This study is to develop a simulation model of a hot water system taking into account the time dependent phenomena which are important for the operational management of district heating systems. First, an exhaustive list of all dynamic phenomena occurring in district heating systems has been drawn and analyzed. Considering this list, this paper proposes that a model which satisfies the criteria listed above can be developed by superposing four sub-models which are a dynamic model of the heat generation plant, a steady state model of the hydraulic calculation of the distribution network, a dynamic model of the thermal behavior of the network and a dynamic model of the heat consumers. A computer code containing these four models has been developed. The entire district heating system of the city of Lausanne with more than 1000 nodes and 19 circulation loops has been simulated on a personal computer. In conclusion the comparison of measured and calculated values has shown that the proposed superposition of the four sub-models is adequate for the development of a simulation model satisfying the initial goals of this study.