Ventilation Rates by Room — Part F Reference

Approved Document F (2021) specifies minimum extract rates for wet rooms in dwellings. For intermittent extract fans: kitchen 30 l/s (adjacent to hob — ducted cooker hood over hob) or 60 l/s (elsewhere in kitchen — no hood or recirculating), bathroom 15 l/s, WC 6 l/s, utility room 30 l/s. For continuous mechanical extract (MEV): kitchen 13 l/s, bathroom 8 l/s, WC 6 l/s. These are minimum rates and may need to be increased for larger rooms or higher moisture loads.

Intermittent extract fans run when needed (switched on by the occupant, pull cord, or automatic sensor) at a higher flow rate. Continuous extract (MEV — mechanical extract ventilation) runs constantly at a low background rate and boosts to a higher rate when the room is in use. MEV systems are increasingly common in new-build housing as they provide better overall air quality. Part F 2021 accepts either approach as System 1 (background ventilators + intermittent fans) or System 3 (continuous MEV).

Yes. Under System 1 (intermittent extract fans), background ventilation is provided by trickle ventilators in windows and doors. Part F 2021 Table 1.7 requires a minimum equivalent area (EA) of 8000 mm² per habitable room (including kitchens) and 4000 mm² per wet room. Under System 3 (continuous MEV), trickle vents are not required as the MEV system provides continuous air exchange. Under System 4 (MVHR), the building should be relatively airtight and trickle vents are not used.

MVHR (Mechanical Ventilation with Heat Recovery) is System 4 under Part F. It provides supply and extract ventilation through a heat exchanger that recovers 70-95% of the heat from exhaust air. MVHR is most effective in well-sealed buildings (air permeability below 5 m³/hr/m²) and is commonly specified for Passivhaus and high-performance new builds. It is not suitable for leaky older buildings as the recovered heat is negligible compared to losses through the building fabric.