Abstract: Longitudinally spaced sections of a collapsible plastic tube are supported on vertically spaced support surfaces. The supported sections of the tube serve as liquid reservoirs, these reservoir sections being laterally supported at their outlet ends by an overflow support. When the depth of liquid exceeds the height of the overflow support, the liquid flows downwardly through an overflow section of the tube into the succeeding reservoir section. Air is heated or cooled by flowing it through spaces which are bounded by the reservoir sections. A solar collector has its inlet connected to the lowermost reservoir section, and its outlet connected to the uppermost reservoir section.