A stairwell AOV is a smoke ventilator 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
  • to provide replacement air for a lobby smoke extract system in large buildings

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.

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. This allows residents to escape safely and firefighters to enter the building and tackle the fire.

Our Guide to Stairwell AOVs  was developed to help you understand when a building can benefit from a Stairwell AOV system. In it, we answer the most common questions we have received and give detailed instructions on when Stairwell AOVs are suitable for use.

Download the Guide to Stairwell AOVs

How big should a stairwell vent be?

The graph 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:

  • 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.
  • 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.

Under the Construction Products Regulations (CPR) it is a legal requirement to use only certified products for smoke ventilation. It is the responsibility of the installer to ensure installations meet the relevant standards. For more information see SCS Group’s Technical Bulletin here.


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