This report describes a series of trials designed to assess the effectiveness of Positive Pressure Ventilation (PPV) when used to clear smoke and gases from large buildings. Cold smoke was used to smoke-log variously sized compartments - from 91 to 7,000 cubic metres in volume - to 100% obscuration. The times taken to increase visibility to a working level using natural ventilation, and PPV, were measured and compared. It is concluded that PPV can make a useful reduction in clearance times, particularly when the natural wind is light. The improvements due to PPV were greater in the smaller compartments - reducing the clearance time to about 25% of the natural ventilation time in the smallest compartment, and to about 50% in the 1,000 cubic metre compartment. A single PPV fan had no discernible effect, overall, in the 7,000 cubic metre compartment although two fans, properly used, could roughly halve the clearance time when the wind was very light.