Question 1

What are the Part F extract rates?

Accepted Answer

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.

Question 2

What is the difference between intermittent and continuous extract?

Accepted Answer

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).

Question 3

Do I need trickle vents with extract fans?

Accepted Answer

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.

Question 4

What is MVHR and when should it be used?

Accepted Answer

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.

Room	Minimum Extract Rate (l/s)	Notes
Kitchen (adjacent to hob — ducted cooker hood)	30	Hood or fan directly above the hob, extracting to outside
Kitchen (elsewhere — no hood or recirculating)	60	Extract point not adjacent to hob, or recirculating hood
Bathroom (with or without WC)	15	Operated by light switch with overrun timer
WC (separate)	6	Operated by light switch with overrun timer
Utility room	30	If tumble dryer is not externally vented

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

Room Type	Min Equivalent Area (mm²)
Habitable room (incl. kitchen)	8000
Wet room (bathroom, WC, utility)	4000

Ventilation Rates by Room — Part F Reference

Intermittent Extract Rates (System 1)

Continuous Extract Rates — MEV (System 3)

Whole-Dwelling Ventilation Rates

Background Ventilator (Trickle Vent) Requirements

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