DW172 Ductwork Standard Explained

What does DW172 require?

It requires the system to be sized from the cooking line rather than from the room. The 2nd edition publishes an appliance schedule giving an extract coefficient in cubic metres per second per square metre of canopy surface, and a surface temperature, for each type of appliance. A fryer and a solid top range pull different amounts of air, so a canopy is calculated appliance by appliance rather than from a single figure per cover.

The 2nd edition also covers, in its own sections:

• Grease separation. What the trade used to call grease filters are called grease separators, and they may be certified to LPS1263, VDI 2052, Underwriters Laboratories or equivalent independent laboratory testing.
• Solid fuel appliances. A dedicated canopy, an independent stainless steel duct and fan with the motor kept out of the airstream, a 300 mm canopy overhang past an open oven door, and a carbon monoxide interlock on every solid fuel system.
• Demand controlled kitchen ventilation. Systems over 2.5 m³/s total extract should be considered for demand control, with a background rate of 20 air changes an hour maintained and carbon monoxide and carbon dioxide monitors able to override the control system.
• Canopy lighting. Around 500 lux, IP55, 80 CRI minimum, 3000K or 4000K, flush mounted LED protruding no more than 10 mm, with emergency lighting at a tenth of operational illuminance.
• Cleaning and maintenance. Access positioned so that every surface of the system can be cleaned and visually inspected.

Figures circulate widely for minimum duct gauge, ductwork leakage class and a universal duct velocity range, usually attributed to DW/172. No primary source could be found for any of them, so none of them appears on this page. If your designer quotes one, ask which clause it comes from.

Which edition is current?

The current version is the 2nd edition, published on 1 October 2018, with an addendum dated May 2024. The earlier printing of the 2nd edition that incorporated an April 2020 addendum is listed as withdrawn in the construction industry document registry, so a copy on a shelf is worth checking before anyone designs from it.

The lineage runs like this:

The publisher changed name along the way, which is why older documents cite three different organisations for the same specification. The Heating and Ventilating Contractors’ Association became the Building and Engineering Services Association in 2012 and the Building Engineering Services Association in 2016. Same body, same document series.

Is DW172 a legal requirement?

No. DW/172 is a voluntary industry specification. What the law requires is different: under the Regulatory Reform (Fire Safety) Order 2005 the responsible person for the premises has to carry out a fire risk assessment and take reasonable fire precautions. DW/172 and TR19 Grease are the recognised way of demonstrating that the extract system was designed and maintained to a competent standard, and insurers commonly ask for them by name. That is a strong practical reason to comply. It is not the same thing as the specification being law, and any page that tells you it is law has not read it.

Where do access doors go?

DW/172 frames this as an outcome. Access has to sit wherever it needs to sit for every internal surface of the system to be cleaned and inspected, and the specification cross-refers to TR19 for the cleaning practice itself. On a real system that usually means access at every change of direction, at each floor level on a vertical riser, at both sides of any attenuator or plant item, and at intervals along straight runs.

On the interval along a straight run, sources disagree. Contractor pages commonly state 2 metres. BESA’s own published guidance says roughly 2 to 3 metres. A consulting engineer’s specification filed with a planning authority for a scheme designed to DW/172 sets a minimum of every 3 metres and at every change of direction. None of the three cites a clause number. So the test worth applying on site is the question the specification actually asks. Can a cleaning engineer reach and see every internal surface? If the answer is no, the access is wrong whatever the tape measure says.

One related point that catches people out: fire dampers are generally not fitted inside kitchen extract ductwork, because grease deposits stop them operating. Fire resisting ductwork is used through compartment lines instead.

How do DW172 and TR19 Grease fit together?

They cover different halves of the same system. DW/172 is about how it is built. TR19 Grease is about how it is kept clean and how that is recorded.

The two meet at the access panels. A system built without proper access cannot be cleaned to TR19 Grease later, and the cleaning intervals TR19 Grease publishes are baseline figures subject to a competent person conducting a cleanliness risk assessment.

Where can I get the DW172 PDF?

From BESA Publications, and it is not free. DW/172 2nd edition is an 88 page paid specification listed at £500 for non-members, and free to BESA contractor members. There is no legitimate free download: copies that turn up on file sharing sites are unauthorised, and a copy of unknown vintage is exactly the problem described above, given the 2020 addendum printing has been withdrawn.

If you need to know whether a specific kitchen complies rather than to read the whole document, a survey answers that faster than the specification does. Kitchen extraction installation, and canopy installation where the capture area is the question.