For freeze protection, a variation of the indirect water-heating system, called
the drain-back system, is used. Drain-back systems are generally indirect water
heating systems that circulate water through the closed collector loop to a heat
exchanger, where its heat is transferred to potable water. Circulation continues
as long as usable energy is available. When the circulation pump stops, the collector fluid drains by gravity to a drain-back tank. If the system is pressurized,
the tank also serves as an expansion tank when the system is operating; in this
case, it must be protected with temperature and pressure relief valves. In the
case of an unpressurized system (Figure 5.13), the tank is open and vented to the atmosphere. The second pipe directed from the collectors to the top of the
drain-back tank is to allow air to fill the collectors during drain-back.
Because the collector loop is isolated from the potable water, no valves are
needed to actuate draining and scaling is not a problem; however, the collector array and exterior piping must be adequately sloped to drain completely.
Freeze protection is inherent to the drain-back system because the collectors
and the piping above the roof are empty whenever the pump is not running.
A disadvantage of this system is that a pump with high static lift capability is
required in order to fill the collector when the system starts up.