As was seen in the previous sections of this chapter, the control system should be capable of handling all possible system operating modes, such as the heat collection, heat rejection, power failure, freeze protection, and auxiliary heating. The basis of solar energy system control is the differential temperature controller (DTC). This is simply a fixed temperature difference thermostat with hysteresis. The differential temperature controller is a comparing controller with at least two temperature sensors that controls one or more devices. Typically, one of the sensors is located at the top side of the solar collector array and the second at the storage tank as shown in Fig. 32. Most other controls used in solar energy systems are similar to those for building services systems.