Evaluation of Residential Drain Water Heat Recovery Unit
An assessment of the energy recovery capabilities of a drain water heat recovery (DWHR) unit was conducted at the Archetype Sustainable House at the Living City Campus at Kortright. A DWHR unit removes waste heat from drain water to preheat the incoming cold municipal water. Heat transfer is accomplished by wrapping a copper coil, carrying cold municipal water, around the drain pipe. The study was initiated in 2010 and assesses the performance of a single DWHR unit by utilizing a pre-determined water use profile for a single family home with four occupants. Shower and bathroom events were simulated for cold and pre-warmed incoming water scenarios.
Through a determination of two common DWHR measures, effectiveness and number of heat transfer units (NTU), it was determined that the unit performed well, with increasing amount of recovered energy during shower events. This is explained by the large volume of water over longer period of time available for heat exchange through simultaneous flow. Additionally, the unit performed better when it has been recently in use and has already warmed up. Therefore, DWHR units perform better when used often, and it follows that they are good energy efficiency solutions for busy family households. The relatively large amount of energy recovered annually, approximately 790 kWh, and their low cost results in payback periods of as little as 3 years when installed to recover heat from shower drain water.