The present manuscript means to analyse a thermal plant which combines, in one unique system, floor radiant heating with air ventilation and conditioning. Such combination is attained by means of the insertion of a channel for the air flow within the floor radiant system. The work is focused on the research and development of a calculation method for predicting the thermal output of such plant. In order to achieve this, the estimating algorithm for traditional floor systems developed by Professor Bernd Glück (Berlin, 1990) has been considered as the starting point. Furthermore the heat transfer phenomenon and the air flow behaviour inside the channel have been studied by means of CFD simulations, using an open source freeware, OpenFOAM. This analysis allowed to evaluate the convective heat transfer coefficient of the air, and thus to complete the modified algorithm, elaborated starting from the Glück’s method. Finally, the reliability of the developed method has been checked by means of a calculation on a test room.