Why 50% solar fraction? Because, reaching 50% solar fraction would mean that the auxiliary heating system that covers the heat load, in periods when solar energy is not available, becomes really an auxiliary. The thesis should therefore outline if a 50% solar fraction is feasible for the buildning where the Solar Energy Research Center (SERC) in Sweden is situated. By defining the heat load of the building it turned out that the realisation of optimization potentials at the ventilation system could reduce the heat load by 33% ( to 64.8 MWh). THe sizing procedure itself was divided in a rough and detailed sizing. For finding the optimum system size a compromise between given building realities and a system cost optimum has to be found. In addition, different temperature levels for space heating and domestic hot water were considered. The final decision was done in favour of a reduced temperature level since significant influences on system size and costs were outlined within the simulation.