In Europe, hydronic concrete core systems are being increasingly used for room conditioning systems. The concrete slab acts as heat accumulator and permits dissipation of the load using, for instance, cooling towers. When using such systems the external climatic conditions limit the achievable water temperature. The convective loads, however, directly affect the room air temperature and reduce the storable part of the load. The ventilation system also has a large influence on the energy related operation of such a system. Moreover, the dimensions of the concrete slab and the layout geometry of the water pipes, especially the spacing, are important factors for the design of the system. A model is described which can be used to illustrate the transient two dimensional heat flow in such a construction. This method is suitable for simple hand calculations, but can also be integrated into existing building simulation programs without having to modify the program code. Thus, the complete system can be designed for practical applications to ensure optimum operation. In addition, this paper describes the interrelationship between heat storage capacity and pipe geometry. Finally, criteria are listed for suitable application of concrete slab cooling and further aspects are listed that need to be considered in connection with these systems.