Natural Smoke Ventilation Systems
Natural Smoke Ventilation Systems (NSVS) use natural airflow to extract smoke and heat from buildings during a fire. They play a vital role in life safety and fire protection, particularly in residential and low-rise commercial buildings. In the UK, they are guided by Approved Document B, BS EN 12101-2, and BS 9991 (among others).
Purpose of Natural Smoke Ventilation
- Maintain smoke-free escape routes for safe evacuation.
- Assist fire and rescue teams by improving visibility.
- Reduce the build-up of smoke and hot gases.
- Minimise damage to structure and contents of the building.
How Natural Smoke Ventilation Works
Natural smoke ventilation relies on the buoyancy of hot smoke, which rises and exits through high-level vents, while cooler air enters from below.
Basic process:
- A fire triggers smoke detection or manual control.
- High-level vents (AOVs) automatically open.
- Rising smoke escapes through these vents.
- Cooler replacement air flows in through lower-level openings.
This creates a natural airflow path that extracts smoke without powered fans.
Key Components of a Natural System
| Component | Function |
|---|---|
| Automatic Opening Vents (AOVs) | High-level windows, roof hatches or louvres that open during fire conditions. |
| Smoke Shafts | Vertical ducts that collect and exhaust smoke from corridors/lobbies in multi-storey buildings. |
| Control Panels | Manage signals from detectors or manual inputs to activate vents. |
| Detectors / Manual Switches | Trigger the system in the event of a fire. |
| Low-Level Air Inlets | Allow fresh air in to support smoke extraction. |
Where Natural Smoke Ventilation is Used
- Apartment stairwells and corridors (≤18m building height).
- Low-rise offices, schools, and healthcare premises.
- Commercial buildings with atriums or rooflights.
- Protected lobbies and escape routes.
📏 Note: According to BS 9991:2024, natural systems can be used in buildings up to 30m tall under strict conditions. Beyond this, mechanical ventilation is usually required.
Relevant UK Standards & Regulations
| Standard / Document | Purpose |
|---|---|
| Approved Document B | Building Regulations guidance on smoke control design. |
| BS EN 12101-2 | Product standard for smoke and heat exhaust ventilators (NSHEVs). |
| BS 9991 (Residential) | Design guidance for residential fire safety (updated 2024). |
| BS 9999 (Non-Res) | Fire safety design framework for commercial/non-domestic premises. |
Advantages of Natural Systems
| Pros | Cons |
|---|---|
| Simple, passive design – fewer moving parts | Performance depends on external conditions |
| No fan power or backup generator needed | Not suitable for tall or complex buildings |
| Cost-effective installation and operation | Requires strategic vent placement and sizing |
| Lower maintenance requirements | May not meet modern performance demands alone |
Design Considerations
- Aerodynamic Free Area (AFA): Vents must meet minimum AFA requirements (usually 1.0 m² for lobbies).
- Location of Vents: Typically installed at the highest point (roof or top-floor wall).
- Travel Distances: Corridor length and layout must allow occupants to reach fresh-air zones safely.
- Vent Control: System should integrate with fire detection and allow manual override.
- Shaft Requirements: If used, shafts must be vertical, fire-rated, and unobstructed.
Maintenance & Testing (Basic Overview)
Natural smoke systems must be maintained regularly to remain compliant and operational.
| Test Type | Frequency | Details |
|---|---|---|
| Weekly Visual Inspection | Weekly | Check for physical damage or obstruction to vents. |
| Monthly Functional Test | Monthly | Simulate smoke input and confirm AOV activation. |
| Six-Monthly Inspection | Every 6 months | Engineer checks of panels, detectors, actuators, and control logic. |
| Annual Service | Annually | Full-duration test and certification of system performance. |
🔧 Vent actuators, batteries (if present), and manual call points should be function-tested to ensure reliability during an emergency.
Best Practice Tips
- All vents should have manual override for fire service use.
- Ensure vents are clearly labelled and unobstructed at all times.
- Maintain an on-site logbook for all inspections and servicing.
- Use tested and certified components in line with BS EN 12101-2.