In the present work, we develop a theoretical scheme that establishes the fundamentals of the first construction for a solar collector system. In particular, the first construction is divided in two elements: a solar cavity and the fluid that is heated. For the cavity, we assume a natural convective regime that for small values of Rayleigh number conducts to that the heat transfer is dominated only by a conductive heat transfer regime. Meanwhile the other region, where the fluid is circulating in a circular tube, the forced convective mode, is prevailed. The dimensionless temperature profiles, in both regions, are calculated theoretically and the minimization conditions for the dimensionless heating time of the fluid is found by this first construction.