The third and last approach to passive heating is the isolated gain system. In this system, the solar collector and the storage are thermally insulated from the rooms that are to be heated up. The system can run apart from the building which draws energy only when heat is required. Where systems are completely passive, the energy transfer from the collector to the room or to the storage and from the storage to the room happens only by natural processes and not by forced processes such as convection and radiation. The most common technique is the one to create natural circulation systems composed of a flat plane collector and a thermal accumulator tank. The thermal vector fluid is normally air. An air radiator system (Fig. 76) uses a glazed collector located in the most suitable position to get the greatest quantity of the Sun’s radiation, but it must be distant and below the thermal storage tank.

The absorbed heat warms up the air which, because of the density gradient, moves up and enters the storage (made of either a compact conglomerate mass or an incoherent bed of stone) thereby heating it up. The stored heat is then distrib­uted over the air in the room by convection. The thermal storage mass can be put below the floor of the building, below the windows or inside pre-fabricated plug­ging elements. The space orientation of the building is less important for the sys­tem’s efficiency compared with the other kinds of solar gain [1, 3, 4].