Abstract: A solar water heating system under normal operating conditions permits water to be circulated by a pump from a hot water storage tank through a one-way check valve and a solar collector back into the hot water storage tank when energy can be transferred from the solar collector to the water passing through. When the solar collector drops to a temperature where it is no longer desirable to transfer energy to the water, the pump ceases operation. If the outside ambient temperature drops further to a level where the water in the system affected thereby would be in danger of freezing, the pump is electrically energized by means of a freeze sensor to circulate the warmer water from the storage tank through the solar collector back to the tank. The water storage tank has a standby electrical heating unit.