Recently, the more accurate comparisons between the existing building behaviour and the simulations have shown that a more realistic model of the whole system has to be considered. In fact, the coupling effects and feedback loops between building, HVAC equipments and the control, are essential and can only be taken into account by a large scale simulation. This observation emerged more and more during the last three years. But, unfortunatly the direct coupling of the existing models in the same simulation software was not often possible. Moreover, when the coupling was possible, the resulting coupled set of equations could not be numerically processed, due to numerical difficulties. As a matter of fact, during the modelling of each component of the whole system, the further coupling has to be considered and adequate models for this purpose must be designed. This paper analyses this aspect of the problem and proposes some solutions and examples for the component model formulation.