Question 1

What is background ventilation under Part F?

Accepted Answer

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.

Question 2

How do I calculate the whole dwelling ventilation rate?

Accepted Answer

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.

Question 3

What is equivalent area (EA) for a trickle vent?

Accepted Answer

Equivalent area is a standardised measure of the airflow capacity of a background ventilator, measured in mm². It represents the area of a sharp-edged orifice that would pass the same airflow. A trickle vent with 5000 mm² EA provides approximately 5 L/s at 1 Pa pressure difference. EA is marked on the ventilator and tested to BS EN 13141-1.

Question 4

What is purge ventilation?

Accepted Answer

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.

Question 5

Do I need trickle vents if I have MVHR?

Accepted Answer

Generally no. System 4 (MVHR) 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.

Background Ventilation Calculator — Part F Trickle Vents

How We Calculate This

ADF 2021 Table 1.3 — whole dwelling rates

ADF 2021 Table 1.7 — trickle vent EA

Frequently Asked Questions

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