What is a stairwell AOV system?

Stairwell automatic opening ventilators (AOV) are used in the stairwells of flats and office buildings where protecting the common escape routes is of paramount importance. 

A stairwell smoke ventilator is an AOV which provides at least 1.0m2 of free area when open. It usually serves one of two purposes;

  • to evacuate any smoke that gets in the stairwell of small buildings,
  • or, more commonly, it provides replacement air for a lobby smoke extract system in large buildings.

Easivent has written a comprehensive guide on stairwell smoke ventilation systems which you can download here.

If a fire breaks out, and pushes smoke into communal lobbies or corridors adjoining the escape stairwell, there needs to be some means of evacuating smoke from these areas. Solutions to this include the installation of an AOV, or a natural/mechanical smoke shaft system. As smoke is extracted through this system, the stairwell ventilator provides replacement air to allow people to escape safely, and firefighters to enter the building easily to tackle the fire.

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Our technicians are often asked how big a stairwell vent should be.

Below shows the required calculation of this free area, which also features in the Building Regulations 2013. The specific free area of a smoke ventilator, may be measured by either;

  1. The declared aerodynamic free area in accordance with BS EN 12101-2-2003 Smoke and heat control systems. Specification for natural smoke and heat exhaust ventilators.
  2. 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 air flow.

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 Download your stairwell AOV pocket guide

Under the Construction Products Regulations (CPR) it is a legal requirement to use only certified products for smoke ventilation. For more information see SCS Group's Technical Bulletin here.


Find out more about automatic opening vents on our information page here.

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