it passes through pipes on the way to a hot water heater.