Abstract: An apparatus and method for circulating a heat transfer liquid from a heat collector, which is generally but not essentially a solar collector, to a heat exchanger or heat storage means which is located at a level below that of the heat collector without the use of any externally powered pumps. The heat transfer liquid is at least partially vaporized in the steeply sloped collector and the vapor/liquid rises in a series of "slugs" to a condenser located adjacent the top end thereof. The vapor is condensed and the hot liquid is forced downwardly to the heat exchanger by the pressure of the rising slugs of vapor and liquid. After giving up useful heat in the heat exchanger the now cooled liquid is recirculated to the condenser and thence to the collector.