Frequently Asked Questions

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Q: How do I choose the right size chain length?

A: 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, or
  •  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.

 Q: How can I specify the best actuator for the job in hand?

A: Chain actuators are most commonly used to open corridor windows, whilst folding arm actuators tend to be used for opening fire doors or large vents. However, the choice of actuators does vary and different actuators can be used to carry out the same function.

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