What is an AOV in fire safety?

An AOV (Automatic Opening Vent) is a ventilator that opens automatically when triggered by smoke detection or a manual call point. In residential buildings, roof-mounted AOVs are typically required at the top of common staircases to vent smoke and allow escape. They must provide a minimum 1m² free area.

When is a smoke ventilation system required?

Approved Document B requires smoke ventilation in common staircases of blocks of flats with a single staircase, basement car parks, firefighting shafts, and corridors/lobbies in buildings over 18m tall. BS 9991 and BS 9999 provide additional guidance for complex buildings.

How big does a staircase AOV need to be?

For a common staircase in a block of flats, the AOV must provide a minimum 1m² of aerodynamic (geometric) free area when fully open. This is typically achieved with a vent approximately 1100mm x 1100mm or larger, depending on the product's free area ratio.

How often should smoke vents be maintained?

Smoke ventilation systems should be inspected and tested at least every 6 months, with a full annual service by a competent engineer. The control panel, smoke detectors, AOV actuators and manual override buttons must all be tested. Records must be kept under the Fire Safety Order.

How much does a staircase AOV system cost?

A typical staircase AOV system including the vent unit, control panel, smoke detectors and installation costs £2,500-£4,000. Smoke shaft systems for taller buildings cost £5,000-£15,000. Mechanical smoke extract for basement car parks can cost £15,000-£50,000+ depending on size.

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Smoke Vent Calculator

Calculate smoke ventilation requirements for staircases, corridors and basements per Approved Document B and BS EN 12101. Estimate AOV sizes, costs and maintenance.

Vent TypeAOV is most common for domestic

ApplicationDetermines minimum free area

Stairwell / Room Area

Floor area of stairwell or room

Number of Vents

AOVs or vent openings

Price per Vent

Supply cost per vent unit

How We Calculate This

This calculator sizes smoke ventilation systems per Approved Document B and BS EN 12101.

Requirements

Staircase AOV: minimum 1m² free area at head of stairs

Staircase AOV: 1.0 m² free area
Corridor/lobby AOV: 1.5 m² free area
Basements: typically 10% of floor area
Control panel with smoke detectors and manual override

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.