Extractor Fans & Ventilation

by Steve on November 5, 2009

Extractor fans & Ventilation- Building Regs part F

The most common method of ventilation in domestic dwellings uses ‘Background Ventilation’ (Trickle vents to windows) and ‘Intermittent Extract Fans’.

Extractor fans are required in rooms where water vapour and pollutants are present to prevent them spreading to other parts of a building.

The Building Regulations relate to ventilation requirements in the following:


Cooker Hood Fan-30 litres per second

Usually 650-750mm above hob.

Wall/Ceiling Fan60 litres per second

Utility Room

Wall/ Ceiling Fan30 litres per second


Wall/ Ceiling fan15 litres per second

If located in Zones 1 or 2 must be IPX4

Fans with over-run facility must have TP isolation (switch outside Zones 0,1 & 2).

Separate WC

Wall/ Ceiling Fan6 litres per second

Extract Fans General

Building Regulations state that extract fans may be manually or automatically controlled.Humidity control can reduce the risk of condensation and mould growth but is not suitable for sanitary accommodation.

Any automatic control must also provide manual over-ride to turn on if required.

For rooms with no natural light the fan must operate with the main light and have a 15 minute over-run.

It is recommend all fans have independent isolation for safety & ease of maintenance.

The installation of extract fans and ducting can also compromise the integrity of fire barriers.

Fan Calculations

Under Part F (Ventilation) and L (Fuel and Power) of the Building Regulations there is a requirement for mechanical ventilation systems to be designed to minimise power usage.


Fan power rating is called ‘Specific Fan Power’ or SFP. It’s calculated by dividing the wattage (W) of the fan by the airflow (litres per second).

To comply with Part L (2010) the SFP should not be worse than 0.5 Watts per litres per second.


For example typical 4 inch low energy 5 Watt wall fan extracting 21 litres per second (75 cu.m/hour) would achieve 0.24W/L/S .

5 Watts / 21 = 0.24 SFP

(1 litres per second = 3.6 (cubic metres) per hour)

Source information:

As detailed in Electrician’s Guide to the Building Regulations


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