The present paper deals with the classification of the suitability of combined sewers for the installation of heat exchangers and with assessment of the theoretical potential of wastewater in the sewer system for heating of buildings. A classification scheme involving criteria like theoretically available heat, sewer diameter, number of the heat exchanger parallel modules in the sewer cross-section, hydraulic conditions (hydraulic capacity of the sewer, pressurized flow), and potential fouling by biofilm growth was developed. First, individual sewers in the pilot catchment were assessed based on monitoring the flow characteristics and wastewater temperatures and on pipe flow modelling. Second, connectivity of the suitable and partly suitable sewers was examined with respect to the length necessary for the installation of the heat exchanger with the minimum required power of 100 kW. For the continuous sewer sections, the maximum potential power was calculated. The presented approach is generally applicable, however, for other heat exchanger types and other climatic and economic conditions, values of the suitability criteria for the heat exchanger installation must be adapted.