Natural ventilation of all the floors of a multi-story building may require a tall stack. To provide satisfactory ventilation flow rates for a multi-story building and to restrict the height of the ventilation stack, this paper proposes a hybrid ventilation scheme of mechanical ventilation in the upper-level floors and natural ventilation of lower-level floors with the stack pressure induced by the warm discharge flows from all of the floors. This scheme is achieved by connecting the lower-level naturally ventilated floors to the upper-level mechanically ventilated floors via a common stack. A model is developed to optimize the quantity of mechanically ventilated floors required to provide satisfactory ventilation rates for the lower naturally ventilated floors and also minimize the energy consumption. The applicable conditions associated with the ventilation scheme are expressed explicitly. The model predictions are validated against a computational fluid dynamics (CFD) simulation. The results indicate that the proposed scheme is effective at energy savings if the key engineering parameters are appropriately designed. A larger exhaust opening of the stack and an optimized vent area of the ventilated rooms are helpful to both the increase in the quantity of naturally ventilated floors and the decrease in the energy consumption. (C) 2015 Elsevier B.V. All rights reserved.