Dual ventilation/smoke extract systems
Dual ventilation/smoke extract systems can operate in two different modes to provide an economical solution for ventilation and smoke extraction.
- Operates as a conventional ductwork system under normal conditions
- Operates as a smoke extract system under fire conditions
Smoke extract ductwork must retain at least 75% of its cross sectional area, both in the fire compartment as well as adjacent compartments.
In the diagram
The ductwork (shown ) must be suitable for clearing smoke within the compartments and normally rated for one hourfor stability and integrity to resist hot smoke and fire from inside (Type B). Insulation fire ratings are not required in the first compartment.
Where the duct crosses the escape corridor (shown ) it must maintain stability, integrity and insulation fire rating for the same period of time as the compartment through which it passes. These standards may be required for the remainder of the ductwork's route through the building to the outside.
In some circumstances insulation requirements may be waived or varied by the Regulatory Authority if there are no combustible materials within 500mm of the duct, such as in the plant room (shown ) BS 9999:2008, clause W.4. Stability and integrity must still be maintained for the relevant period of time.
In order that the 75% cross sectional free area criteria be maintained, fire rated ductwork would be used within the service shaft requiring stability and integrity (shown ). However, if there are combustible materials within 500mm of the duct, insulation would still be needed.
All fire rated ductwork and penetration seals must be tested to BS 476-24/ISO 6944 or BS EN 1366-1 and classified in accordance with BS EN 13501-3 by a recognised NAMAS/UKAS accredited laboratory.
Note: Final approval must always be obtained by the local building control officer/fire department prior to commencement of construction