An analysis of a high-temperature circuit of a building energy system is applied with respect to the thermodynamic- and thermoeconomic point of view. The building energy system is supplied by a micro CHP unit and two boilers in parallel. Following the second law of thermodynamics, in the thermodynamic analysis parameters can be found, which affect the magnitude of thermodynamic inefficiencies. The thermoeconomic analysis combines thermodynamic analysis with economic aspects to identify factors involved in the generation of energy costs. The used economic model is based on the Total Revenue Requirement method. As input for the analysis, data from a model of the energy system is used. To capture dynamic effects of the energy system, the system is modelled and simulated with the help of the object-oriented modelling language Modelica. The results of the simulation are afterwards analysed with the help of an evaluation tool implemented in MatLab. Highest inefficiencies and costs are related to the processes in which natural gas is burned, hence in the CHP unit and boilers. Furthermore, the charging and discharging cycles of the heat storage tank and the heat consumption units cause high inefficiencies and costs. The presented analysis can be extended to other energy systems. (C) 2016 Elsevier Ltd. All rights reserved.