One of the ways of diminishing energy consumption for hot water heating is the use of Drain Water Heat Recovery (DWHR) units. The aim of the use of these devices is thermal energy recovery from warm drain water and transferring it to incoming cold water. This paper presents the calculation model that allows the estimation of the financial efficiency of the project involving the construction of a shower Drain Water Heat Recovery system in a single-family dwelling house. The presented method of investment risk assessment can be used for decision making by individual users, designers and others. The study of the financial performance was carried out for the various parameters of the installation and the different heat recovery system configurations. From investors point of view the most beneficial option of heat recovery system installation is the system in which preheated water is fed to both the hot water heater and shower mixing valve. Additionally, it was proved that obtained financial results are affected by showering time and water consumption. DWHR units will be therefore particularly beneficial to apply in case of swimming pools, sports facilities or fitness clubs, where high rotation of users is observed. (C) 2013 Elsevier B.V. All rights reserved.