Heating, cooling and domestic hot water supply represent the biggest share of energy demands in residential buildings. Usually for these demands fossil fuels are used, however in the last years international energy policies recommended the use of less valuable energy. This paper investigated the possibility to use sewer water as alternative source of heat. Wastewater deriving from buildings is characterized by higher temperature than clean water because inside the buildings 60% of water is heated. The amount of energy that can be obtained from wastewater and the optimal design of heat recovery systems depend on knowledge of the flow rate and the temperature. In this paper the sewer system in Bologna (Italy) is taken as case study. Results of a monitoring period have showed the variability of wastewater flow rate and temperature and their correlation. Data analysis allowed to identify the daily trend for the wastewater flow, whose coefficients in relation to the average flow (where average flow = 1) vary between 0.25 and 1.50, and for the wastewater temperature in which the coefficients ranging from 0.90 to 1.05. This study can be useful to map the potential thermal energy of sewage systems and to design of heat recovery system. (C) 2013 Elsevier B.V. All rights reserved.