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An experimental setup is realized to study the behavior of a confined fire when the fire source is placed in combustion products trapped by a soffit, or located at the lower part of the rear wall of an enclosure. As a function of the source position, the flame, the heat release rate and the ventilation factor are drastically modified. In the first configuration, different flame behaviors are identified corresponding to jet flames, horizontal flames on the whole room surface at the level of the soffit, or extinction. In the second one, the wall fire interacts with the burnt gases close to the ceiling. The outwards hot smoke layer at the aperture always occupies one third of the open height. Based on the analysis of Babrauskas et al , a two zones hydrostatic model is developed, and a relation between the input air mass flow rate entering the room, the ventilation factor Fv and the heat release rate Q is obtained : mair = f(F 2 / 3Q1/ 3 ). For a fixed heat release rate, mair only depends on the ventilation factor but stays independent of the flame structure in the room. This relation corroborates the experimental data and the theoretical analysis to results available in the literature and can be used to model for the structure of such flames.