Part F Ventilation Rates Reference

System 1 is intermittent extract fans with background ventilators (trickle vents). System 2 is passive stack ventilation (PSV). System 3 is continuous mechanical extract ventilation (MEV). System 4 is continuous mechanical ventilation with heat recovery (MVHR). AD-F Volume 1 (2021 edition) sets out the requirements for each system in its Table 1.1/1.2/1.3 series (Table 1.1 intermittent extract rates, Table 1.2 continuous extract rates, Table 1.3 whole-dwelling rates).

For System 1 (intermittent extract fans), a kitchen requires a minimum of 30 l/s (litres per second) adjacent to the hob (ducted cooker hood over hob), or 60 l/s if located elsewhere in the kitchen (no hood, or a recirculating hood that does not extract outside). For Systems 3 and 4 (continuous extract), the minimum continuous rate is 13 l/s with a boost rate of 30 l/s. These values are from AD-F 2021 Vol 1 Table 1.1.

Purge ventilation provides rapid air exchange to dilute pollutants from decorating, new furniture, or accidental releases. Every habitable room must have purge ventilation capability — typically an openable window with a free area of at least 1/20th of the room floor area. Purge ventilation is required in addition to the normal extract/background ventilation strategy.

For System 1 (intermittent extract with background ventilators), the total equivalent area (EA) of background ventilators depends on the number of bedrooms: 1 bed = 25,000 mm² EA total, 2 bed = 35,000 mm² EA, 3 bed = 45,000 mm² EA, 4 bed = 55,000 mm² EA, 5+ bed = 65,000 mm² EA. These are distributed across habitable rooms and wet rooms per AD-F 2021 Vol 1 Table 1.4.

Scotland has its own ventilation requirements under Technical Handbook Section 3 (Environment) which has broadly similar principles but different specific rates and requirements. Northern Ireland follows Technical Booklet K. The 2021 AD-F update is specific to England; Wales follows its own edition of Approved Document F.