During summer, a large number of air-conditioners at the same time could be rejecting condenser heat into a recessed space formed by the external walls outside many high-rise residential buildings in Hong Kong. Prediction of the temperature and flow field of a powerful rising hot air stream formed in a 30-story high recessed space is performed by using FLUENT 5.0, a computational fluid dynamics (CFD) code. For a 30-story high recessed space of "I" type with four window-type air-conditioner units per story, this paper describes the findings for two depths of the recessed space. For the buoyant air stream, the results show that the mass-flow rate and temperature inside the recessed space are not significantly influenced by the recessed space depth. For air streams used by the condenser units, deeper recessed space yields an increase in the enthalpy in the air stream used by the outer units, and a decrease in the enthalpy in the air stream used by the inner units, which draw the hottest air from the recessed space.