Presently, the building performance analysis requires to take accuratly into account the coupling effects between building, HVAC and control. For large scale systems, this implies the intensive use of the simulation tools.Today, the existing tools are not flexible enough for this purpose. The present paper describes the development of a more adequate tool : CSTBt. This tool is built on the TRNSYS core (University of Madison), but offers a more adapted library of solvers and component models concerning the building envelope and the hot water heating network. These developments are based on a very powerful and generic base of models, allowing different levels of analyses from very simple to very detailed. The model parameters are directly issued from the standard characteristics. The software structure extensively uses hierarchical and modular principles and a user interface gives the possibility of a natural description of complex systems. The present tool was developed at the CSTB in the frame of a cooperative action with Gaz de France for the detailed analysis of a large viariety of heating systems.