Ventilation Rate Calculator — Extract Rates per Document F

Under Approved Document F (2021) Tables 1.1 and 1.2, a bathroom or shower room requires an intermittent extract rate of 15 litres per second (l/s), or a continuous mechanical extract high rate of 8 l/s (dMEV/System 3). The fan should be capable of achieving this rate at the point of extract, accounting for duct resistance — keep flexible duct runs to 1.5m or less and use rigid duct where possible (ADF para 1.76–1.80). A 15-minute fan overrun is required by ADF para 1.51 only where the room has no openable window; it is widely fitted in all bathrooms as good practice.

Intermittent (or boost) ventilation is the higher rate that operates when the room is in use (e.g., while showering or cooking). Continuous (or background) ventilation runs at a lower rate 24/7 to maintain air quality throughout the dwelling. Most homes use a combination: trickle vents or MEV for continuous ventilation, and extract fans for intermittent boost.

Under Approved Document F (2021) Table 1.7, with a natural ventilation system the minimum background ventilator (trickle vent) equivalent areas are: habitable rooms (bedrooms, living rooms) AND kitchens 8000 mm² (10,000 mm² in single-storey dwellings); bathrooms 4000 mm²; utility rooms and WCs/sanitary accommodation have no minimum. Kitchens are treated the same as habitable rooms, not as wet rooms. These are total equivalent areas per room and can be achieved with one or more trickle vents in the windows. (With continuous mechanical extract instead, para 1.64 requires 4000 mm² per habitable room and no ventilators in wet rooms.)

Per ADF 2021 Vol 1 Table 1.3, the whole-dwelling continuous ventilation rate depends on the number of bedrooms: 1 bed = 19 l/s, 2 bed = 25 l/s, 3 bed = 31 l/s, 4 bed = 37 l/s, 5 bed = 43 l/s (formula 13 + 6 × bedrooms). Para 1.24 requires the rate to meet BOTH this bedroom figure AND a minimum of 0.3 l/s per m² of total internal floor area (all floors added together) — use whichever is greater. This can be achieved through MEV, MVHR, or passive stack ventilation.

Approved Document F (para 1.51) only requires a 15-minute fan overrun in a room with no openable window. Where the room has an openable window an overrun is not mandatory, but a run-on timer is widely fitted as good practice to clear residual moisture — and where one is provided it must be set to at least 15 minutes (para 4.6c). A humidistat (humidity sensor) is a common alternative that boosts the fan when moisture is detected and stops when humidity drops. For kitchens, the fan should operate while cooking is in progress; a cooker hood extracting to the outside achieves 30 l/s when positioned directly over the hob.