Abstract: Redundant freeze protection having primary and secondary protection modes is provided for a solar heating system, such as a solar water heating system, of the kind whose normal operation involves circulation of a heat transport liquid through a solar collector and a solar energy storage tank to store solar thermal energy in the tank. The primary protection mode is triggered in response to the collector liquid temperature, T.sub.c, dropping to a selected freeze protection trigger level, T.sub.f, and involves termination of liquid circulation through and draining of liquid from the collector. The secondary or back up freeze protection mode occurs upon failure of the collector drainage means to drain the collector in the primary mode in response to the freeze protection trigger condition T.sub.c .ltoreq. T.sub.f and involves continued circulation of liquid through the collector and solar energy storage tank to warm the collector.

Abstract: Solar energy is utilized to augment the heater of a water heating system, such as a domestic hot water system, having a water storage and heating tank by conducting water to the tank through a solar energy storage tank and circulating a heat transport liquid through a solar collector and selectively through the solar tank and water tank in response to the collector temperature, the solar tank temperature, and the water tank temperature, in a manner such that heat transport liquid flow occurs in a normal solar energy storage mode through the collector and the solar tank under a first set of temperature conditions, and cascade fashion through the collector, the water tank, and then the solar tank in a cascade solar energy storage mode under a second set of temperature conditions and in a maximum solar energy storage mode under a third set of temperature conditions.