Therefore, many buildings incorporate smoke and heat-control systems. These systems may use natural ventilation, mechanical extraction or a combined arrangement.

At

AK Fire Safety
,
we provide professional

AOV and smoke-control inspections, servicing, repairs and commissioning

for residential and commercial buildings throughout the United Kingdom.

Natural and Mechanical Smoke Ventilation Compared

What Is Natural Smoke Ventilation?

Natural smoke ventilation uses the movement of hot smoke to remove smoke and heat from a building. Because hot gases rise, smoke can collect at high level and escape through an appropriately positioned opening.

Natural smoke ventilation equipment may include:

• Automatic opening windows
• Automatic rooflights
• Natural smoke and heat exhaust ventilators
• External wall louvres
• Natural smoke shafts
• Smoke-shaft dampers
• Replacement-air windows or doors
• AOV control panels
• Smoke detectors and fire alarm interfaces
• Manual firefighter controls

Although it is called a natural system, it may still depend on electrically powered actuators, control panels and standby batteries. The word “natural” refers to the airflow method rather than the absence of electrical equipment.

How Does Natural Smoke Ventilation Work?

Advantages of Natural Smoke Ventilation

Limitations of Natural Smoke Ventilation

Natural systems are affected by the available opening area, external wind, internal temperature and building geometry.

Potential limitations include:

• The building may not have a suitable external wall or roof opening.
• Wind direction may affect airflow around an external vent.
• Long or enclosed corridors may require a different solution.
• Suitable replacement air must be available.
• Roof vents and windows may be exposed to weather.
• Large openings may be needed to achieve the required performance.
• Building alterations may change the original airflow path.

For these reasons, an automatic window should not be added and described as a smoke-control system without a suitable design assessment.

What Is Mechanical Smoke Ventilation?

Mechanical smoke ventilation uses powered fans to extract smoke from a building. It may serve a smoke shaft, basement, car park, corridor, lobby, atrium or another protected area.

A mechanical system may include:

• Powered smoke-extract fans
• Duty and standby fans
• Smoke-resistant ductwork
• Smoke-control dampers
• Fan control panels
• Variable-speed drives where suitable
• Motorised replacement-air vents
• Pressure sensors
• Fire alarm interfaces
• Manual firefighter controls
• Primary and secondary power arrangements
• Fan-run and fan-fault monitoring

How Does Mechanical Smoke Extraction Work?

Advantages of Mechanical Smoke Ventilation

Limitations of Mechanical Smoke Ventilation

Mechanical systems can deliver controlled performance. However, they normally contain more equipment that must work together during an emergency.

Potential considerations include:

• Dependence on suitable electrical power
• More complex control panels and interfaces
• Fan, motor, belt or bearing failures
• Smoke-control damper failures
• Blocked or altered ductwork
• Higher servicing requirements
• Noise and vibration
• Greater commissioning complexity
• The need to prove replacement-air operation
• The need for fan-run and fault feedback

What Is Replacement Air?

Replacement air is the air that enters a building to replace smoke being released or mechanically extracted.

Without sufficient replacement air, extraction performance may be reduced. Doors may also become difficult to open if excessive pressure differences develop.

Replacement air may enter through:

• Automatic opening doors
• Low-level windows
• Wall louvres
• Dedicated air inlets
• Open stair doors where specified by the design
• Supply-air fans in an engineered system

The inlet should be included within commissioning and routine testing. An extract fan should not be tested separately from the replacement-air arrangement.

Which System Is Better?

Neither natural nor mechanical smoke ventilation is automatically better for every building.

Natural ventilation may be suitable where the building has appropriate external openings and sufficient space for the required vent area. In contrast, mechanical extraction may be more suitable for enclosed or complex areas where natural airflow cannot provide the required performance.

The selection should consider:

• The building height and layout
• The size and shape of the protected space
• Corridor and lobby arrangements
• Available roof or external wall openings
• The evacuation strategy
• Firefighter access requirements
• Available replacement air
• Power-supply arrangements
• Maintenance resources
• The approved fire strategy
• Any fire-engineering analysis

Can a Building Use Both Natural and Mechanical Ventilation?

Yes. Some buildings use a combined or hybrid smoke-control arrangement.

Examples include:

• A mechanical extract shaft with natural replacement-air inlets
• Natural corridor vents with a powered basement extract system
• Mechanical extract serving one stair core and natural ventilation serving another
• Natural roof vents supported by powered extract in selected areas
• A system that uses powered fans during a fire but natural windows for everyday ventilation

In a combined system, the sequence and control priorities must be clearly documented. Each device should operate according to the approved cause-and-effect schedule.

Where Are Natural Systems Commonly Used?

Natural smoke ventilation may be found in:

• Apartment-building corridors
• Protected stairways
• Natural smoke shafts
• Atriums
• Warehouses
• Industrial buildings
• Shopping centres
• Large single-storey premises
• Commercial buildings with suitable external openings

Where Are Mechanical Systems Commonly Used?

Mechanical smoke extraction may be found in:

• Basements
• Enclosed car parks
• High-rise apartment buildings
• Mechanical smoke shafts
• Long internal corridors
• Underground areas
• Complex atriums
• Large commercial developments
• Buildings where natural openings are impractical

However, these are only typical examples. The building’s approved design determines the correct solution.

What Happens During a Fire Alarm?

Both natural and mechanical systems may connect to the building fire alarm or use dedicated smoke detection.

A typical cause-and-effect sequence may include:

• A detector activates on the fire floor.
• The fire alarm sends a signal to the smoke-control panel.
• The affected floor’s smoke damper opens.
• Dampers on unaffected floors remain closed.
• The roof vent opens or the extract fan starts.
• The replacement-air inlet opens.
• Position and fault signals are monitored.
• Manual firefighter controls remain available.

Read our guide:

Can AOV Systems Be Connected to a Fire Alarm System?

How Do Maintenance Requirements Differ?

Smoke Ventilation Maintenance Checklist

Common Natural Smoke Ventilation Faults

• Failed AOV standby batteries
• Damaged window or roof-vent actuators
• Corroded hinges and brackets
• Obstructed vents or louvres
• Water ingress
• Failed smoke-shaft dampers
• Incorrect fire alarm interface operation
• Rain or wind controls interfering with fire mode
• Vents opening only partially

Common Mechanical Smoke Ventilation Faults

• Extract fan failing to start
• Duty or standby fan changeover failure
• Damaged fan belts or bearings
• Contactor or variable-speed drive faults
• Smoke-control dampers failing to open
• Missing fan-run confirmation
• Blocked or altered ductwork
• Replacement-air vents remaining closed
• Power-supply or control-panel faults
• Incorrect cause-and-effect programming

What Standards Apply?

The applicable requirements depend on the building and system design. Relevant standards and guidance may include:

• BS EN 12101-2 – natural smoke and heat exhaust ventilators.
• BS EN 12101-3 – powered smoke and heat-control ventilators.
• BS EN 12101-7 – smoke duct sections.
• BS EN 12101-8 – smoke-control dampers.
• BS EN 12101-10 – power supplies for smoke and heat-control systems.
• BS 7346-8 – recommendations for planning, design, installation, commissioning and maintenance of smoke-control systems.
• BS 5839-1 – fire detection and alarm systems where a fire alarm interface is provided.
• Approved Document B – government fire-safety guidance supporting the Building Regulations in England.
• The approved building fire strategy.
• The equipment manufacturers’ instructions.

BSI provides information about

BS EN 12101-2 for natural smoke and heat exhaust ventilators
.

Information about

BS EN 12101-3 for powered smoke and heat-control ventilators

is also available from BSI.

Current government guidance can be accessed through the

Approved Document B publication page
.

Who Is Responsible for Maintaining the System?

Responsibility commonly rests with the building owner, landlord, employer, managing agent, housing provider or facilities manager acting as the responsible person.

The responsible person should ensure that:

• Routine checks are completed.
• Professional maintenance is arranged.
• Faults are investigated promptly.
• Fire alarm interfaces are tested end to end.
• Smoke vents and air inlets remain unobstructed.
• Fans and dampers are tested.
• Documentation remains accurate.
• Repairs are followed by successful recommissioning.
• Maintenance records are retained.

Article 17 of the Regulatory Reform (Fire Safety) Order 2005 requires necessary fire-safety facilities, equipment and devices to be suitably maintained and kept in efficient working order.

Read

Article 17 of the Fire Safety Order
.

Frequently Asked Questions

Professional Smoke Ventilation Services

AK Fire Safety provides professional inspection, testing, servicing, repairs and commissioning for natural and mechanical smoke-control systems.

Our services include:

• Natural AOV system inspections
• Mechanical smoke-extract inspections
• AOV control-panel servicing
• Smoke-extract fan testing
• Smoke-control damper testing
• Window and roof-vent actuator testing
• Replacement-air inlet testing
• Fire alarm interface testing
• Cause-and-effect testing
• Standby battery testing
• Manual firefighter control testing
• Fault diagnosis and repairs
• System upgrades and recommissioning
• Planned preventative maintenance
• Nationwide multi-site support

Conclusion

Natural smoke ventilation releases smoke through automatic windows, roof vents, louvres or smoke shafts. Mechanical smoke ventilation uses powered extract fans, ductwork and dampers to move smoke from the building.

Natural systems may contain fewer powered components and can suit buildings with suitable external openings. In contrast, mechanical extraction can provide more controlled airflow in enclosed or complex spaces.

However, neither system is automatically better. The correct choice depends on the building layout, evacuation strategy, smoke-control calculations and approved fire strategy.

Most importantly, both types require regular testing and professional maintenance. Every detector, interface, control panel, vent, damper, fan and replacement-air opening should work together according to the approved cause-and-effect sequence.