The paper analyzes the effect of hydrodynamic viscous boundary layer growth over the performance of a thermal storage tank during the discharge cycle. As well established in Fluid Mechanics textbooks, a viscous boundary layer modifies the velocity profile across the tank section. Consequently, in a discharge cycle the warm water flowing outside the boundary layer (i.e., in the core region) reaches the tank bottom exit faster than that if the down flow water were flowing at the mean velocity based on the discharge flow rate, which is the usual designing and tank selection assumption. Consequently, the storage tank height must be greater than that determined using the simple mean flow velocity. Two controlling parameters appear naturally in the analysis: the Reynolds number based on tank diameter, ReØ, which is also associated with the hydrodynamic entry length, and f, which defines the position of a given contact surface from the tank entrance to the hydrodynamic entry length. Results show that the tank loss of capacity due to viscous effects may not be negligible and the selection of a height-to-diameter tank ratio is essential for minimizing those effects.