How do I choose the right size chain length?

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