In order to achieve the EU's climate and energy goals, it is essential to continuously improve the efficiency of energy systems and to integrate renewable energy sources. In this context, the heating and cooling sector presents great potential, with significant progress expected in district heating systems. At present, fossil fuels are used in most district heating systems in Croatia, and while progress has been made in the use of biomass, the use of solar heat is limited to only one district heating system. At the same time, solar heat is used to a greater extent in households, hospitality and process industry. In this thesis the integration of the solar thermal system into a low capacity district heating system in the Zagreb climate zone is discussed. A model consisting of a calculation of solar radiation on an inclined surface, and models of solar collectors and heat storage tank in accordance with the standards HRN EN 15 316-4-3: 2017 and HRN EN 15 316-5: 2017 have been developed. Based on the known data on weather conditions, space and technical constraints, thermal needs and technical characteristics of the components, the solution with the most favourable payback period was selected. According to the results of the calculation, a solution with a total of 280 vacuum plate solar collectors with a total area of 548.8 m2 and with storage tank 45 m3 in capacity was selected. The collectors are mounted upright at an angle of 30° to the horizontal, with row spacing of 4.54 m. The total annual amount of collected heat equals 414.69 MWh, which covers 14.05% of the district heating systems' heat demand. The average efficiency of solar collectors in the case described is 0.5293, and the simple return on the investment equals 18.39 years. Comparison of the results obtained by the model with the auxiliary heat exchanger located inside or outside the tank revealed differences in the achieved temperatures in the tank as well as the energy collected during the characteristic periods during the year. However, the difference in the amount of heat collected on an annual basis is very small (<1%). The main reasons for this are the slight slope of the curve of vacuum plate solar collectors, high return temperature which prevents high temperature difference in the storage and low heat demand in the summer period. Comparison of the results with the EnergyPRO also resulted in small differences in the amount of energy collected annually.