Smoke ventilation systems protect escape routes and support firefighting. Here's how they work, when they're required, and what the fire strategy must address.
30 April 20244 min readFire Safety Services
What Is a Smoke Ventilation System?
A smoke ventilation system is a fire safety installation designed to remove or limit the movement of smoke within a building in the event of a fire, with the primary purpose of maintaining tenable conditions along escape routes and firefighting access routes for long enough to allow safe evacuation and effective firefighting. Smoke ventilation is a critical element of fire safety in multi-storey buildings, particularly residential buildings operating a stay-put evacuation strategy where the protection of staircase lobbies and corridors from smoke infiltration is essential.
Smoke ventilation systems range from simple natural ventilation openings — windows or roof vents that open automatically on fire detection — to sophisticated powered extract systems with carefully designed airflows and make-up air provision. The appropriate system depends on the building type, height, configuration, and the fire safety strategy for the building.
Natural Smoke Ventilation
Natural smoke ventilation relies on the buoyancy of hot smoke gases to drive smoke upwards and out of the building through openings at high level. In residential buildings, natural ventilation of lobbies and corridors is commonly achieved through automatically opening vents (AOVs) at the top of each staircase lobby or at the end of each corridor, which open automatically when the fire alarm activates.
Natural ventilation is simpler and less expensive than mechanical ventilation and requires no power supply to function — which is an advantage in a fire where power supplies may be compromised. However, it is less reliable in tall buildings where stack effect and wind pressure can affect performance, and it requires careful design to ensure that the vent sizes and positions achieve the required smoke removal performance.
Automatically opening vents (AOVs) are one of the most critical — and most frequently defective — fire safety components in residential buildings. If an AOV fails to open in a fire because it has not been maintained, the corridor or lobby it is designed to protect may fill with smoke, threatening the means of escape.
Mechanical Smoke Extract Systems
Mechanical smoke extract systems use powered fans to extract smoke from corridors, lobbies, stairwells, and other areas. They provide more reliable and controllable performance than natural ventilation, particularly in tall buildings where natural ventilation performance is affected by external conditions. Mechanical systems are required in many taller residential buildings and in buildings where natural ventilation cannot achieve the required performance.
A mechanical smoke extract system typically comprises extract fans at high level in each zone, make-up air supply to replace extracted air and maintain pressure differentials, automatic dampers to isolate zones not affected by the fire, and a control system that integrates with the building's fire alarm system. The system must be designed by a mechanical engineer working in coordination with the fire engineer.
Smoke Control in Common Areas of Residential Buildings
In purpose-built blocks of flats operating a stay-put evacuation strategy, smoke ventilation of the common parts — corridors, lobbies, and staircases — is particularly important. The means of escape for residents in the flat of fire origin passes through these common areas. If they fill with smoke, the escape route is compromised and the stay-put strategy cannot function safely.
The smoke ventilation strategy for the common areas must be set out in the fire strategy and must demonstrate — either by reference to standard design guidance or by CFD modelling — that adequate smoke-free conditions are maintained for the required time.
Maintenance of Smoke Ventilation Systems
Smoke ventilation systems require regular maintenance and testing to ensure they will function correctly in a fire. AOVs must be tested to confirm they open correctly on activation of the fire alarm. Mechanical extract systems must be tested under fire conditions to confirm that the correct zones extract and that the airflow rates meet the design specification. Maintenance records must be kept as part of the fire safety management records for the building.
Frequently Asked Questions
What is the difference between natural and mechanical smoke ventilation?
Natural smoke ventilation relies on the buoyancy of hot smoke gases to drive smoke out through high-level openings (AOVs). Mechanical smoke ventilation uses powered fans to extract smoke. Natural systems are simpler and require no power, but are less reliable in tall buildings. Mechanical systems provide more controllable performance.
Do all residential buildings need smoke ventilation systems?
Residential buildings above a certain height require smoke ventilation of common areas — corridors, lobbies, and staircases — to protect the means of escape. The specific requirement depends on the building height, configuration, and the applicable code (BS 9991 or Approved Document B).
How often should AOVs be tested?
Automatically opening vents should be tested at least annually, with monthly checks recommended for critical units. The testing should confirm that each AOV opens correctly when the fire alarm activates and that the opening size meets the design specification. Records of all tests must be maintained.
What is a common corridor smoke ventilation system?
A common corridor smoke ventilation system is a system designed to maintain tenable conditions in the corridors of a residential building in the event of fire. It typically comprises AOVs at the ends of corridors or at the top of staircase lobbies, which open automatically on fire detection to allow smoke to escape and make-up air to enter.
Does smoke ventilation need to be shown in the fire strategy?
Yes. The smoke ventilation strategy — whether natural or mechanical — must be described in the fire strategy and must demonstrate that adequate smoke-free conditions are maintained along escape routes. For complex systems or where performance-based approaches are used, CFD modelling may be required to support the strategy.
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