Three typical combistores are shown in Figure 6.10. In the first, shown in Figure 6.10a, an immersed heat exchanger is used, mounted on the whole inside surface of the mantle and top of the store. In the second store, shown in Figure 6.10b, the preparation of hot water is based on a natural circulation (thermosiphoning) heat exchanger, which is mounted in the upper part of the store. The third one, shown in Figure 6.10c, is the tank-in-tank case, where a conical hot water tank is placed inside the main tank, as shown, its bottom part reaching nearly the bottom of the store. Typical tap water volumes in heat exchanger are 15, 10, and 150–200 L for the three tanks, respectively (Druck and Hahne, 1998).