One of the most effective and simplest methods to maintain thermal stratification of water storage tanks during charge and discharge is the use of a rigid porous manifold. This type of manifold does not require any moving parts or special materials and thus is particularly appealing for solar hot water systems. It has a series of vertical hydraulic resistance elements placed within a perforated tube. At the design operating conditions, the manifold ensures that fluid entering the tank is delivered to the vertical position where it is the same temperature as the tank fluid. In this paper we assess the utility of a rigid porous manifold over a range of operating conditions. The primary result is the development of a simple relationship between a dimensionless performance indicator and a second dimensionless parameter that represents the operating condition. This relationship can be used as a tool to predict undesired flow values over the range of operating conditions.