The LWF Blog

Fire Engineering Design and Risk Assessment – Fire and Smoke Ventilation – Part 13 – Fan Assisted Ventilation

April 12, 2017 9:22 am

In our most recent blogs for Architects and others in the building industry, we have been looking at fire and smoke ventilation and in part 12, at firefighting shafts. In part 13, we’re going to be talking about Fan-assisted ventilation.


In residential buildings where fan-assisted ventilation is used, there will be two or more vertical shafts which are the full height of the building. These will be used for the purpose of either extracting air/smoke or providing replacement air.


An automatic opening vent would connect the vertical shafts to the corridor or lobby and the vent becomes operational on the floor of fire origin only.


The size of the fan at the top of the shaft is commonly proven by computational fluid dynamics (CFD) and this enables control of the conditions in the corridor to be maintained at no worse a level than the comparable conditions in a code-compliant design. Care should be taken when working out the size (extract rate) of the fan and replacement air requirements, as overkill in this area could lead to smoke being drawn from the fire and into the lobby.


Some of the most common methods of fan-assisted ventilation follow:


Mechanical double reversible push-pull system – commonly used where the stair is the centre of a common corridor with smoke shafts at each end of the corridor. Double reversible fans are provided at the head of each shaft and smoke detectors are present at each end of the corridor, so that when a smoke detector is activated, the fan closest to that detector will automatically begin to remove air and smoke from the area. The fan at the other end of the corridor will, in turn, begin to replace the air. Because of the need to stop smoke spreading to other areas of the corridor, it is important that the fans are set with a supply/extract rate.


When the fire service attend, this system has the advantage of being able to place both fans on extract, with supply air being provided from the top and bottom of the stair from a roof vent and the stairs door at the bottom. To prevent negative pressures, the vent which is placed at the head of the stairs will open automatically when the system begins operation.


CFD is a common method of proving a system based on the bespoke arrangements, corridor geometry and purpose.


In next week’s blog, we will continue looking at types of fan-assisted ventilation, starting with combined mechanical and natural systems. 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.


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