The computer program TRNSYS was used to simulate the heating system of the Bourgoin-Jallieu experimental solar building and added quantitative results to the survey of several years of monitoring. The building, situated near Lyon, France, consists of two identical three-storey blocks; the south face is a curtain wall with water solar collectors (the parameters of the solar system can be varied). The floors are equipped with coils of water imbedded in the concrete. The heating system was simulated by a model which took into account the independent electric auxiliary heating and the thermal capacitance of the floor and of the walls. It gave good information about the energy balance according to the experiments. The overall performance of the system has been investigated by the Chang model and the concept of decay constant was analyzed. It was shown to be relevant only for intermediate solar radiation, to be a function of the utilizability and to be inversely proportional to the load of the building. It was also shown that the most interesting solution is to use the largest storage allowed by the tanks, although the direct use of the concrete mass of the floor could achieve nearly equivalent energy performance at a lower cost.