How many air changes per hour does a car park smoke ventilation system need?

Approved Document B Volume 2 asks for a mechanical system designed to operate at 10 air changes per hour during a fire, running in two parts, each capable of 50% of that rate and each with an independent power supply. BS 7346-7 sets the same 10 air changes per hour figure for ducted mechanical extract and for impulse smoke clearance. That figure is a prescriptive clearance rule, not a performance target. If the fire strategy asks the system to keep an approach route clear for firefighters or to protect escape routes, the extract rate is calculated from a design fire and usually comes out higher.

What is the difference between smoke clearance and smoke control in a car park?

Smoke clearance shifts smoke during and after a fire so the fire service can check for secondary seats of fire and the building can be returned to use. BS 7346-7 is explicit that a clearance system is not intended to keep any area of the car park clear of smoke, to limit smoke density or temperature, or to assist means of escape. Smoke control is the higher duty: either a clear approach route for firefighters, or protection of escape routes on the fire storey. Control designs need a design fire, calculation and normally CFD modelling.

Do car park jet fans need to be F300 rated?

BS 7346-7 recommends that all fans intended to exhaust hot gases in a car park ventilation system are tested to BS EN 12101-3 and suitable for 300°C for at least 60 minutes, which is class F300. Ancillary electrical and mechanical equipment exposed to the same gases should hold up to the same time and temperature. A higher class such as F400 may be required where the fire strategy or the calculated smoke temperature demands it. Check the classification on the Declaration of Performance, including the mounting arrangement it was tested in.

What natural ventilation opening area does an enclosed car park need?

For fire, Approved Document B Volume 2 asks for permanent openings at each parking level with an aggregate free vent area of at least 1/40 of that level's floor area, at least half of it provided equally by two opposite walls. The separate 1/20 figure applies to open sided car parks and buys a fire resistance concession rather than being a ventilation minimum. Approved Document F sets the day to day pollution requirement, and for a naturally ventilated car park that is an equivalent area of 1/20 of the floor area with at least 25% on each of two opposing walls.

Car Park Smoke Ventilation: Jet Fans and Extract

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Publishing Sunday 1 November 2026

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How enclosed and basement car parks are ventilated for smoke: the Approved Document B figures, what BS 7346-7 adds, and jet fans against ducted extract.

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