The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 14 – Fan Assisted Ventilation

April 20, 2017 3:46 pm

In this recent Fire Engineering Design and Risk Assessment blog series for architects and those who work in building design, we have been looking at fire and smoke ventilation solutions. In Part 13, we began to discuss fan assisted ventilation and today, we’re going to explore combined mechanical and natural systems in multi-occupancy residential buildings.


A combined mechanical and natural system can have various formats in order to achieve your fire safety aims. It would usually have two smoke shafts in cases where a stairway is positioned at one end of a corridor, but they can also be provided where the stairway is in a central position and three smoke shafts are used.


Equally, there can be variation of the system composition in terms of natural and mechanical elements. It is common to see mechanical supply with natural ventilation, or to have a mechanical extract with natural replacement air. The version with mechanical supply is often favoured slightly as it has a side effect of slightly pressurising the corridor. A mechanical extract conversely can suck smoke into the area.


With the system laid out so that there is a smoke shaft at each end of the corridor, vents removing smoke from the corridor area, by either natural or mechanical means, at the end furthest from the stair entry, there should be replacement air being provided via the vent adjacent to the stairs.


The system would be dormant until the smoke detector in the corridor is triggered, upon which a signal is received and the system would automatically begin to operate.


Mechanical ventilation systems have two distinct advantages over a natural ventilation system. The first and most relevant is that they provide improved conditions within the common corridors and the second is that they are likely to require less floor space than a natural system.


Fan-assisted ventilation can help to compensate for any shortfall there might be by straying from the regulatory guidance, subject to appropriate design and approvals.


A CFD model will consider conditions in the area from the means of escape period and the early stages of fire and track the process through to the firefighting stage, which will mean that doors to the stairs and flats are likely to be opened. A system designed in such a way should satisfy the requirements of the fire-fighting team attending, despite slightly different operational practices undertaken in various areas of the country.


In Part 15 of this series, we will be taking a look at fire-fighting shafts in commercial buildings. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.


Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.



Share this post