Residential blocks of flats with a top storey floor level above 11 metres require an automatic smoke ventilation system in the common areas under Approved Document B and BS 9991. The system must ventilate protected lobbies and the common stair to maintain a tenable evacuation environment. AOV components include optical smoke detectors, a control panel, 24 V DC actuators, roof vents or dampers, and a battery backup unit.

Why Residential Flats Over 11m Need AOV

In a residential block, residents must be able to evacuate through the common stair without encountering life-threatening smoke concentrations. Below 11 m, natural ventilation of the stair via an openable vent at the top is often sufficient. Above 11 m, wind pressure, stack effect, and the greater volume of smoke that can accumulate make passive ventilation unreliable. Automatic smoke ventilation — triggered by detectors and opened by powered actuators — provides a dependable, tested means of maintaining clear escape routes.

Following the Grenfell Tower fire and the resulting changes to building safety legislation, there is heightened scrutiny of smoke ventilation provision in residential blocks. The Building Safety Act 2022 creates additional obligations for buildings over 18 m, but the 11-metre AOV trigger remains the baseline for all purpose-built residential blocks.

What the Regulations Require

Approved Document B

ADB Volume 1, clause 2.27 requires smoke ventilation of common corridors and lobbies in residential blocks above 11 m. The AOV system must achieve a minimum free area at the vent of 1.5% of the floor area of the corridor or lobby served. The stairwell itself must also be ventilated — typically by a vent at the head of the stair with a minimum free area of 1 m².

BS 9991

BS 9991:2021 (and the 2024 amendment) provides the detailed design standard for residential AOV systems. It specifies vent sizing methodology, detector placement, wind pressure allowances for natural vents in exposed locations, and the interface between the AOV system and the fire alarm. It also sets testing and maintenance requirements.

System Components for a Typical Residential Block

Smoke Detectors

Optical smoke detectors are placed in each protected lobby and at the head of the stair. They connect to the AOV control panel on conventional zone circuits. In addressable fire alarm buildings, the AOV panel may receive a trigger from the fire alarm panel rather than using its own dedicated detectors — but the AOV system should have its own detection circuit as a backup.

AOV Control Panel

The smoke control panel receives detector signals and drives the actuators. For a single-stair block it may be a single-zone panel; for multi-stair blocks or buildings where each floor’s lobby is independently controlled, a multi-zone panel is required. The panel includes a key-switch for fire service override (manual open and manual close), a mains supply indicator, and fault monitoring outputs.

Roof Vent or Damper

At the head of the stair, a powered roof vent with a 24 V DC actuator opens on receiving the signal from the panel. The vent must provide the required free area to exhaust smoke from the stairwell. Lobby vents may be dedicated openings in the lobby ceiling or walls, or they may be controlled dampers in the supply/exhaust ductwork if the building uses a mechanical shaft system.

Battery Backup Unit

A battery backup unit conforming to BS EN 12101-10 provides power to the entire system during mains failure. It must sustain the system for a minimum of 72 hours standby followed by 60 minutes full activation. The BBU is sized to match the total current draw of all connected actuators, detectors, and panel electronics.

Common Installation Arrangements

Natural Shaft System

Each lobby connects via a duct to a shared vertical shaft that terminates at a powered roof vent. On alarm, the lobby damper for the affected floor opens and the roof vent opens, creating a natural stack-effect exhaust path. This is the most common arrangement for blocks up to around 30 m.

Dedicated Lobby Vents

Some designs use individual roof vents above each lobby rather than a shared shaft. This simplifies the ductwork but requires more vents and actuators. It is more common in lower-rise blocks or where the building geometry makes a shared shaft impractical.

Mechanical Extract System

Taller buildings or those in locations with high wind pressure may use a mechanical extract fan rather than natural stack effect. The fan pulls smoke out of the shaft at high volume flow rate regardless of external wind conditions. BS 9991 provides design criteria for fan sizing in mechanical systems.

Maintenance and Testing Requirements

AOV systems in residential blocks must be tested and maintained at regular intervals. BS 9991 and the Regulatory Reform (Fire Safety) Order 2005 require:

  • Weekly — visual check that the panel shows no fault condition
  • Monthly — functional test of at least one detector zone and confirming that vents open correctly
  • Six-monthly — full system test with smoke test of all detectors and full activation of all actuators
  • Annual — comprehensive inspection by a competent engineer including battery capacity test

Records of all tests must be kept as part of the building’s fire safety log.

Supply Your AOV System from AOV Direct

AOV Direct supplies all components for residential AOV installations — roof vents, dampers, smoke control panels, and battery backup units — for next-day UK delivery. All products conform to BS EN 12101 and are compatible with standard 24 V DC AOV systems.

Frequently Asked Questions

Does a three-storey block of flats need AOV?

It depends on the floor-to-floor height and whether the top habitable floor exceeds 11 m above ground level at the fire service access point. A typical three-storey block with 3 m floor-to-ceiling heights will have a top floor at around 6–7 m and will not require AOV under ADB. A building with tall ground-floor commercial space and two residential floors above may exceed 11 m and will require AOV.

Who is responsible for maintaining the AOV in a block of flats?

The Responsible Person under the Regulatory Reform (Fire Safety) Order 2005 — typically the building owner, management company, or residents’ association — is responsible for ensuring the AOV system is maintained and tested. This obligation cannot be delegated away, though the work itself can be contracted to a specialist fire protection company.

Can the AOV system be connected to my existing fire alarm?

Yes. AOV control panels include a volt-free input for a trigger signal from the fire alarm system. When the fire alarm panel detects smoke, it sends a signal to the AOV panel, which activates the vents. The AOV system typically retains its own dedicated detector circuit as a backup, ensuring it operates even if the main fire alarm panel is not functioning.