Standard solar systems for DHW production in residential buildings usually consist of a solar collector coupled with a water storage tank. Most of these systems also include auxiliary elements such as circulation pumps, sensors, controllers and safety equipment, giving a sort of complexity to the final installation. Furthermore, maintenance costs as well as space consumption represent some of the main drawbacks of this technology. For these reasons, even though conventional systems are currently well established on the market, "passive" solar systems are slowly gaining attention from both the practical and theoretical point of view, especially for their easy installation, smaller sizes and maintenance-free approach.