Understanding Ductwork Leakage Classes In DW/143
Air leakage in ductwork may seem insignificant, but across a large building, it can waste thousands of kilowatt-hours of energy every year. Even small leaks increase fan energy demand, compromise comfort levels, and can make it harder for a system to meet performance targets.
That’s why the Building Engineering Services Association (BESA) developed DW/143, the UK’s authoritative standard for ductwork airtightness. It defines clear leakage classes – A, B, C, and D – that allow designers, contractors, and clients to specify and verify system performance with confidence.
By assigning a leakage class at the design stage, teams can reduce risk, control energy costs, and demonstrate compliance with regulations such as Part L of the Building Regulations. This ensures airtightness is measured, not assumed, and that every component of the ventilation system performs as intended.
Why Leakage Classes Matter
Every joint, seam, or penetration in a ductwork system presents a potential source of air leakage. Even well-fabricated systems can experience energy losses if they are not sealed and tested properly.
The leakage class framework brings clarity. It ensures that system integrity is measured, not assumed, and that contractors are accountable for meeting the airtightness level specified by the designer or client. This is especially critical in buildings where ventilation performance is linked to safety or regulatory obligations.
Class A to D: What They Represent
Each class in DW/143 corresponds to a specific level of air leakage, measured at a defined pressure. Class A is the most lenient, typically used in noncritical, low-pressure systems. Class D is the tightest, required in high-risk environments like hospitals or laboratories.
Class C is the most commonly used in commercial building services, balancing performance and practicality.
| Leakage Class | Typical Use | Test Pressure Range (Pa) | Max Leakage Rate (L/s·m²) |
|---|---|---|---|
| Class A | Low-pressure, noncritical systems | Up to 500 Pa | 2.75 × 10⁻³ × p⁰·⁶⁵ |
| Class B | Standard commercial systems | Up to 1000 Pa | 9.18 × 10⁻⁴ × p⁰·⁶⁵ |
| Class C | High-performance systems | Up to 2000 Pa | 3.05 × 10⁻⁴ × p⁰·⁶⁵ |
| Class D | Critical or high-pressure systems | Up to 2500 Pa | 1.01 × 10⁻⁴ × p⁰·⁶⁵ |
The Role Of System Pressure
As system pressure increases, so does the likelihood of air escaping from poorly sealed joints. That’s why high-pressure systems – such as those in tall buildings or cleanrooms – often require Class C or D.
DW/143 links system pressure directly to airtightness performance, giving designers a practical rationale for selecting tighter classes when needed.
When And How To Specify A Leakage Class
Leakage classes should be specified at the design stage and clearly documented in project specifications. This sets expectations for fabrication, sealing, and testing early in the project.
For example:
- Use Class C or D for systems above 1000 Pa or where high energy performance is required.
- Use Class A or B in low-pressure, noncritical systems to control cost without compromising safety.
Testing For Compliance
The only way to confirm compliance is to test the ductwork using the methodology provided in DW143. This involves isolating a duct section, applying test pressure, and measuring leakage using calibrated equipment.
The result is compared with the maximum permissible leakage rate for the specified class, offering assurance that both design and regulatory expectations are met.
Explore more in:
- How to Conduct a Ductwork Air Leakage Test
- Ductwork Air Leakage Testing: Why It Matters and How to Get It Right
Aligning Projects With DW/143
DW/143 provides a practical framework for defining and verifying ductwork airtightness. It gives everyone involved, from designers to clients, a shared language to manage performance and compliance.
By using the right class for the right system and testing effectively, you can avoid costly remediation and ensure systems perform as intended. Whether your goal is net zero, safety, or energy savings – airtightness matters.
Download DW/143 today from BESA Publications and build your next project on the right foundation.