Abstract: An apparatus for heating water with solar radiation incorporates an auxiliary heating unit, external to the hot water reservoir and bypassing the solar heating element, for providing auxiliary water heating whenever the temperature in a selected portion of the reservoir decreases below a predetermined level. A common return conduit in combination with a single supply pump and a selector valve enable the auxiliary heating "by-pass" circuit to be simply and quickly added to already existing installations.

Abstract: The safety device disclosed herein is adapted for use with low power gas burning water heaters which are unvented. In order to limit the duty cycle of the heater, a thermal mass having a preselected thermal inertia is heated from the burner through a probe of limited heat conductivity so that the temperature of the thermal mass rises relatively slowly during operation of the heater and cools relatively slowly in the intervals between operation. If the thermal mass reaches a predetermined threshold temperature, the heater is turned off or sharply throttled back so as to limit the emission of combustion products into the atmosphere.