Purge ventilation calculator

Have the room dimensions and the opening type and geometry for each room you are checking. Purge ventilation is about clearing pollutants and water vapour quickly, and it is assessed room by room rather than for the dwelling as a whole.

The 2010 and 2021 editions state the purge requirement differently. Establish which applies to the work before entering anything; the tool follows the edition you choose rather than averaging the two.

Floor area and, where the method needs it, the room height. The opening area required follows from the room, so a large room with a small window is the case this calculation exists to catch.

The opening type and its geometry, because a hinged window opening 30 degrees does not deliver what the same window opening fully does. Enter the actual openable geometry rather than the frame size.

The tool gives the openable area required and compares it with what the opening provides. A shortfall is a design problem, not a rounding problem: more openable area, a different opening type, or mechanical purge are the routes out of it.

Background ventilation, extract rates and whole-dwelling rates are separate calculations. So are security, safety, restrictors and anything that limits how far an opening can actually be opened in use — all of which can make a geometrically compliant opening unusable.

The PDF needs your name and company. It records the edition and the room-by-room working. Save inputs writes a .briaryvent file. For problems, email toolsuite@briary.co.uk.

Purge is the one Part F requirement written as an air change rate, so the arithmetic behind it is in the tool: ach = flow × 3.6 ÷ volume. Work either way round — the rate a known fan achieves in a room, or the flow that delivers a rate you are aiming at. Enter the dimensions or a volume you already have, using internal dimensions and, for a sloping ceiling, the average height over the floor area.

The table converts the Approved Document F rates into air changes per hour for the volume you entered, so a flow can be judged against them directly. Read it carefully: Part F states its extract rates as flows, not as rates. Fifteen litres a second in a bathroom is fifteen litres a second whatever the room holds, so a large bathroom shows a low air change rate for the same compliant fan. The fan still complies. Only purge is specified as a rate, at four an hour.

Approved Document F states its rates in litres per second; product data, commissioning sheets and imported equipment rarely do. The Airflow units panel converts between all of them and puts the intermittent extract rates alongside, which is usually the reason for converting. Every conversion is exact rather than approximate: one litre per second is 3.6 cubic metres an hour by definition, and one cubic foot is 0.028316846592 cubic metres exactly, so one cubic foot per minute is 0.4719474 litres per second.