Narrow rectangular channels play a crucial role in various engineering applications, particularly in the cooling systems of research nuclear reactors. The thermal-hydraulic code CATHARE is widely used to simulate these systems and has been extensively validated for two-side heating conditions. However, its accuracy in one-side heating scenarios remains less explored. This study investigates the predictive performance of CATHARE under one-side heating conditions by simulating selected experimental tests. Two experiments with different operating conditions were considered, using available experimental data to compare and assess the numerical results. The analysis demonstrates that CATHARE provides accurate predictions in single-phase flow and at high Peclet numbers, while showing limitations in other operating regimes.