If you have a system that is operating in areas with subzero temperatures then freeze protection must be implemented. The easiest means of preventing freezing is to use a controller with a low-temperature setting, so when the manifold temperature drops below a certain pre-set temperature (5°C/40°C), the pump will circulate, warming the collector with water from the bottom of the storage tank. The pump will not run continually, just periodically, the frequency of which will depend on the outside temperature. In extremely cold areas, a closed-loop using a glycol/water mix may be appropriate.