The focus in the present Ph.D. thesis is on the active use of solar energy for domestic hot water and space heating in so-called solar combi systems. Most efforts have been put into detailed investigations on the design of solar combi systems and on devices used for building up thermal stratification in hot water storage tanks. A new stratification device has been developed and patented. The device is a two fabric layer stratification inlet pipe. The strategy used in the thesis is a combination of experimental and theoretical investigations. The experimental investigations are used to study the thermal behaviour of different components, and the theoretical investigations are used to study the influence of the thermal behaviour on the yearly thermal performance of solar combi systems. The experimental investigations imply detailed temperature measurements and flow visualization with the Particle Image Velocimetry measurement method. The theoretical investigations are based on the transient simulation program TrnSys and Computational Fluid Dynamics. The Ph.D. thesis demonstrates the influence on the thermal performance of solar combi systems of a number of different parameters such as the varying weather conditions in Denmark, the domestic hot water consumption, the space heating demand and the size of the space heating system etc. through a detailed parameter sensitivity analysis. Further the calculations show that high thermal performances of solar heating systems are achieved by highly thermal stratified heat storages. Furthermore, it is demonstrated that thermal stratification can be build up in a nearly perfect way by using stratification devices. Different opertation conditions were applied in the experiments that showed that different stratification devices are suitable for different operation conditions. Tests, simulating both the thermal behaviour of a stratifier in a solar collector loop and in a space heating loop, have been carried out. The thermal behaviour of the stratifiers is demonstrated both with forced flow rates in the range from 2 – 10 l/min and with a volume flow rate based on thermosyphoning, the latter with both an external plate heat exchanger and with an imerged heat exchanger spiral.