What are the Part F extract rates?

Approved Document F (2021) specifies minimum extract rates for wet rooms in dwellings. For intermittent extract fans: kitchen 30 l/s (adjacent to hob) or 60 l/s (cooker hood), 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.

What is the difference between intermittent and continuous extract?

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).

Do I need trickle vents with extract fans?

Yes. Under System 1 (intermittent extract fans), background ventilation is provided by trickle ventilators in windows and doors. Part F 2021 requires a minimum equivalent area (EA) of 5000 mm² per habitable room and 2500 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.

What is MVHR and when should it be 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.

Ventilation Rates by Room — Part F Reference

Required extract ventilation rates and air changes per hour by room type per Approved Document F (2021). Intermittent and continuous extract rates for UK dwellings.

Intermittent Extract Rates (System 1)

Reference: Approved Document F Volume 1 (2021), Table 1.2.

Room	Minimum Extract Rate	Notes
Kitchen (adjacent hob fan)	30 l/s	Extraction at ceiling or wall level
Kitchen (cooker hood)	60 l/s	Capture hood above hob
Bathroom (with or without WC)	15 l/s	Operated by light switch with overrun timer
WC (separate)	6 l/s	Operated by light switch with overrun timer
Utility room	30 l/s	If tumble dryer is not externally vented

Continuous Extract Rates — MEV (System 3)

Reference: Approved Document F Volume 1 (2021), Table 1.3.

Room	Low Rate (continuous)	Boost Rate
Kitchen	13 l/s	30 l/s (as intermittent rate)
Bathroom	8 l/s	15 l/s (as intermittent rate)
WC	6 l/s	6 l/s
Utility room	8 l/s	30 l/s (as intermittent rate)

Whole-Dwelling Ventilation Rates

Reference: Part F Table 1.1. Minimum whole-dwelling ventilation rate.

Number of Bedrooms	Minimum Whole-Dwelling Rate (l/s)	With 2 Occupants + 1 per Bedroom
1	19	3 occupants assumed
2	25	4 occupants assumed
3	31	5 occupants assumed
4	37	6 occupants assumed
5	43	7 occupants assumed

Background Ventilator (Trickle Vent) Requirements

Minimum equivalent area per room (System 1 only). Reference: Part F Table 1.4.

Room Type	Min Equivalent Area (mm²)
Habitable room	5000
Wet room (kitchen, bathroom, WC, utility)	2500

Note: These are minimum requirements from Approved Document F Volume 1 (2021 edition). Always refer to the current edition of Approved Document F for definitive requirements. System 2 (passive stack) and System 4 (MVHR) have separate requirements not fully covered here.

See the Part F Ventilation Rates, the Part F Extract Fan Requirements, and the Duct Sizing Tables.