Natural vs Mechanical Smoke Ventilation Systems

Smoke Ventilation

Natural vs. Mechanical Smoke Ventilation Systems

By keeping escape routes such as corridors and staircases clear, smoke ventilation systems allow the safe, compliant evacuation of residential buildings, while also providing suitable access into the building for the fire services.

Here, we look at the range of systems available to architects, developers and contractors and the benefits that they can bring to a project.

Often a building’s design will determine which ventilation system is most appropriate, with British Standard (BS) requirements varying depending on a building’s height and the distance from the furthest apartment entrance door to the nearest escape route.

These requirements state that in residential buildings that stand four storeys or more in height (i.e. contain a floor over 11m from the access level), maximum travel distances of 7.5m are permitted. To achieve code compliance the corridor should be provided with either a 1.5m² automatic opening vent (AOV) window or a 1.5m² natural smoke shaft, in order to provide a safe, protected escape route for a building’s occupants by protecting its staircases from smoke ingress.

If the property’s common corridor has an external wall, natural smoke ventilation systems are a particularly cost-effective method of achieving compliance as existing windows can be utilised for this purpose.  Natural systems also offer the advantage of easy maintenance due to the small amount of mechanical parts included, often simply consisting of fire doors and an actuator. This lack of required maintenance makes natural systems ideal for use in multi-occupancy properties where maintenance budgets are limited, or where systems may be vandalised.

The alternative solution is a mechanical smoke vent shaft system. There can be distinct advantages to installing a mechanical smoke ventilation system in place of a natural one, a technique which is permitted under British Standard 9991:2011, which provides guidance on fire safety in the design of residential buildings.

smoke ventilation system

The standard states that mechanical smoke shaft systems can be used, provided they offer equivalent performance to their 1.5m² natural counterparts. Due to the greater efficiency of mechanical systems, this can enable the use of shafts as small as 0.25m² in some circumstances, although typically these are 0.5m² or 0.6m², providing substantial space saving benefits over natural systems, creating more saleable space within a property.

As with natural systems, the equivalent mechanical alternative should also be utilised to protect the building’s staircase, however these systems can also be implemented in buildings that contain travel distances in the common corridor that extend over the code recommended 7.5m since they offer greater performance for means of escape, assisting in clearing smoke from the building’s common corridors, mitigating this non-compliance.

Prices of Mechanical vs. Natural

Although mechanical smoke ventilation systems are more expensive in terms of initial kit and installation costs due to the inclusion of fans, dampers and their requirement for a secondary power supply, the costs involved equalise due to the improved efficiency offered by mechanical systems, allowing the maximisation of saleable space and the resultant design benefits that can achieved (e.g. extended travel distances and removal of secondary staircases).

Both natural and mechanical systems can offer significant advantages to a project, both in terms of smoke clearance and cost/space savings. Deciding whether a mechanical or natural smoke ventilation system will offer the greatest advantages depends upon a number of factors, including the property’s design and the building owner or developer’s objectives.

By working with experienced fire engineers from a project’s early stages, the most suitable system, or combination of systems can be selected, forming a bespoke solution which produces the best possible result for the property in question.