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Background Ventilation Calculator — Part F Trickle Vents
Calculate minimum whole dwelling ventilation rate, trickle ventilator equivalent areas per room, and purge ventilation openings per Approved Document F 2021 Tables 1.3, 1.7 and 1.4.
Sets whole-dwelling rate (Table 1.3) - 8000 mm² EA each
8000 mm² EA each
8000 mm² EA each (10,000 single-storey)
4000 mm² EA each
No ADF minimum (sanitary accommodation)
8000 mm² EA each
For 0.3 L/s per m² check
Natural ventilation with background ventilators needs intermittent extract fans in wet rooms (ADF 2021)
Openable windows for rapid ventilation
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How We Calculate This
This calculator determines background ventilation requirements per Approved Document F 2021 Volume 1 for natural ventilation with intermittent extract fans and background ventilators (the approach formerly numbered System 1, before the 2021 edition replaced the numbered systems with named system types).
ADF 2021 Table 1.3 — whole dwelling rates
- 1 bedroom: 19 L/s
- 2 bedrooms: 25 L/s
- 3 bedrooms: 31 L/s
- 4 bedrooms: 37 L/s
- 5 bedrooms: 43 L/s (add 6 L/s per extra bedroom)
- Or 0.3 L/s per m² internal floor area (whichever is greater)
ADF 2021 Table 1.7 — trickle vent EA
- Habitable rooms & kitchen: 8000 mm² EA each (10,000 in single-storey dwellings)
- Bathroom: 4000 mm² EA; utility room & WC: no minimum
Frequently Asked Questions
Background ventilation provides continuous low-level air exchange through trickle ventilators (small openable slots in window frames). ADF 2021 Vol 1 Table 1.7 specifies minimum equivalent areas (EA): 8,000 mm² per habitable room, 8,000 mm² for a kitchen (treated the same as a habitable room), 4,000 mm² for a bathroom, and no minimum for a utility room or WC. Trickle vents should be controllable by occupants but designed to be left open.
ADF 2021 Vol 1 Table 1.3 gives the minimum rate based on 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 (add 6 L/s per extra bedroom). Also calculate 0.3 L/s per m² of internal floor area and use whichever is greater. This is the total ventilation rate for the whole dwelling, provided by background ventilators and/or mechanical systems.
Equivalent area is a standardised measure of the airflow capacity of a background ventilator, measured in mm². It is the area of a sharp-edged circular orifice that would pass the same airflow at a 1 Pa pressure difference. As a rough guide, a 5,000 mm² EA vent passes about 4 L/s at 1 Pa. EA is marked on the ventilator and is tested to BS EN 13141-1.
Purge ventilation is the ability to rapidly ventilate a room — for example, after decorating, spillage, or smoke. ADF requires an openable window (or external door) with an opening area of at least 1/20th of the room floor area in each habitable room. This is separate from background and extract ventilation requirements.
Generally no. Mechanical ventilation with heat recovery (MVHR) — the system formerly numbered System 4 before the 2021 ADF edition adopted named system types — provides both supply and extract ventilation mechanically, so trickle ventilators are not normally required. However, purge ventilation (openable windows) is still needed in habitable rooms. If the MVHR system is turned off, the dwelling would have no ventilation without trickle vents, so some designers include them as a failsafe.
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Last updated: June 2026
Verified against UK standards · estimates only, confirm with your supplier.