The thesis research and development have been carried out in the Technical University of Denmark in Copenhagen. The solar combisystem of the REBUS project will have a plate heat exchanger for transferring the solar energy from the collector loop to the solar store. Based on previous investigations at DTU, the heat exchanger might operate without a pump in the secondary loop, driven just by thermal forces between cold and hot water. The purpose of this study is to calculate the hydraulic loop of a thermosyphon system. Theoretical and experimental investigations have been carried out in order to find out all the important parameters of the secondary loop of the system that consists basically of a tank and a plate heat exchanger. First of all, a theoretical study of how to calculate a thermal driven hydraulic loop for different boundary conditions has been done. Then a test stand has been built up and it has been tested under typical operating conditions in a solar thermal system. A further study to find out the pressure droå of the heat exchanger has been done. In order to carry out all this research, an Excel tool has been developed.This Excel tool calculates the hydraulic loop from some necessary basic input data. The final goal of that tool is to calculate the “?” value of the heat exchanger, necessary to calculate the entire secondary hydraulic loop. The above mentioned tool will be very useful for the REBUS project and for future studies of thermosyphon systems since it will make it easier to calculate all the important parameters of the hydraulic loop.