The paper deals with the simula tion of indoor environment in a da ta center a t breakdown of its cooling sys - tem. Individual components of the cooling system during the breakdown were studied in detail with respect to their influence on the course of the emergency situation, in particular the influence of the cooling circuit thermal capacity. A complex numerical model was set up in TRNSYS simulation environment to predict energy flows and air temperatures in the data center. The simulation model describes in detail the cooling system including control elements and it takes into account heat accumulation in room constructions. The model represents a real data center and it was calibrated using data from a real emergency situation. The simulation results showed that the backup of water circuit makes possible to extend the operation time of the data center at emergency