The simulation of urban energy systems, e.g. district heating or cooling (DHC) systems often in combination with co- or trigeneration and smart grids has to account for a huge number of different buildings (50 ... >500) to be provided with energy, e.g. heat and electricity. In that context is not sufficient to model each building including their dedicated energy system (e.g. hydraulic circuit of a heating or cooling system) in detail. The alternative modelling and simulation approach described in this paper is based on the idea to subsume the building energy system into an equivalent one-node model that accounts for heat exchanger characteristics, domestic hot water generation as well as space heating. Rather than building performance simulation this paper is focusing on the simplified modelling and simulation of the building energy system (i.e. space heating and domestic hot water (DHW) system) itself. Consequently the paper describes a method how to estimate return temperature and flow rate in the pipe connecting the building with a DH system. The underlying models have been validated mainly using empirical data gained at real buildings. Finally results of the co-simulation of large district heating system with simplified building models will be presented and discussed.