A collector core for a solar water-heating-system includes a plurality of
heat-absorbing pipes each of which surrounds a cooler-water return-pipe.
The heating-pipes may connect directly to an insulated hot-water
storage-tank from which cooler water descends through the return-pipes
into the heating-pipes. Upon reaching the end of the return-pipes, the
cooler water flows outward into the space between the surrounding
heating-pipes and the inner return-pipes. Upon warming, water between the
two pipes rises upward back to the hot-water storage-tank thus completing
the thermosyphon flow cycle. Preferably, the inner return-pipe is made of
polyvinyl chloride ("PVC"), polybutelene ("PB"), or other compressible
material which permits collector core operation both in freezing and
non-freezing environments. Alternatively, the collector core may be added
to an existing solar water-heating panel to improve its operation.