The present study investigates the effect of inclination on the buoyancy driven systems. There are extensive studies on the cavity systems which exhibit a heat transfer coefficient jump upon the change in the inclination as the system moves from one steady state to another. The present numerical study makes use of 2-D and 3-D CFD simulations to investigate if a similar behaviour occurs in natural circulation systems such as a Natural Circulation Loop and a Coupled Natural Circulation Loop. The study identifies a combination of flow pattern rearrangement and flow direction reversal as the phenomena responsible for the heat transfer coefficient jump. The study also examines carefully as to why the same was not reported previously. The CFD study is validated thoroughly with the numerical and experimental data available in the literature. A parametric study is also conducted to examine the effects of the aspect ratio, the Rayleigh number, the Prandtl number and the inclination with the plane of the Natural Circulation Loop. © 2020 Elsevier Ltd