The time dynamics of Rayleigh-Benard convection originating from different heating rates for a Boussinesq fluid of Prandtl number Pr = 0.71 (air) has been studied numerically inside a three-dimensional (3D) rectangular box of aspect ratios Gamma(z) = 1.25, Gamma(y) = 2. The heating rate, introduced through a time dependent Rayleigh number R(t), drives a how transition at least in the range of supercritical Rayleigh numbers (R(s)) here considered to solve the governing equations; R(s) = 3.6 x 10(3), 5 x 10(3), 9 x 10(3), 1.3 x 10(4) and 1.6 x 10(4). The flow transition, identified by a change of the rotation sense of a two-roil fluid pattern, was found when perfectly conducting side walls were used. This kind of how transition has been reported in recent experiments.