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DW/144 explained: ductwork standards and air leakage testing

Updated 23 October 2026 · SEO Dons Editorial

What is DW/144?

DW/144 is the specification for sheet metal ductwork published by BESA, the Building Engineering Services Association. BESA describes it as the recognised standard for how sheet metal ductwork is designed, made and installed in building services, across low, medium and high pressure and velocity air systems.

BESA's listed scope includes leakage limits and performance classes, construction rules for rectangular, circular and flat oval duct, how smoke and fire dampers are integrated, and duct linings and thermal insulation. The publication page also records an April 2024 addendum aligned with the revised TR19 Air cleanliness specification, which the ventilation duct cleaning guide explains. BESA sells the document, so this page summarises only what BESA states publicly.

DW/144 is a specification, not a statute. BESA's own guidance says the law does not mandate it, although the industry widely uses it to show compliance. The link to the Building Regulations in England runs through Approved Document L Volume 2, which says ventilation ductwork should be built and assembled to be reasonably airtight, and should meet either DW/144 or the combination of BS EN 1507, BS EN 12237 and BS EN 13403.

What are the pressure and leakage classes?

Pressure class comes first. BESA gives the limits as up to 500 Pa for low pressure, up to 1,000 Pa for medium and up to 2,000 Pa for high pressure, and says the class affects sheet thickness, jointing and leakage limits. Underspecifying risks leakage or collapse, while overspecifying adds cost and complexity.

Approved Document L Volume 2 sets the same ladder out as classes A to D in its Table 8.1. The design static pressure rises from 500 Pa for class A to 1,000 Pa positive and 750 Pa negative for class B, and 2,000 Pa positive and 750 Pa negative for classes C and D. The leakage coefficient falls from 0.027 for class A to 0.009, 0.003 and 0.001, so each step allows about a third of the leakage of the one before.

Leakage classWhere BESA places it
Class AThe most lenient, typically for noncritical, low-pressure systems
Class BSuggested with Class A for low-pressure, noncritical systems, to control cost
Class CThe class most often used in commercial building services
Class DThe tightest, required in high-risk settings such as hospitals or laboratories

BESA suggests Class C or D above 1,000 Pa or when energy performance matters most, and Class A or B for low-pressure, noncritical systems. It advises fixing the class at design stage and recording it in the project specification, so every bidder prices the same number.

When is leakage testing required?

  • Approved Document L Volume 2: leakage tests should be carried out on ducted systems whose fans have a design flow above 1 cubic metre per second, following BESA's DW/143 and DW/144.
  • DW/144 itself: BESA reads it as requiring tests on high-pressure ductwork, with testing optional for low and medium pressure unless the job specification asks for it.

One cubic metre per second is 1,000 litres per second. Approved Document F Volume 2 sets an office supply rate of 10 litres per second per person or 1 litre per second per square metre of floor, whichever gives the higher total, so at the per-person rate that is the supply for roughly 100 people.

A separate paragraph is often misread. Where low-pressure ductwork has at least 10 per cent tested at random and it meets the low-pressure standard in DW/143, Approved Document L allows a calculated improvement in the building's primary energy rate and emission rate. That is an energy-calculation credit, not a cap on how much has to be tested.

How much energy does leaky ductwork waste?

BESA says leakage can waste thousands of kilowatt-hours a year across a large building, and that even small leaks push up fan energy, hurt comfort and can stop a system reaching its performance targets. Approved Document L Volume 2 also limits specific fan power, which is fan input power divided by airflow, by system type.

A worked example shows the mechanism. It is hypothetical: a supply system must deliver 5,000 litres per second to the rooms, the fan runs at an assumed 1.8 watts per litre per second, and it operates 2,600 hours a year (10 hours a day, five days a week, 52 weeks).

ScenarioAir to the roomsAir the fan must moveFan inputEnergy a year
Sealed ductwork5,000 l/s5,000 l/s9.0 kW23,400 kWh
10 per cent leaks out5,000 l/sabout 5,556 l/s10.0 kW26,000 kWh
Differencenoneabout 556 l/s1.0 kW2,600 kWh

The sum is a straight line at fixed specific fan power. Real systems also depend on the fan curve, where the leaks sit and the controls, so read it as the scale of the issue, not a prediction. Air lost from a supply duct can also have been filtered, heated or cooled already, so the waste is not only fan energy.

How is a ductwork leakage test done?

  1. Finish the work first. BESA says the test comes after everything is in place (fittings, dampers, access panels and supports) and before final commissioning starts.
  2. Take the section out of service and seal every opening, including terminals, grilles and access points, with blanking plates, end caps or similar covers.
  3. Choose representative sections. BESA's description of DW/143 puts the surface area of a test section at generally ten to one hundred square metres.
  4. Use calibrated equipment: a fan that can hold steady airflow at the test pressure, a pressure gauge and an airflow measuring device. BESA says DW/143 requires all instruments to be calibrated.
  5. Raise the pressure to the figure for the leakage class and hold it for at least 30 seconds.
  6. Work out the leakage per square metre of duct surface, in litres per second, and compare it with the limit for the class.
  7. If the result is over the limit, find and seal the leaks and retest, keeping every reading.

What should you ask for at handover?

A test only protects the client if the paperwork exists. Ask for:

  • The leakage class specified, and the document that records it.
  • Which sections were tested, their surface areas and the test pressure.
  • The calculated leakage for each section against the class limit, with retest results for any that failed.
  • Calibration records for the instruments used.
  • Who carried out the tests. Approved Document L names membership of the BESA Specialist Ductwork Group or ADCAS as one way to show competence at ductwork pressure testing.
  • The commissioning notice. Approved Document L says a commissioning notice goes to the building control body and the owner, and Approved Document F says ductwork commissioning should take account of DW 144, DW 154 and DW 143.
  • The wider air-side commissioning record. CIBSE Commissioning Code A covers setting to work, functional testing and regulation of ventilation and ductwork systems, and the commissioning and handover guide sets out what a full handover pack contains.

The commercial ventilation and MVHR page covers the systems it serves, and the spiral, rectangular or flexible ductwork guide compares the duct types. You can send an enquiry through the form on this site.

Sources

Common questions

BESA's own guidance says the law does not mandate DW/144. Approved Document L Volume 2, the Building Regulations guidance for England on energy performance, says ductwork should comply with either DW/144 or BS EN 1507, BS EN 12237 and BS EN 13403.

What is the difference between DW/143 and DW/144?

DW/144 is the specification for building sheet metal ductwork, including leakage limits and classes. DW/143 is the companion guide to testing, which BESA describes as the UK's official method for testing how airtight ductwork is. Approved Document L Volume 2 cites both.

Does the 10 per cent figure mean only a tenth of the ductwork needs testing?

Not on its own. The 10 per cent in Approved Document L paragraph 8.11 is the share of low-pressure ductwork that has to be tested at random and pass if the project wants to claim a calculated energy improvement. When tests should be carried out is set separately, so ask the designer which paragraph the project relies on.

What happens if a section fails its leakage test?

Approved Document L says the ductwork should be put right until retests perform satisfactorily, and that further sections should then be tested as DW/143 sets out. A failure can therefore widen the testing, so the records should show each retest.

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