The direct gain system is the simplest way of achieving passive heating. In it, sunlight enters through windows, clerestories or skylights and is absorbed by the inside surfaces. It concerns conventional openings with preferably double glazing, south orientated ± 25o. Solar energy is collected through the glass surface. A part of it is transmitted into the internal space and the rest is stored in the mass of the building elements (floors, walls, roofs), which must have sufficient heat capacity.
