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Abstract Falling-film drain water heat recovery systems have proven to be a cost-effective and reliable class of heat exchanger for reducing primary energy consumption in residential and commercial buildings and in industrial buildings and processes. It is fitting, therefore, that regulatory bodies are preparing standards by which various products can be characterized, both for rating purposes and to provide data for building energy simulation. Unfortunately, standards development is progressing in the absence of measured performance data that characterize how these heat exchangers perform. The intention of the current work is to examine drain water heat recovery performance at various equal-flow conditions. The effectiveness of three drain water heat recovery systems was examined in a counter flow as a function of volumetric flow rate. The drain water heat recovery systems represented products from two manufacturers and two lengths. One of the drain water heat recovery systems was also tested in parallel flow. While the performance characteristics generally mirrored theoretical performance, there were some key differences. For the units tested, there was a clear transition region occurring between flow rates of 5 and 10 L/min (1.3 and 2.6 gpm). While the presence of this region did not impact the proposed rating process, it could significantly impact the applicability of the data analysis to building simulation. It was also shown that the number of transfer units for the collector changed significantly with flow rate, but in a predictable manner. By fitting the number of transfer units versus flow rate data, and using this correlation in conjunction with theoretical ϵ–number of transfer units equations, the drain water heat recovery performance could be well predicted over the entire range of operation. Acknowledgments The principle author (MRC) acknowledges the support of RenewABILITY Energy Inc., of Kitchener, Ontario, Canada, for supplying the DWHR units used in this study. Michael R. Collins, P.Eng., PhD, Member ASHRAE, is Professor. Gerald W. E. Van Decker, P.Eng., MA.Sc., Member ASHRAE, is Engineer. Joel Murray, P.Eng, is Engineer. Notes 1 PJ = 1015 J, 1 GJ = 109 J. The manufacturer's quoted accuracy for this device is better than the accuracy used in the current analysis; the manufacturer's quoted accuracy was deemed to be unreasonably low. Insulation was added in accordance with the NRCan testing protocol (NRCan Citation2008). The new CSA standard (CSA Citation2012) dictates that the DWHR system remain uninsulated. As of January 2013, the new CSA test standard requires flow rates of 5.5, 7, 9, 10, 12, and 14 L/min (1.5, 1.8, 2.4, 2.6, 3.2, and 3.7 gpm). Based on the principle author's experience of having tested more than 100 DWHR systems in the past 4 years. Fig. 8 Application of rating procedure to DWHR units tested. Nominal effectiveness determined at flow rate of 9.5 L/min (2.5 gpm) mains-side inlet temperature of 8°C(46.4°F) and drain-side inlet temperature of 36°C (96.8°F). Display full size