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Approved Document F: Ventilation

by Austin Williams
NBS Content Development Manager

The new Approved Document Part F: Ventilation contains more guidance on domestic mechanical and natural ventilation systems than the existing ADF L: 1995. Due to come into force on 6th April 2006, the main changes between the new Approved Document F (ADF) and the old document are:

• a performance specification approach has been adopted through much of the new recommendations;
• it assumes that well constructed buildings will have an air permeability of 3m²/h/m² at 50 Pa pressure or worse;
• ventilator areas now describe 'equivalent area' as opposed to 'free area';
• basement ventilation requirements have been spelled out in some detail;
• the required air supply rates for offices is increased from 8 l/sec to 10 l/sec;
• replacement windows should now be fitted with trickle ventilators; and
• a range of Good Practice recommendations have been included to help with on-site conditions and to ensure that the completed building complies with the drawings and calculations.

ADF: 2006 is split into three distinct parts:

• Section 1: Dwellings;
• Section 2: Buildings other than dwellings; and
• Section 3: Existing buildings.

The ventilation rates throughout are predicated on a higher level of airtightness than that specified as a target value in ADF.

Most ventilation rates in dwellings remain unchanged when compared with the previous ADF, but there is a new requirement for the ventilation of basements. In order to assess the ventilation requirements, Diagram 1 in the document sets out a straightforward diagrammatic representation of four ventilation system options:

• background ventilation and intermittent extractor fans;
• passive stack ventilation;
• continuous mechanical extract; and
• continuous mechanical supply and extract with heat recovery.

Each of these four 'types' relates to a separate page that describes a two or three-step approach to reading off the clear equivalent ventilation area. For example, Table 1.2a in the document provides a read-off of the equivalent ventilator area for a given number of rooms and room sizes. Table 1.2b provides a read-off of the cross-sectional area of passive ventilation ducts for different domestic room types.

The phrase 'purge ventilation' has been introduced as a reworking of the previous ADL's 'rapid ventilation' which relate to specific and intermittent ventilation required to clear air of paint fumes, vapour from heaters, cooking smells, etc. There is a requirement for purge ventilation in sanitary accommodation. For example, it recommends that there be a clear opening area equivalent to 1/20th floor area, catered for by a fully openable window(s), regardless of the mechanical, passive or trickle ventilation requirements provided.

The additional requirement for basements is new and straightforward. Where a basement is connected via an open stair to the main dwelling, ventilation must be as per the dwelling. Where basements have one clear exposed wall, mechanical ventilation, with or without heat recovery, is the 'preferred' option.

Internal rooms can be vented through a secondary room provided that background ventilation and purge ventilation (min 8000mm²) requirements are met. If the outer room is a conservatory, the area of the closable partition between the two rooms – and from the secondary room to external air – must meet the clear ventilation opening equivalents relative to the floor area of both rooms, for purge ventilation purposes.

In Section 2: Buildings other than dwellings, the whole building ventilation rate for offices requires an air supply rates increases from 8 l/s in the current ADF to 10 l/s here.

A lot of helpful guidance is given on passive stack ventilation systems (PSV). These can be used as an alternative to a mechanical extract fan for office sanitary areas, washrooms and food preparatory areas. Recommendations are given on the location and extract rates but two pages are given over to referencing external compliance documents. The CIBSE Guide B2:2001 figures quite extensively.

There are some complicated aspects to the performance specification calculations, although a number of worked examples in the Appendices cover many likely scenarios.

Written August 2006

 

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