the various operation modes are achieved by the use of the dampers shown. Usually, in air systems, it is not practical to have simultaneous addition and removal of energy from the storage. If the energy supplied from the collector or storage is inadequate to meet the load, auxiliary energy can be used to top up the air temperature to cover the building load. As shown in Figure 6.11, it is also possible to bypass the collector and storage unit when there is no sunshine and the storage tank is completely depleted and use the auxiliary alone to provide the required heat. A more detailed schematic of an air space heating system incorporating a subsystem for the preparation of domestic hot water is shown in Figure 6.12. For the preparation of hot water, an air-to-water heat exchanger is used. Usually a preheat tank is used as shown. Details of controls are also shown in Figure 6.12. Furthermore, the system can use air collectors and a hydronic space heating system in an arrangement similar to the water-heating air system described in Section 5.2.3 and shown in Figure 5.14.