Natural circulation in a two-phase water loop with imposed wall power, is investigated at atmospheric pressure both experimentally and numerically, using the system-scale code CATHARE. Low-pressure loops are prone to instability due to strong differences of density between the liquid and the vapor. Particular attention is devoted to analyzing the boundaries of the flow reversal regime with respect to flow cross section aperture (related to friction forces and experimentally performed using regulation valves) upstream and downstream of the heated test-section. At low heat flux, the flow is stationary, whereas at higher heat flux, flow reversal appears. The system-scale code is amenable to reproduce the flow reversal regime boundaries explored experimentally. An analytical criterion is used to highlight that boiling may become unstable and flow reversals appear. Results show that the stable flow region boundaries can be extended by increasing the upstream pressure loss coefficient (valve closing) in order to reach large exit void fractions α ≈ 1. In configurations where the upstream pressure losses dominate, stable boiling points with high mass flux and high heat flux are observed. On the opposite, larger downstream pressure loss coefficients strongly favor instability and flow reversal.