This paper describes the transient ventilation of two interconnected rooms. One of them has a negative buoyancy source located at ceiling level, similar to an overhead split type air-conditioning system with ductless distribution, whereas the other has a vent at floor level. The flow evolution from short-and longterm analytical models was determined and confirmed with scale-model salt bath experiments. The shortterm model predicts the depth of the dense-layer 'first front' until it reaches the ceiling of the first room, and the long-term model predicts the density evolution of the fluid until it reaches a steady value. The advance of the first front depends on the location of the interior opening but is independent of the size of the exterior vent. The long-term density evolution depends on both parameters. As the interior opening is lowered, the volume implicated in the dynamics decreases and the dense fluid density increases at a higher rate. Furthermore, increasing the size of the exterior vent causes a turbulent exchange flow to develop through that vent, which inhibits the increase of the average density by introducing ambient fluid to the rooms.