Abstract: Ice thermal energy storage (ITES) is one of the key solutions for peak-load shifting in commercial and industrial cooling systems. Natural convection during phase change is crucial to efficient energy management. However, the performance of the heat transfer can depend strongly on the complex process. It is still lacking in the contrasting role in solidification and melting. This study aims to numerically simulate the asymmetric regulation of the natural convection throughout a full charging–discharging cycle in a vertical ice-on-coil unit. A systematic investigation was also made to quantify the influence of the natural convection during solidification and melting. A theoretical framework was then provided for the dual promotion–inhibition behavior. A three-dimensional model of a concentric tube unit was developed using the enthalpy–porosity technique in the ANSYS Fluent platform. Furthermore, the nonlinear density–temperature relationship of the water (including the maximum at 4°C) was precisely incorporated to capture the buoyancy-driven flows. A series of tests was carried out to validate using published experimental data on the stearic acid solidification. The results showed that the better performance was achieved in the high accuracy (average relative error less than 0.2%), indicating a reliable framework for the coupled conduction–convection–phase change. Natural convection also exhibited a dual promotion–inhibition behavior during charging (solidification). Initially, the density-driven dual vortices enhanced the heat transfer. Among them, the peak storage rate increased by 15.6%, and the phase onset was advanced by 15.1%. This flow induced the nonuniform ice growth. Thus, an inverted-cone interface was formed with the top layer, nearly twice as thick as the bottom. The resulting uneven thermal resistance extended the total solidification time by 3.6%. The convection influence factor (θ) declined to about 0.95 in the later stage, indicating an overall inhibitory effect. Conversely, the natural convection was provided a sustained enhancement during discharging (melting). Expanding liquid regions promoted the stable multi-scale vortices that continuously transported the heat to the melting front. Complete melting time was shortened by 6.1%, whereas the 80% energy release was accelerated (φ = 0.2) by 13.1%. There was the expression of θ>1 and peaks at 1.08 during melting, indicating the positive and dominant role. The asymmetric behavior of the natural convection was governed by the phase-change direction. Specifically, the solidification was used to constrict the liquid domain, thereby forming the self-limiting feedback that suppressed the flow, whereas the melting was expanded to generate a self-amplifying loop that sustained the convection. The suppression of the natural convection during charging (e.g., via fins) was enhanced during discharging, in order to maximize the overall cycle efficiency. These findings can offer theoretical insights for the ITES system optimization.