Low-temperature thermal solar systems, which have been described until now and in which the energy transfer from the storage place to its utilization is realized by fluids moved by pumps and ventilators, are also called active systems. By the
expression ‘passive heating systems’ we generally mean all applications where the thermal hygrometric well-being conditions are obtained only by solar energy which is used without employing any conventional heating systems requiring elec­tricity or fuel. In merely passive systems, even the heat distribution and removal are realized by natural the phenomenon of conduction, convection or radiation, rather than using forced systems. Passive heating systems require the installation of wide glazed surfaces for solar energy interception and also structures with high thermal capacity storage function.

The efficiency of these systems is limited to the width of the glazed area which has to be correctly oriented, to the efficiency of the thermal storage realized by the walls and inner floors and eventually to the stored heat distribution towards the building parts characterized by scarce solar radiation. Currently, the realization of passive solar heating systems capable of guaranteeing the comfort conditions required in every inner room of a building seems to be an impossible goal both in cold climate zones and in mild climate zones such as Italy. However, its contribu­tion to the reduction of the yearly heating requirement could be relevant. Passive solar heating systems can be classified on the basis of the mechanism of energy transfer towards the heated room as follows [1, 3, 4]:

• Direct gain systems

• Indirect gain systems

• Isolated gain systems