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Experiments have been conducted to determine critical Froude numbers associated with required inflow of air to a fire space through wall and ceiling apertures to prevent escape of smoke. The experiments were conducted mostly on reduced, model scale, with verification i n a 2.4 m high test room. Critical Froude numbers, as formulated, were insensitive to aperture geometry. Those for wall apertures varied slowly and predictably with the vertical temperature distribution i n the room, consistent with a discharge coefficient of 0.64 for the inflow. Those for ceiling apertures exhibited a dependence on an aperture Grashof number, with both a high- Grashof number asymptote and an aperture low-Grashof number asymptote. While discharge coefficients for wall apertures can be considered constant near 0.64 for aperture Froude numbers larger than critical , the discharge coefficient for ceiling apertures increased from 0.19 near the critical Froude number, toward the familiar isothermal value for sharp-edged orifices of 0.61 near a Froude number seven times larger than the critical.