The purpose of this work is to experimentally investigate the impact of mains-side and drain-side water temperatures on the effectiveness of falling film drain water heat recovery (DWHR) systems. DWHR systems are heat exchangers that recover energy from a building's greywater, and use it to preheat incoming mains water. According to standards, the effectiveness of such systems is typically determined at controlled mains-side and drain-side inlet temperatures. Four DWHR systems of varying diameter and length are tested at inlet temperatures of 25, 30, 35, 40 and 45 degrees C on the drain-side, and 5, 10, 15, 20, and 30 degrees C on the mains-side. The effectiveness for each DWHR system is calculated in each test. Experiments show that heat exchanger effectiveness is strongly linked to fluid inlet temperatures, and that increases in either of these temperatures would lead to an increase in effectiveness, even if the temperature difference between the inlets is kept constant. These changes in effectiveness occur regardless of diameter, length, or flow rate. A semi-empirical correlation is established to correct system effectiveness for different inlet temperatures. This correlation represents one step in the development of a building simulation model of falling film DWHR systems. (C) 2016 Elsevier B.V. All rights reserved.