This paper summarises the results of previous work in the area and then goes on to describe the results of a more detailed simulation study of a number of different system designs. For these systems, the boundary conditions were: modern Swedish single family house in Stockholm (255 MJ/m2.year heating load); moderate DHW load (11.3 GJ/year); and typical Swedish single glazed collector with selective surface absorber. For the majority of the parametric studies, these boundary conditions were kept constant, but a section of the paper describes the influence of these boundary conditions on the system performance. All system designs and their variations are compared to a reference solar combisystem, the most common combisystem type in Sweden. The simulation work is based on models that have been validated against detailed measurements on several stores and components. The results show that the fractional energy savings can be increased by over 30% from the reference combisystem, given the same system size, collector type and load size. The two major improvements in performance are achieved by using an external DHW unit instead of the two heat exchangers in the store, and by having a low return temperature from the heating circuit. Several other variations are shown to have lesser influence on the energy savings.