In the present paper the heat transfer in a vertical cylinder heated from below has been analysed; the influence of the Rayleigh number, Prandtl number, aspect ratio and lateral wall thermal boundary condition has been examined in the range of 1 less than or equal to A less than or equal to 5 and 1.10(-2) less than or equal to Pr less than or equal to 1.10(5). The time-dependent flow onsets and their fundamental frequencies are indicated for several low Prandtl numbers (Pr less than or equal to 5.10(-2)). For Pr > 1.10(-1), no evidence of oscillatory flow has been found in the studied range of parameters. In order to simulate the lateral walls with thermal boundary conditions different than the two idealised cases (adiabatic wall and perfectly conducting wall), the heat transfer through the solid wall is included by the simultaneous calculation of both fluid and solid domain (conjugated heat transfer).