Part F Ventilation Calculator — Extract Rates & Trickle Vents

System 1: Background ventilation (trickle vents) + intermittent extract fans in wet rooms. System 2: Passive stack ventilation (natural draught ducts). System 3: Continuous mechanical extract ventilation (MEV). System 4: Continuous supply and extract with heat recovery (MVHR). Any system can be used for any dwelling.

Per ADF 2021 Vol 1 para 1.24, the whole-dwelling rate has to satisfy both conditions, so it is the greater of: 0.3 l/s per m² of internal floor area, or the Table 1.3 rate set by the number of bedrooms (1 bed 19, 2 bed 25, 3 bed 31, 4 bed 37, 5 bed 43 l/s, which is 13 + 6 per bedroom). For a 100m² 3-bed house that is the greater of 30 l/s and 31 l/s, so 31 l/s. Part F sets no rate per occupant.

Per ADF 2021 Vol 1 Table 1.1: 30 l/s intermittent adjacent to the hob (a ducted cooker hood sited over the hob meets this) OR 60 l/s intermittent elsewhere in the kitchen (no hood, or a recirculating hood that does not extract outside). The continuous-extract alternative is 13 l/s. A recirculating hood does not satisfy Part F on its own — the room still needs a separate 60 l/s extract path.

MVHR (Mechanical Ventilation with Heat Recovery) supplies fresh air and extracts stale air through a heat exchanger recovering up to 90% of heat. Best for airtight buildings (air permeability below 3 m³/hr/m²). Costs £2,500-£5,000 installed but saves £100-£200/year on heating.

With MVHR (System 4), trickle vents are not required for background ventilation as the MVHR provides continuous supply air. However, openable windows are still needed for purge ventilation (1/20th of floor area). With Systems 1, 2 and 3, trickle vents are mandatory.