The vacuum tubes trap incident sun rays, and convert it to heat inside the inner tube. Water, now being heated its temperature rises, density changes, and hot water sections rise through the tube to the top of the tank, while cold water from the bottom of tank replaces it in the tube, to start the cycle again. This natural cycling of water driven by temperature difference keeps water hot when stored in an insulated tank.

The vacuum tubes trap incident sun rays, and convert it to heat inside the inner tube. Water, now being heated its temperature rises, density changes, and hot water sections rise through the tube to the top of the tank, while cold water from the bottom of tank replaces it in the tube, to start the cycle again. This natural cycling of water driven by temperature difference keeps water hot when stored in an insulated tank.