The scope of this thesis includes the modelling and simulation of a heating and cooling system operation of a nZEB (nearly zero energy building) educational institution, as well as the on-site electricity production. The net surface of the analyzed building is 1590 m2. Hourly heating and cooling demands have been calculated in TRNSYS, with the annual heating demand of 16 kWh/m2a, and cooling demand of 22 kWh/m2a. A thermally-active building system has also been modeled in TRNSYS, and it represents a heating/cooling energy emitter in the building. Based on the analysis of similar systems in practice, the supply water temperature has been set to 30°C for heating and 17°C for cooling. Furthermore, an air – water heat pump, which represents a heating/cooling energy production system, has been modeled. The model is based on the data from the heat pump manufacturer. The model has resulted in an equation that calculates the COP and the EER for any operating condition. Specifically, if the supply water temperature, ambient temperature and a heat pump part-load ratio are known, it is possible to calculate the COP, the EER and the electricity consumption for each hour in a year. For the analyzed system, the heat pump annual electricity consumption is 9 kWh/m2a. Finally, photovoltaic panels have been modeled as an on-site electricity production system. The panel slope has been optimized so that it enables the maximum electricity production. With the optimal slope of 25° the system produces 124 kWh/m2a of electricity. The panel surface has also been optimized, with the goal of finding the surface area needed to achieve the net-zero energy standard (the energy consumption equal to the energy production of a building). For the analyzed system that surface is 111 m2.