In this paper the thermal behaviour of a direct ground cooling system located in Milano, Italy, isstudied by means of dynamic simulations performed in the TRNSYS environment. The simulationmodel consists of a reference building equipped with radiant panels connected to a vertical groundheat exchanger. Room thermostats and chilled surface condensation sensors provide system control.The ground heat exchanger size is adjusted in order to provide summer comfort conditions in thebuilding as well as sustainable operation over a long period. The effects of ground thermal propertieson the system performance and sizing are investigated. Specific injected energy values (thermalenergy per year per unit length of the ground pipes) are found and discussed.