Buffer storage tank This kind of storage tank can be made of steel (pressurized tanks) or plastic and it is mainly used for room heating. In this case, the fluid inside the storage tank is withdrawn, heated inside the boiler and put back inside the tank; at this point, the warmed fluid is once again withdrawn from the tank and sent to the radiators. This solution is adopted to improve the boiler working conditions and so the boiler is not forced to work in the stop-and-go mode. Actu­ally the boiler warms up the stand-by water volume which is kept at a certain temperature inside the storage tank and so it can remain switched off for a long period of time [9].

‘Tank in tank’ storage tank The combined solar systems are conjointly used for both warming up sanitary water and room heating. This system often uses a type of storage tank called tank in tank, which consists of a buffer tank inside which there is a storage tank for drinkable water. The latter tank gets the heat through its own sides from the fluid contained inside the outer tank. The storage tank with sanitary water is located in the upper part of the buffer tank where the water is maintained at a certain temperature by an integrative heating system. At the bottom of the buf­fer tank, instead, there is the solar heat exchanger (see Figs 67 and 68) [9].

Figure 68: Scheme of a combined solar system.