For freeze protection, a variation of the direct circulation system, called
the drain-down system, is used (shown schematically in Figure 5.10). In this
case, potable water is also pumped from storage to the collector array, where
it is heated. When a freezing condition or a power failure occurs, the system
drains automatically by isolating the collector array and exterior piping from
the make-up water supply with the normally closed (NC) valve and draining it
using the two normally open (NO) valves, shown in Figure 5.10. It should be
noted that the solar collectors and associated piping must be carefully sloped to
drain the collector’s exterior piping when circulation stops (see Section 5.4.2).