Abstract: Harmful smoke is drawn from rooms in an air conditioned building by way of duct work which supplies conditioned air to the rooms during normal operation. When a potentially harmful level of smoke is detected, supply fan motors associated with the duct work are reversed in direction so as to evacuate the smoke through registers in the rooms and into the duct work. The smoke is then exhausted away from the building occupants through an outside ventilating unit. Accordingly, the building occupants will have sufficient time to evacuate the building safely without succumbing to smoke inhalation.