Buoyancy-induced flow regimes are investigated numerically for the basic case of a horizontal cylinder centred into a long co-axial square-sectioned cavity. In the frame of the 2D assumption, the threshold for the occurrence of time-dependent behaviour is explored. Stable symmetric and non-symmetric steady-state solutions, as well as unsteady regimes are observed, depending on the Rayleigh number, Ra, and the aspect ratio of the cavity, d. Four d-values are considered (d = 0.2, 0.4, 0.6, and 0.8). Heat transfer results are correlated by a single equation covering the full subcritical region. (C) 2007 Elsevier Ltd. All rights reserved.