Smoke extract ductwork systems
Smoke extract fire ductwork is used to extract smoke from a building.
- Ductwork used in smoke extract systems should normally be rated for stability, integrity and insulation, at least equal to the compartment wall through which it passes. (Approved Document B Fire Safety, Notes To Table A1, Appendix A).
- Smoke extract ductwork must retain at least 75% of its cross sectional area, both in the fire compartment as well as in adjacent compartments.
- If the duct is contained within a dedicated shaft or there is at least 500mm separation between the ductwork and combustible materials (e.g. in car parks) stability and integrity ratings may be required within the area to be protected.
In the diagram
Office extract ductwork (shown) must be suitable for clearing smoke within the compartments. This is normally rated to provide one hour of stability and integrity - resisting hot smoke and fire from inside (Type B). Insulation fire rating is not required in the first compartment.
Where the office extract 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 - this may be required for the remainder of the ductwork 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).
In order that the 75% cross sectional free area criteria be maintained, fire rated ductwork should be used within the service shaft requiring stability and integrity, (shown ). However, if there is combustible material within 500mm of the duct, insulation would still be required.
Building Regulations and BS 476-24/ISO 6944 or EN 1366-1 stipulate that all fire rated ductwork should be tested and installed to comply with the full ‘cellulosic’ time/temp fire test curve (ISO 834 Fire Curve). A fire engineered solution designed by a qualified fire engineering consultant for lower smoke temperature may be allowed by the regulatory authority, thus reducing the insulation requirement. However, the duct must still be tested and installed to maintain stability and integrity to the full test temperature.
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