To provide an air conditioning system having a configuration capable of easily air conditioning spaces in a building communicated through an open ceiling space at a uniform temperature even with only one air conditioner, a building with the air conditioning system, and a method for operating the air conditioning system.SOLUTION: An air conditioning system according to the present invention is used for a building having a downstairs space and an upstairs space positioned at upper and lower sides in a vertical direction while having a floor therebetween, and an open ceiling space connecting the downstairs space and the upstairs space. A part of the periphery of the open ceiling space is defined by a side wall surface extending along the vertical direction, an open ceiling surface continued to a ceiling surface of the upstairs space, and an open ceiling floor surface continued to a floor surface of the downstairs space. A convection type air conditioner supplying air to the open ceiling space is provided in at least one of the downstairs space and the upstairs space.SELECTED DRAWING: Figure 4
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