A description is given of a model which predicts the motion of combustion products and fresh air caused by a fire located in one room of a two-room structure. The gas in each room is assumed to be divided into two homogeneous layers, a layer next to the ceiling which contains hot combustion products and one next to the floor which contains fresh air. The model predicts the motion of the interface separating these layers and the density of the hot layers as a function of time and an arbitrarily specified fire heat release rate.