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There are different options for modelling indoor and outdoor long-wave radiation exchange in thermal building models for simulations at urban scale. For improving these building models, a good trade-off between accuracy and simulation time is a major challenge. To evaluate different radiation models for thermal network building models, we compared four outdoor radiation and two indoor radiation models. For the comparison, we set-up three test cases on a generic room and a single family dwelling and analysed surface temperatures, heat demands, and simulation times. The results favoured an outdoor radiation exchange model according to the German Guideline VDI 6007 with modified parameter calculations. It includes important simplifications that lead to short computing time while keeping a sufficient accuracy. For indoor radiation exchange modelling at constant temperatures, a linear approach significantly reduces simulation time without any major accuracy losses.

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In the context of district energy systems, changes in a building's heat demand also affect the energy efficiency of the whole district heating network. We present an integral dynamic model of a district heating network branch with three buildings. By comparing simulations of a reference case and a retrofit scenario we show that reducing the buildings' heat demands also reduces the mass flow rate in the network branch. We discuss the effects of this mass flow reduction on the energy efficiency of the district heating network and deduce that a district-level system perspective can lead to more comprehensive evaluations of retrofit efforts.