This paper presents a feasibility study of a fresh air load reduction system by using an underground double floor space. The fresh air is introduced into the double slab space and pass the opening bored through footing beam. The air is cooled by the heat exchange with inside surface of the double slab space in summer, and heated in winter. Then the heat is transferred to the adjacent soil through foundation slab and wall and to the room just above the mat through floor slab. The initial cost of

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... the system can be reduced, because it doesn't need a pipe or duct in the ground as does the cool-tube system. The object of this paper is examination of performance of fresh air load reduction system by using underground double floor space. Performance index are reduction rate of fresh air load, HVAC energy consumption, running cost and environmental load such as CO2 emission. In this paper, the system assumed to be applied for office building and these indexes are examined comparing with total enthalpy heat exchange system.