The simplified thermaI simulation of buiIdings is particularly useful for both bioclimatic design and the selection of the most suitable HVAC systems. The LEGO computer program was prepared in order to simulate the conventional HVAC plant-building systems and the complex systems Iike seasonal storage tank and solar collector plants. Therefore, it is possible with this kind of program to study the interaction among the various systems by means of combination of different simulation subroutines. In this way, a more accurate dimensioning of the systems can be achieved. The computer code Is framed within simplif led programs and was conceived to realize simulations (for long period of time, too) on IBM compatible PC. The computer program is entirely written in Fortran and is composed of several and different subroutines. The software structure has been done in a way so that it can be modified, Implemented or personalized by skilled users. In order to make easier the introduction of data input and computer graphics output, it has been conceived a particular Interface in the EXCEL (Microsoft) frame which allows the use of LEGO in a piloted manner. At the end of this report, two simulation results will respectively be presented for an air conditioning system design at ENEL building (with heat pump storage tank plus solar collectors), and for a direct gain seasonal storage tank bui It in Catania.