The renovation of energy-inefficient buildings in the public sector has been recognized as a crucial factor in achieving the European Union's climate goals. In Croatia, as an EU member, this issue has been addressed, among other measures, through the publication of A Public Call for Energy Renovation of Public Sector Buildings, worth 40 million euros. The funded measures achieve energy and financial savings as well as improved quality of living in the premises. This thesis analyzes the renovation and expansion of the Đurđevac Elementary School, with a usable area of 5,919 m², as a representative of public sector buildings. Since the public call finances the introduction of renewable energy sources, various heat pump designs as sources of heating/cooling energy have been analyzed. The first part briefly explains heat pumps with air, ground, and water as heat source. The next sections detail the areas, construction elements of the analyzed building, and describe the existing and planned thermotechnical system. The sizing of heating bodies for the extended part of the building was carried out according to the HRN EN 12831 standard and cooling bodies according to VDI 2078. Calculations were performed using the BricsCAD AX3000® software. For the comparison of different thermotechnical system designs, the annual required energy for heating (QH,nd) and cooling (QC,nd) of the entire building was calculated using TRNSYS® software and MATLAB® programming language, employing The algorithm for calculating the required energy for heating and cooling building spaces according to HRN EN ISO 13790. The required energy for the entire building for heating is 436,113 kWh/year and 63,831 kWh/year for cooling, including the energy needed to prepare air in water heaters and coolers for mechanical ventilation. In the next step, hourly values of heating factor (COP) and cooling factor (EER) of different heat pump designs were calculated using manufacturer data and MATLAB® programming language. Using the hourly values of COP and EER, the electricity consumption and seasonal heating factor (SCOP) and seasonal cooling factor (SEER) for each heat pump design were calculated. An energy and economic analysis of the air-to-water, ground-to-water in combination with a gas boiler and water-to-water heat pump designs was conducted. For high-temperature heating needs, an energy and economic analysis of using a high-temperature “Booster” water-to-water heat pump and gas boiler was also conducted. The calculated investment costs of the high-temperature heat pump amount to 74% of the gas boiler's investment costs. The high-temperature heat pump requires higher operating costs, where the gas boiler’s operating costs are 69% of the high-temperature heat pump’s operating costs. The analysis also accounted for the operating costs of a low-temperature heat pump used to raise the supply medium temperature. Based on the conducted techno-economic analyses, a water-to-water heat pump system was chosen due to the numerous limitations of air-to-water and ground-to-water designs with a gas boiler. The water-to-water design showed the lowest annual primary energy values (33 kWh/m²a) and associated CO2 emissions, a significant advantage from an ecological standpoint. The water-to-water heat pump demonstrates high year-round values of heating (COP) and cooling (EER) factors due to the stability of the heating/cooling source (water). Although maintenance costs are the highest compared to other heat pump designs, they ensure more reliable and durable equipment, maintaining high system efficiency. Future plans include expanding the cooling system to the existing part of the school, which would increase the cooling factor (EER) and reduce operating costs. The final chapter describes the chosen technical system and basic equipment at the level of the machine room/heat source. Additionally, an analysis was conducted to determine the minimum outside temperature at which it is possible to ensure the design indoor air temperature for the existing part of the school using a low-temperature heating regime. The analysis was performed for classrooms facing north and south orientation using available building data and the IntegraCAD® and MATLAB® software programs. The recommended limit temperature for selecting the bivalent temperature ranges between 5 and 10 °C. The limit temperature is a setpoint in the control system and the user can change its value during system operation depending on the building's needs.