Solar combi-storages are used in many countries for storing solar heat for space heating and domestic hot water (DHW) in one device. When a combi-storage is used in combination with a heat pump, the temperature stratification efficiency of the storage is a decisive factor for the overall efficiency and thus, for the consumed end-energy of the system. In particular, fluid that is entering the storage with a high velocity may cause considerable mixing, thus, destroying stratification and leading to poor system performance. This work presents computational fluid dynamics (CFD) simulations of direct horizontal inlets at midheight of a typical solar combi-storage of about 800 L volume. Different inlet diffusor designs were simulated, and laboratory measurements were used to validate CFD experiments. For the given tank geometry, mass flow rates, and inlet position, it is found for a fluid inlet temperature of 30 °C that fluid velocities should be below 0.1 m/s and Reynolds numbers below 3000–5000 at the outlet of the diffusor in order to avoid the disturbance of a hotter 50 °C zone above the inlet. Furthermore, the fluid path within the diffusor must exceed a minimum length that corresponds to three to four times the hydraulic diameter of the diffusor.