The oscillatory instability of a 3D natural convection flow in a cold cubic enclosure containing a tandem of cold and hot vertically aligned cylinders is investigated in detail as a function of the distance between the cylinders. The study is performed by applying time integration of slightly supercritical flows. The flows undergo a transition to unsteadiness via either reflectional symmetry breaking or reflectional symmetry preserving Hopf bifurcation as a function of the distance between the cylinders. An extensive discussion of the observed instability scenarios is presented, and the different instability mechanisms are characterized in terms of the values of the main oscillating harmonics and the spatial distribution of the oscillating amplitudes of all the fields obtained for slightly supercritical flows. (C) 2019 Elsevier Ltd. All rights reserved.