BESA DW/144 Explained: The UK Ductwork Installation Standard

If you're procuring ductwork installation for a commercial project in the UK, you will encounter the term DW/144 in almost every specification. But what does DW/144 compliance actually mean in practice — and how do you verify it?

This guide explains the standard, the leakage classes, the testing requirements and what questions to ask your ductwork subcontractor before you award the package.

What is BESA DW/144?

DW/144 is the Building Engineering Services Association (BESA) specification for sheet metal ductwork. It is the primary UK standard governing how commercial ductwork is manufactured, sealed and installed.

The current edition covers:

• Construction requirements for rectangular and circular ductwork
• Joint types and sealing methods for different pressure classes
• Support spacing and hanger requirements
• Flanged connection requirements
• Fire-rated ductwork requirements
• Stainless steel, pre-insulated and other specialist duct types

DW/144 is referenced in Building Regulations Approved Document F (Ventilation) and is required by virtually all commercial building specifications in the UK. It applies to HVAC supply and extract systems, smoke extract, kitchen extract, toilet extract, and most other mechanical ventilation ductwork.

Pressure Classes and Leakage Classes

One of the most important aspects of DW/144 is how it categorises ductwork by pressure — because the sealing requirements increase significantly with pressure class.

Class	Design Pressure	Typical Applications	Sealing Requirement
Class A (Low)	Up to 500 Pa	General supply/extract, toilet extract, simple office ventilation	Joints sealed with mastic or tape at M&E engineer's discretion
Class B (Medium)	500–1000 Pa	Kitchen extract, smoke extract, some central plant systems	All transverse joints and seams sealed
Class C (High)	Above 1000 Pa	High-velocity systems, large commercial plant	All joints and seams sealed, including longitudinal seams

The pressure class is specified by the M&E engineer in the contract documents. The ductwork contractor must install and seal to the specified class.

Air Leakage Testing — DW/143

DW/144 sets the installation standard. Air leakage testing is carried out under the companion standard DW/143 — the BESA guide to air leakage testing and measurement of air leakage rate in ductwork systems.

DW/143 testing involves:

1. Isolating a section of ductwork (typically 10% of the system by surface area)
2. Pressurising the isolated section to 1.25× the design pressure
3. Measuring the leakage rate using a calibrated flowmeter
4. Comparing the measured rate against the maximum permitted for the pressure class

Where a section fails, additional sealing is applied and the section re-tested. A passing test result generates a leakage test certificate, which becomes part of the O&M handover documentation.

When is DW/143 testing required?

Testing is typically required when:

• The project specification explicitly calls for it (most common on Class B and C systems)
• Building Regulations or the M&E engineer requires verification of the as-installed leakage rate
• The design relies on achieving a specific leakage class for energy performance calculations
• The system includes smoke extract or other life-safety critical ductwork

For simple Class A low-pressure systems in smaller commercial projects, visual quality inspection by a competent person may be sufficient. Always check the project specification.

What DW/144 Compliance Looks Like in Practice

When a ductwork contractor says they are "DW/144 compliant," this should mean:

• All ductwork fabricated or procured to the dimensional and construction requirements of DW/144
• All joints and seams sealed to the appropriate class (mastic, tape, flanged connections)
• All ductwork supported at the correct intervals using appropriate hangers and brackets
• Penetrations through fire compartments using the correct fire-rated access doors and fire sleeves/dampers
• All work carried out by competent, CSCS-carded installation personnel
• RAMS (Risk Assessment & Method Statement) provided prior to site works
• Commissioning and, where specified, leakage test certificates issued on completion

Common Ductwork Types We Install

Type	Typical Use
Rectangular galvanised steel	General supply/extract in commercial buildings, office fit-out, retail
Circular / spiral	Plant rooms, exposed installations, kitchens
Flat oval	Where headroom is limited — under floor plates, behind suspended ceilings
Fire-rated (Class A/B/C fire)	Penetrations through compartment floors and walls, smoke extract
Stainless steel	Commercial kitchen extract, laboratory, food production
Pre-insulated (PIR board)	External ductwork, cold areas, roof plant rooms

Questions to Ask Your Ductwork Subcontractor

When evaluating ductwork subcontractors for a commercial project, these are the right questions to ask:

1. Do you install to BESA DW/144? Any competent subcontractor should confirm this immediately.
2. Can you provide air leakage testing to DW/143 if specified? Not all installers offer this — important for Class B and C systems.
3. Do you provide RAMS prior to works commencing? Required on any commercial site. Should be project-specific, not generic.
4. What commissioning documentation do you issue? Commissioning certificates should be provided for every system.
5. Are all site operatives CSCS carded? Required on most commercial sites.
6. Can you supply O&M documentation at handover? Required under Building Regulations for most systems.
7. What is your quote turnaround from receipt of drawings? A quality subcontractor should be able to provide a detailed, itemised quote within 48 hours on most projects.