Direct circulation systems can be used in areas where freezing is infrequent. For extreme weather conditions, freeze protection is usually provided
by recirculating warm water from the storage tank. This loses some heat but
protects the system. A special thermostat that operates the pump when temperature drops below a certain value is used in this case. Such recirculation freeze
protection should be used only for locations where freezing occurs rarely (a
few times a year), since stored heat is dumped in the process. A disadvantage
of this system occurs in cases when there is power failure, in which case the
pump will not work and the system could freeze. In such a case, a dump valve
can be installed at the bottom of the collectors to provide additional protection.