Air systems are indirect water heating systems because air, circulated through air
collectors and via ductworks, is directed to an air-to-water heat exchanger. In the
heat exchanger, heat is transferred to the potable water, which is also circulated
through the heat exchanger and returned to the storage tank. Figure 5.14 shows a
schematic diagram of a double storage tank system. This type of system is used
most often because air systems are generally used for preheating domestic hot
water and hence the auxiliary heater is used in only one tank, as shown.