Energy flexibility strategies in buildings require calibrated thermal energy models capable of representing real operational behaviour. This thesis presents the development and calibration of a detailed building energy model for the Energy Center, an office building in Turin serving as the Italian pilot case within the European ENTRANCE project. This work integrates monitored data, HVAC system modelling, and a calibration procedure to ensure realistic representation of the building performance. The model, developed in EnergyPlus, includes district heating supply, radiant ceiling panels, and a Dedicated Outdoor Air System (DOAS), enabling comprehensive simulation of thermal generation, distribution, and control strategies. Calibration was performed according to ASHRAE Guideline 14 statistical criteria through an iterative trial-and-error approach.