Recent studies have indicated that wastewater contains relatively large amounts of thermal energy. Recovering this thermal energy can be used to decrease the CO2 footprint of the water cycle. This paper describes the development of a model to simulate the heat balance and predict the temperature in a sewer system. The model can be used to estimate the recoverable thermal energy and its dynamics. The model was verified with field data. It was concluded that the model is a powerful and accurate tool to simulate the heat balance of a sewer system at the urban district level. It was found that the recoverable heat show highly dynamic patterns, directly related to water consumption patterns. The recoverable heat depends on technical aspects as well as regulations for maximum acceptable temperature differences due to heat abstraction.