Abstract: A grey water to clean water heat exchanger and methods of use. A heat exchanger having a copper coil carrying clean water inside a grey water discharge pipe is described. The heat exchanger can use counterflow geometry. The heat exchanger has a controller that directs grey water through a bypass pipe whenever the grey water has too low a temperature to provide useful thermal energy. When the grey water is hot enough, the controller directs it through the heat exchanger to heat the clean water. A filter is provided to eliminate fouling of the interior of the heat exchanger. The controller provides an indication when the filter needs to be cleaned or replaced.