An experiment is performed to detect the onset of time-dependent flow regimes within a water-filled square-sectioned cavity containing a horizontal cylindrical heat source. Data are recorded on the cavity cross-section, where velocity fields and local temperature values are detected using a 2D PIV rig, and thermocouples, respectively. Numerical predictions of the experimental runs are carried out, where the system is modelled as a 2D feature. Results are provided for nine values of the leading parameter, the modified Rayleigh number Raq, ranging from 1.48x10^4 to 1.21 x10^5. The system is witnessed to undergo a transition from steady-state, laminar flow to unsteady oscillatory flow at Raq = 5.88x10^4. Experimental data and numerical predictions are compared and cross-validated. The evolution of the flow throughout the bifurcation is described by velocity and temperature profile plots, 2D field visualizations and Fourier analysis of experimental time series. The influence of the flow regime on the overall heat transfer rate of the system is finally discussed.