DW/144 Pressure Classes Explained for HVAC Ductwork
Choosing the correct ductwork pressure class is an important part of designing a reliable commercial ventilation system. DW/144 provides recognised guidance for the construction of sheet metal ductwork according to the pressure it must withstand during operation.
The pressure class affects the materials, reinforcement, joints and sealing methods required during manufacture.
What is a ductwork pressure class?
A ductwork pressure class identifies the level of internal pressure that a duct system is designed to handle.
Ventilation systems operating at higher pressures place greater force on duct walls, joints and supports. The ductwork must therefore be manufactured using suitable material thicknesses, reinforcement and jointing methods.
Why does pressure classification matter?
Using ductwork that is unsuitable for the system pressure can lead to excessive air leakage, vibration, movement and reduced ventilation performance.
Correct pressure classification helps provide:
- Reliable structural strength
- Controlled air leakage
- Stable joints and connections
- Efficient airflow
- Long service life
- Compliance with project specifications
Low pressure ductwork
Low pressure ductwork is commonly used in general comfort ventilation systems where air is moved at relatively modest pressure.
Typical applications include offices, schools, shops and other commercial buildings. Although the operating pressure is lower, the ductwork must still be correctly manufactured, sealed and supported.
Medium pressure ductwork
Medium pressure systems require stronger construction than typical low pressure systems.
Additional reinforcement, stronger joints or different material thicknesses may be required depending on the duct dimensions and operating conditions.
Medium pressure ductwork is often used in larger commercial ventilation systems where air must travel longer distances or serve several areas of a building.
High pressure ductwork
High pressure ductwork is designed for ventilation systems operating under greater internal pressure.
These systems require careful attention to joint strength, reinforcement, sealing and material selection. Accurate manufacture is particularly important because weaknesses can affect system efficiency and durability.
Positive and negative pressure
Ductwork can operate under either positive or negative pressure.
Positive pressure occurs when air inside the duct pushes outwards against the duct walls. Negative pressure occurs when an extract system creates suction within the ductwork.
Both conditions must be considered when specifying and manufacturing the system.
How duct size affects construction
Pressure class is not the only factor that determines how ductwork should be manufactured. Larger duct sections may require additional reinforcement because wider surfaces are more likely to flex.
Construction requirements can vary according to:
- Duct dimensions
- Operating pressure
- Material type
- Joint design
- Support spacing
- System application
Air leakage and pressure classes
Air leakage becomes increasingly important as system pressure rises.
Poorly sealed joints can reduce airflow at the intended outlets and make fans work harder. This can increase energy use and reduce overall ventilation performance.
Correct manufacture and sealing help ductwork meet the leakage requirements stated in the project specification.
How to specify the correct pressure class
The required pressure class should be confirmed by the ventilation system designer or project engineer before manufacture begins.
The specification should identify the operating pressure, duct dimensions, material requirements and any air leakage testing requirements.
Conclusion
DW/144 pressure classes help ensure sheet metal ductwork is manufactured for the operating conditions of the ventilation system.
Correct classification supports structural strength, airflow efficiency, controlled leakage and long term reliability. Contractors and designers should confirm the required pressure class before ordering ductwork for a commercial HVAC project.