This study presents the numerical investigations to predict the influence of the operating parameters during discharging/consumption mode on the thermal performance of a new designed vertical solar storage tank (SST) with a mantle heat exchanger installed in a solar domestic hot water (SDHW) system. It is assumed that the tank is initially stratified during its previous heat storing/charging operation. Meanwhile, the collector loop is assumed to be active. Three-dimensional (3D) unsteady Computational Fluid Dynamics (CFD) simulation program (Fluent 14.0) was constructed based on the actual operation conditions and the actual geometry of a mantle solar storage tank (MSST). The laminar model was thought for treating convection in the flow field. The governing equations in the cylindrical coordinate have been used in the simulation of flow fields inside the mantle storage tank where realistic boundary and initial conditions were applied and discretised by employing the finite volume method. Validation of the unsteady flow computation results with experimental data found in the literature has shown a good agreement. The obtained results from the computer simulations were discussed and concluded as depending on the thermal stratification parameters. The results show that for higher values of Grashof number, the preestablished thermal stratification is well preserved during the discharging operation mode and thus supplied hot water for usage as long as possible. In order to increase levels of thermal stratification or have least mixing inside the tank during the consumption period, the tank inflow Reynolds number should be kept below certain values. In these cases, the storage tank is enabling to provide proper amount of hot water with a proper temperature for consumption purposes. (c) 2015 Elsevier Ltd. All rights reserved.