In the paper the dynamics of the simultaneously filling and emptying of a box is studied theoretically and experimentally. Both situations, consisting of negatively buoyant (fountains) and positively buoyant (plumes) discharges, have been considered. For the sake of generality, no assumption is made about the value of the density deficit (between the release and the ambient fluid), so that this study deals with the so-called non-Boussinesq general case. Experiments are carried out with buoyant air-helium mixtures continuously released from top (fountain) or bottom (plume) into air in a cylindrical tank with an open bottom boundary and a top vent of variable areas. At steady state, for both fountain and plume configurations, a buoyant layer of constant thickness and density forms under the tank ceiling. Based on mass and buoyancy conservation equations applied on the buoyant layer, theoretical models are proposed to estimate its depth and density at steady state. The theoretical models compare favourably with the experimental data. Subsequently, these models allow us to compare the plume and the fountain configurations for identical source conditions, box size and vent area. Even if, in the majority of situations, the plume configuration allows a better mixing of the buoyant fluid with the environment, it is found that beyond a certain value of the fountain height, the fountain configuration becomes more efficient than the plume configuration for mixing phenomena.