The Direct (Open-Loop) Solar Water System freeze recirculation is both the simplest and most efficient type of solar heating system. In this system, potable water is drawn directly from the storage tank and circulated through the array to be heated and returned to a recirculation port in the tank. The mechanics for this type of system are almost identical to a boiler application, except that the solar array is the heat source.

The major drawback to this type of system is its susceptibility to freeze damage. Although recirculation of fluid by the pump and thermal bleed valves provides a certain level of protection, these systems are typically limited to areas that do not freeze during an average year.