Central air conditioning systems are used in fire-resistive buildings occupied as high rise office buildings and hotels. These systems may serve the entire building with cool air in the summer and heat in the winter. A system of ducts acts as a network to supply this conditioned air. Fire or smoke in a room near a fresh air intake or return air duct will be sucked into the air conditioning system and be pumped throughout the structure. Air ducts of a central air conditioning system penetrate every fire barrier in the type I building.
Ducts pierce the walls, floors, partitions and ceilings.
One such fire in a Nevada hotel resulted in the deaths of 85 people because the central air conditioning system pumped deadly smoke throughout the burning building. There was no smoke detector inside the air conditioning system designed to shut down the system when a fire occurred. So, the first action taken by fire officer in command of a fire inside a fire-resistive building should be to order the air system shut down. Auto-exposure, the vertical spread of flames from windows below to windows above, is another way fire spreads throughout a type I building. Flames erupting out of a heat-shattered window can melt and break the glass window directly above.