Drain water heat recovery systems are heat exchangers designed to recover energy from a building's greywater, and use it to preheat incoming mains water. The rated performance of such systems is determined at controlled mains-side and drain-side inlet temperatures, and under conditions of equal flow. This work aims to experimentally investigate the impact of having unequal mains-side and drain-side flow rates on the performance of these systems. As expected, experiments show that heat exchanger performance is strongly linked to fluid inlet flow rates. However, the findings also showed that variations in either of these flow rates lead to predictable changes in performance, regardless of diameter, length, or temperature. A semi-empirical equation is established that is capable of correcting system performance for different inlet flow rates based on the rated performance results. (C) 2018 Published by Elsevier B.V.