Laboratory experiments were carried out to explore, for the first time, the influence of several parameters on combined forced and density-driven air flows through large openings in a horizontal partition. Such flows may occur, for example, in a stairwell connecting two floors of a detached house. The two-way flows in the opening were monitored using a tracer gas technique. Variable parameters included the direction and rate of the net flow, the temperature difference between the zones, and the dimensions of the large opening. The results of the investigation are presented and discussed.