The potential for Vapor/Air explosions inside manufacturing buildings poses a significant challenge to facility siting. The effectiveness of ventilation systems to minimize the accumulation flammable vapors is often questioned. NFPA 30 ventilation requirements provide adequate turnover of air to prevent accumulation from small, fugitive releases, but are in many cases insufficient for most cases that fall into the area of process safety incidents. The complexities of modeling low velocity releases of gas into confined spaces with ventilation are best handled by computational fluid dynamics. This paper documents a study of several release scenarios involving the release of a flammable gas into a large, enclosed manufacturing building in the presence of ventilation. The resulting flammable mass and explosion consequences are modeled, discussed and compared with more commonly used analysis techniques.