How do I choose the right size chain length?

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The length of the chain is specified by the free area required for smoke to escape. The Approved Document B stipulates that vents within lobbies should have a free area of 1.5m2 while stairwell vents should be a minimum of 1.0m2 should open fully within 60 seconds.

Appendix C of Approved Document B of the Building Regulations specifies how the free area should be calculated, this being either:

  • The declared aerodynamic free area in accordance with BS EN 12101-2 – Smoke and heat control systems.
  • Specification for natural smoke and heat exhaust ventilators
  • The total unobstructed cross-sectional area measured in the plane where the area is at a minimum and at right angles to the direction of flow


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