Solar thermal systems have been identified as a viable solution to meeting domestic thermal energy needs due to their low temperature operating conditions. The complication of these systems is that solar availability and demand side loads are often mismatched. This is especially true for solar space heating systems, where demand is a maximum in winter when solar potential is the lowest. By using a long-term thermal storage, useful gains in the summer may be carried over to the heating season. There are numerous simulation tools capable of studying such solar thermal systems and building-side demands, however each tool possesses both strengths and shortcomings. This paper outlines the strengths of two modelling tools, TRNSYS and ESP–r, through a new co-simulator in order to evaluate the potential contributions of a seasonal solar thermal system at a single-house scale.