PURPOSE: To enable to prevent a mutual heat flow between a high and low heat sources when a carrying device is suspended, by a method wherein the carring device of a working liquid is provided on a closed cicuit obtained by connecting a high and low heat sources by a feed and return pipes with each other and a trap part is provided on a connecting part of the feed and return pipes with the low heat source.

CONSTITUTION: A heat exchanger 4 (low heat source) being contained within a hot water storage tank 3 and the lower part of a collector 1 (high heat source) is connected by a feed pipe 6 in the midst of which is provided with a carrying device 5 such as a pump with each other and the heat exchanger 4 and the upper part of the collector 1 are connected by a return pipe 7 with each other. A closed circuit is constituted by a method wherein trap parts 9a, 9b are provided respectively on connecting parts among the feed and return pipes and the heat exchanger 4 with each other and a working liquid 8 of a latent heat medium such as flon is enclosed in the circuit. Operation of the carrying device 5 is suspended when the temperaure of the collector 1 becomes lower than that of the heat exchanger 4. As vapor of the working liquid boiled and evaporated by the heat exchanger 4 returns up to only the trap parts 9a, 9b and does not enter into the heat exchanger 4 even if the vapor is condensed and liquidized by the collector 1 when the operation of the carrying device 5 is suspended, a heat flow to the collector 1 from the heat exchanger 4 is eliminated.