What This Requirement Covers
Kitchens and bathrooms generate moisture and pollutants that must be removed to maintain indoor air quality and prevent condensation and mould growth. Approved Document F requires extract ventilation in all rooms where moisture or pollutants are produced.
Key Requirements
Minimum Extract Rates (Intermittent Fans)
- Kitchen: 30 litres/second (adjacent to a hob, as a cooker hood) or 13 litres/second (elsewhere in the kitchen)
- Bathroom (with bath or shower): 8 litres/second
- Utility room (with washing machine or dryer): 8 litres/second
- WC (separate, no bath or shower): 6 litres/second
Fan Selection
- Fans must be rated to achieve the minimum extract rates when installed (not just their free-air flow rate)
- The installed performance must account for ductwork length, bends, and the terminal (grille or cowl)
- Axial fans are suitable for short duct runs (up to 1.5 metres)
- Centrifugal fans are suitable for longer duct runs and more complex routing
Ductwork
- Ducts for extract fans should be as short and straight as possible
- Each 90-degree bend reduces the effective capacity of the fan
- Flexible ductwork should be kept to a minimum; rigid ductwork is preferred for performance
- Ducts must terminate to outside (not into the loft, soffit, or cavity)
- Condensation traps or a fall towards the outside should be provided in horizontal duct runs
Controls
- Kitchen extract fans should be manually controlled (switched by the occupant when cooking)
- Bathroom extract fans should be controlled by the light switch or a humidity sensor, with a minimum 15-minute overrun after the occupant leaves the room
- Continuously running fans (System 3) should have a boost control for high-moisture periods
Noise
- Fan noise should not exceed 30 dBA in bedrooms and living rooms, or 35 dBA in kitchens and bathrooms
- Quiet operation encourages occupants to leave fans running for the appropriate duration
Practical Compliance Tips
- Specify fans that achieve the required extract rate at the installed duct length and configuration, not just the catalogue rating
- Duct to outside, never into the loft or soffit; moisture discharged into the loft causes condensation, rot, and mould
- Use smooth rigid ductwork wherever possible; flexible duct has much higher resistance
- Install a condensation trap in the ductwork where the duct runs horizontally for any distance
- Overrun timers on bathroom fans are essential; they are cheap to install and prevent condensation damage
- Consider humidity-controlled fans for bathrooms; they activate automatically when moisture is detected
- Provide clear instructions to occupants on using the extract ventilation; many condensation complaints arise from inadequate fan use