Abstract: A solar collector has a liquid system independent of a household hot water heater system, the two systems transferring heat in a heat exchanger. The heat exchanger has an open top inner tank enclosed in a closed outer tank, a coil connected to the hot water tank system circulates under main pressure in a coil in the annular space between the inner and outer tanks. Liquid from the collector enters at the bottom of the annular space, flows upward around the coil and overflows the top of the inner tank. A pump draws liquid from the bottom of the inner tank for circulation through the solar collector system. The liquid in the collector system drains by gravity into the inner tank at any time that the collector circulating pump is not operating, thus providing freeze protection during all normal operations and also at such time that a power failure may occur.