Ductwork Air Leakage Testing to DW/143: A Practical Guide

If your project specification mentions DW/143 air leakage testing, it means your ductwork subcontractor will need to pressure-test sections of the duct system and produce a certificate proving the installation is airtight enough to meet the DW/144 standard. This guide explains what that involves, when it's required, and what you need to plan for to avoid test failures delaying your programme.

The Relationship Between DW/144 and DW/143

These two BESA standards work together:

• DW/144 — the construction standard that defines how ductwork should be built and what leakage rates it must achieve. It sets the permissible leakage limits for each pressure class (A, B, C).
• DW/143 — the testing specification that defines how to measure whether the installed ductwork actually achieves those limits. It specifies the test methodology, equipment requirements, test pressures and how to calculate and report results.

In other words: DW/144 says what the ductwork must achieve, DW/143 says how to prove it.

When Is DW/143 Testing Required?

Testing is not required on every ductwork installation. Whether it is required depends on:

• The project specification. Most commercial new build and refurbishment specifications on projects above a certain scale call for DW/143 testing on medium and high pressure systems as a standard requirement.
• The pressure class. Class B (medium pressure, 500–1000 Pa) and Class C (high pressure, above 1000 Pa) systems are routinely specified for testing. Class A low-pressure systems may not require formal testing unless the specification says so.
• The building type. Healthcare projects (SHTM 03-01), clean rooms, laboratories and data centres almost always require testing. Standard commercial offices may require testing on higher-pressure zones (main distribution ductwork) but not on low-pressure branch runs.
• Building regulations. Scottish Building Standards do not mandate DW/143 testing for all commercial ductwork, but the Non-Domestic Technical Handbook references commissioning requirements that in practice lead most M&E specifications to include testing on significant ductwork installations.

The DW/143 Test Process

Step 1: Dividing the System into Test Sections

Ductwork systems are divided into manageable test sections, typically of 50–200 m² of duct surface area. Each section is sealed at its open ends using inflatable bladders or temporary plates. All terminals (grilles, diffusers) and dampers within the section are sealed or closed. Fire dampers and control dampers must be confirmed open so the test section represents a continuous duct void.

Step 2: Pressurising the Section

A fan and calibrated flow measurement equipment is connected to the test section at a convenient access point. The fan pressurises the duct to the test pressure specified for the relevant leakage class:

Leakage Class	System Pressure Range	Test Pressure (Pa)
Class A	Up to 500 Pa	400 Pa
Class B	500 – 1000 Pa	700 Pa
Class C	Above 1000 Pa	1000 Pa

Once the target pressure is reached and stabilised, the airflow required to maintain the pressure is measured. This measured airflow is the leakage rate — the air escaping through imperfections in the ductwork joints and seals.

Step 3: Comparing Against the Limit

DW/144 defines the maximum permissible leakage as a function of duct surface area and pressure class. The formula is:

Q_L ≤ f × A_s

Where Q_L is the measured leakage (l/s), f is the leakage factor for the class (0.027 for Class A, 0.009 for Class B, 0.003 for Class C) and A_s is the duct surface area (m²) in the test section. Class C has the tightest limit — approximately 9× more airtight than Class A.

Step 4: Producing the Certificate

If the measured leakage is within the permissible limit, the test section passes and a DW/143 certificate is issued for that section. The certificate records the test date, section reference, surface area, test pressure, measured leakage, permissible limit and pass/fail result. All section certificates are compiled into a test report for the project.

Common Test Failures and Their Causes

Failure Cause	Typical Location	Remedy
Unsealed longitudinal seam	Rectangular ductwork joints	Apply additional mastic sealant; re-test
Poorly sealed flanged joint	Duct section connections	Tighten bolts; apply gasket or mastic; re-test
Unsealed access door	Access panels, clean-out doors	Replace door gasket; apply sealant to frame
Leak at flexible connection	Fan connections, vibration isolators	Re-clamp or replace flexible duct section
Unsealed penetration	Ductwork passing through structure	Apply fire-rated mastic or collar
Pinhole in spiral duct seam	Circular spiral ductwork	Apply mastic externally; re-test

Programme Planning for Testing

DW/143 testing must be done before ceilings are closed. Once the ceiling is closed and the ductwork is inaccessible, a failed test means destructive investigation work. The correct programme sequence is:

1. Ductwork installation complete in test section
2. All sealing and access panels installed
3. DW/143 test carried out
4. Any remedial work completed and re-tested if required
5. Test certificate issued
6. Ceiling or void closure proceeds

Testing time must be allowed in the construction programme as a distinct activity, not squeezed into the same time as ceiling installation. On large commercial projects, testing can run concurrently with installation if the building is phased by floor or zone.

What the M&E Contractor Needs at Handover

At project completion, the M&E contractor (and ultimately the client) should receive:

• DW/143 test certificate for every section of ductwork that was tested
• A test schedule confirming which sections were tested and at which pressure class
• Clear identification of any sections that were accepted without formal testing and on what basis
• Calibration certificates for the testing equipment used

These documents form part of the building's O&M manual and are required for the Scottish Building Warrant Completion Certificate submission on commercial projects.