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Since smoke is carried by the movement of air, you can stop the spread of smoke throughout the
building by lowering the air pressure in the area containing the fire and by raising the air pressure
in the surrounding areas and floors. The difference in air pressure (also called the “Air Pressure
Differential”) between the smoke-filled area and the surrounding areas acts as a barrier to the
smoke, pushing it back into the smoke-filled area. The figure below shows how this works.
FigureTag FD4-465-01
POSITIVE
AIR PRESSURE
POSITIVE
AIR PRESSURE
POSITIVE
AIR PRESSURE
POSITIVE
AIR
PRESSURE
POSITIVE
AIR PRESSURE
POSITIVE
AIR PRESSURE
POSITIVE
AIR PRESSURE
NEGATIVE
PRESSURE
POSITIVE
AIR
PRESSURE
Figure 1-2. Applying Positive Air Pressure to Control Smoke
1. Lower the air pressure in a smoke-filled area by controlling the air flow into it and turning
ON the exhaust fans from the area to full capacity. This “Negative Air Pressure” technique
pulls the smoke out of the area and vents it outside of the building.
2. Pressurize the areas and floors surrounding the fire by turning OFF all exhaust systems
(including closing any exhaust dampers) and forcing supply air to those areas at full capacity,
creating zones of “Positive Air Pressure.” The air in the pressurized areas tends to leak into
the smoke zone, using the same cracks and holes that the smoke would use to get out. This
positive pressure airflow into the burning room keeps the smoke from spreading.
Continued on next page
Controlling Smoke Movement, Continued
Basic Concept
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