We present the dynamic modelling of an absorption system and an application to a large air-conditioned system. This study was performed using the ALLAN.Simulation software, a pre- and post- processor enabling the symbolic description of systems. The absorption system is a direct gas double effect absorption chiller or heater, using LiBr/HB2BO as working fluid. This modelling has two aims. The first one is to optimize the water circuit by determining the volume of a storage tank that maximise the energetical and economic operation performances.