PURPOSE:To improve the heat exchange efficiency by a method wherein a bubble detector is arranged at a place, at which bubbles tend to stagnate, in a part of a circuit and a pump is controlled based upon the detected result. CONSTITUTION:An indirect heat exchange type heat source apparatus 1, an air separator 2, and a variable delivery pump for circulating cold/hot water are connected in series by a main pipe 4 and yet indirect heat exchange type load apparatuses and a closed type pressure tank 8 connected to the air separator 2 are respectively equipped in said series circuit. Further, a portion 4a, at which bubbles in cold/hot water tend to stop, is arranged in a part of the series circuit and a bubble detector 17 is arranged at the portion 4a. When bubbles exist, the flow speed of cold/hot water is raised so as to discharge the bubbles through the air separator 2 to the closed type pressure tank 8. On the contrary, when no bubble exists, the flow speed of cold/hot water is lowered so as to ensure the economical efficiency. Thus, the lowering of the heat exchange efficiency is prevented.
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