This article starts with an overview of the principles of tertiary building control, and some basic points on the application of fuzzy logic to this problem. After a presentation of the TRNSYS simulation program, two new modules are described. The first of these, the TYPE 70, uses a simplified model of a building, based on an electrical analogy. It sets out the heat balance of a thermally homogenous zone, and also studies a multi-zone building made up of a number of adjacent rooms, namely several TYPE 70s connected together. The second module, the TYPE 71, is a fuzzy controller which determines the supply temperature of the water in the heating system. Its originality lies in the fact that it takes into account the ＇thermal state＇ of the building, in addition to the external temperature and the solar gains. A comparison is made between the TYPE 71 and an equivalent, but classical, controller. Simulations carried out with a mono-zone storey of a building showed that the fuzzy controller produced a temperature closer to the set-point temperature, and also a more accurate optimization of the restarting procedure.