KNXnet/IP communication system allows integration of different KNX networks through IP by a particular device called KNXnet/IP Router. Due to the different transmission speeds among KNX networks and the IP medium, a flow control mechanism to prevent the congestion of KNXnet/IP Router has been foreseen by the KNX standard. In this paper we analyse the performance of the KNXnet/IP flow control mechanism in congestion condition, through simulation of a model based on Stochastic Activity Networks (SAN), capable to implement all the specifications foreseen by the KNX standard.