In this simple step-by-step guide, Dan Foster from Be Safe Direct explains the key factors for choosing the correct smoke ventilation system for a residential building.
When specifying and installing smoke ventilation systems for residential flats and apartments there is no one size fits all approach and which option is most suitable will depend on characteristics of the property.
The two main types of smoke ventilation employed within a building are natural and mechanical. Natural Smoke Ventilation systems use the passive forces of wind – and properties of hot air rising – to draw in fresh air and provide a safe escape route for a building’s occupants. In contrast Mechanical Smoke Ventilation Systems (MSVS) use powered fans connected to extract shafts to remove smoke in the event of a fire.
Approved Document B: Fire Safety of the Building Regulations provides guidance on choosing the most appropriate system based on several key factors. This includes the height of the building, internal layout of the property and the distance of each dwelling from the staircase for evacuation.
Height of the building
The first factor that needs to be considered when specifying a system is the height of the building. The guidance provided by Approved Document B (ABD) differentiates between properties with floors over 11 metres from the access level and those below this height.
This is the distance from the furthest flat door to the stairs that would provide the means of escape in the event of a fire. ABD sets out key travel distances that impact the selection of products. The deciding distance is 4.5m for buildings under 11m in height and 7.5 metres for those over 11m. If travel distances are more than this, mechanical, rather than natural smoke ventilation must be employed to ensure occupants can exit safely in the event of a fire.
The internal layout of the building and the flats themselves will decide the type of smoke protection required. If the building has more than two flats per storey or has storey with a floor height above 11m then the staircase must be lobbied – sealed off from the rest of the building by fire doors to protect the escape route. Similarly, a lobbied staircase is required if each floor has fewer flats but they do not have self contained hallways internally that would stop the spread of smoke.
These factors all affect which approach needs to be taken and which system is most suitable. These can be divided up into three categories:
Natural stairway smoke ventilation
For lower rise buildings, below 11m, with short travel distances the key consideration is protecting the stairway from smoke. A stairwell smoke ventilation system, whether lobbied or unlobbied will consist of a vent control unit linked to either a 1m² louvered stair smoke vent or stair window chain actuator. The system will also include orange smoke venting call points and a back up battery to ensure it will continue to function in the event of a power failure. The call points are coloured orange to allow easy differentiation from the red fire alarm call points.
Natural corridor smoke ventilation
Where there is a short corridor to the stairs (less than 7.5m) this must also be protected and here a natural smoke ventilation system is suitable. The same is true for buildings with storeys over 11m where the distance is up to 7.5m. If the common corridor that leads to the staircase has a window at least 1.5m2 this can be used as an automatic opening vent (AOV). If this isn’t available, then a smoke shaft will have to be installed to provide an outlet for the smoke in the event of a fire. Like the stairwell smoke ventilation options, the AOV and natural smoke shaft systems will consist of the window actuator or shaft vent linked to a control system with its own power supply and call points.
Mechanical smoke ventilation
Finally, for buildings with longer travel distances, or those that cannot accommodate natural ventilation options, a MSVS must be utilised. The greater efficiency and ventilation capacity of these systems ensures that the longer exit routes are kept clear of smoke to allow safe evacuation. A properly designed and maintained MSVS will provide a higher life safety standard for residential buildings compared with natural venting alternatives.
A typical MSVS consists of a vertical mechanical extract shaft that serves a building’s common corridor and/or lobby. If smoke is detected within one of these areas, the fire damper to the smoke shaft on the affected floor will open, and the fan at the top of the shaft will activate to extract the smoke.
With so many factors influencing which system is most suitable and, crucially, compliant for a given project it is always recommended that installers seek advice from system suppliers. They will also be able to provide installation guidance to ensure the system will operate as designed.
Be Safe Direct has created a system specifier guide to help installers select the correct system: https://www.besafedirect.com/smoke-venting-systems#specifier