This paper deals with the combined effects of fire and mechanical ventilation on the bi-directional flow occurring at a doorway for a fire scenario involving a three-compartment assembly. Based on large-scale fire tests, the analysis focused on three issues the velocity profiles at the doorway, the location of the neutral plane, and the inflow and outflow rates entering and leaving the fire compartment. The combined effect of fire and mechanical ventilation leads to different flow regimes depending on whether the smoke outflow (or the fresh air inflow) and the forced ventilation are directed in the same direction or in opposite directions. The theoretical description based on the Bernoulli approach for natural convection flow is applied in case of the combined effect of buoyancy and inertia. Five configurations are identified one-directional flow of smoke, bi-directional flow with upper smoke flow as dominant, natural convection case with equal smoke and fresh air flows, bi-directional flow with lower fresh air flow as dominant, and one-directional flow with only fresh air flow. These regimes are observed experimentally and clearly defined theoretically. A satisfactory agreement is obtained between the theory and the experiments. This approach permits identifying the critical conditions under which one-directional flow is encountered at the doorway. © 2015, Springer Science+Business Media New York.