Abstract: Discloses a closed-system, solar heat holder for thin-film heating of heat-transfer fluid in a tank. Solar energy, passing through an insulated glass cover, is trapped between the glass cover and an absorbing surface to heat the absorbing surface and thereby a thin film of heat-transfer fluid passing between the thin-film spacing provided by the absorbing surface and a parallel-disposed, insulated baffle with a bimetallic thermostat valve controlling the passage therethrough of such heat-transfer fluid which, when heated, in turn gives up heat to fluid flowing through a heat exchanger thereby cooling such heated heat-transfer fluid. Another insulated baffle is disposed parallel to the first baffle and the cooled heat-transfer fluid moves downwardly between the baffles to the bottom of the tank. A pneumatic container in the tank contracts to relieve the tank of pressure resulting from the heating and expansion of the heat-transfer fluid.