Heat recovery from water sources such as sewage water or condensation loops at low temperatures (usually between 10 and 30 degrees C) is becoming very valuable. Heat pumps are a potential technology able to overcome the high water temperature lift of the Sanitary Hot Water (SHW) application (usually from 10 degrees C to 60 degrees C with COPs up to 6). This paper presents a model to find the optimal size of a system (heat pump and recovery heat exchanger) based on water sources to produce SHW compared to the conventional production with a gas boiler in order to maximize the benefit. The model includes a thermal and economic analysis for a base case and analyzes the influence of a wide set of parameters which could have a significant influence. Even the uncertainties involved, results point out considerable benefits from this substitution based on the capacity of the system. Thus, demonstrating the importance of the optimal size analysis before an investment is done. (C) 2017 Elsevier Ltd. All rights reserved.