In master's thesis titled „Cost-optimal Old House Energy Renovation“, in the area of the city of Dubrovnik, the most favorable solution was chosen among the three thermotechnical systems. In order to select the most favorable solution, primarily the calculation of standardized heat losses and heat loads were derived, as well as the required annual energy for heating, cooling and hot water preparation and the analysis determined the optimal insulation thickness of the house. According to the obtained results, three conceptual solutions of the system were defined:  Conceptual design Ⅰ: Underfloor central heating and cooling with convector valves using a heat pump, preparation of hot water using solar collectors.  Conceptual design Ⅱ: Underfloor central heating, preparation of hot water and cooling with convector valves using air-to-air heat pump.  Conceptual design Ⅲ: Boiler central underfloor heating on biomass, solar collectors for preparation of hot water, with cooling system using air conditioning. Each conceptual design is described in detail and the appropriate equipment is selected. Then, the calculation of primary and final energy consumption, CO2 emissions for each solution was performed. Finally, a cost analysis of all three solutions for a period of 15 years was made and it turned out that the most acceptable conceptual solution is the number Ⅲ.