What a duct air leakage test measures
A leakage test uses a defined section of ductwork and a controlled pressure difference. The airflow needed to maintain that condition provides a measure of leakage through the test boundary. The result depends on what is included, how the ends are sealed and how the pressure and flow are measured.
It is a different question from whether the complete extract system delivers its design airflow. A system can have acceptable leakage yet still have an incorrect fan selection, poor balancing or unsuitable capture at the cupboard.
Likewise, smoke observed near a joint may help locate a leak, but it does not by itself produce a quantified leakage result at a specified pressure.
Positive and negative duct pressure testing
An extract system can contain sections operating below or above the surrounding pressure. The fan location and system layout determine those conditions. Identify the intended operating pressure and the specified test direction instead of assuming that one pressure test covers every condition.
Under negative pressure, leakage can draw surrounding air into the duct. That added air may consume fan capacity without improving extraction at the intended opening. Under positive pressure, leakage can release duct contents into the surroundings. The consequence depends on the substance, location and operating conditions.
The designer or responsible testing specialist should establish an appropriate method and pressure for the material, joints and components. Avoid applying a pressure-class number from an unrelated HVAC specification to a chemical-extraction installation.
Set the boundary and acceptance criteria
| Item | What to specify |
|---|---|
| Boundary | Drawing references, section ends, branches and included fittings. |
| Pressure | Direction, level, reference and any staged-test requirements. |
| Acceptance | The project criterion, calculation basis and responsible reviewer. |
| Measurement | Equipment, calibration information and recorded test conditions. |
Decide whether access covers, flexible connections, dampers and equipment interfaces are included. Temporary closures can otherwise dominate the result or conceal a boundary that differs from the finished installation.
Specify how an unsatisfactory result will be investigated, repaired and retested. Agreeing this in advance keeps a failed test from becoming an argument about what was actually commissioned.
Prepare chemical ductwork for a safe assessment
Existing extraction duct may contain hazardous residues. Plan the process shutdown, isolation, decontamination and access with the people responsible for the laboratory. Testing must not expose workers or unintentionally pressurise connected equipment.
Give the testing team the material and joint information. Fabricated PVC duct, a flexible connection and a connected cupboard may have different limits. The temporary test arrangement and closures need to be suitable for the proposed pressure.
WorkSafe’s local exhaust ventilation guidance places system design, maintenance and competent assessment within the wider exposure-control task. This page is a planning guide, not a pressure-test procedure for untrained staff.
Keep the leakage result with the commissioning record
Retain the boundary drawing, pressure and leakage readings, calculation method, equipment details, observations and final acceptance decision. Identify the joints or components repaired so later maintenance has a useful history.
After the complete system is assembled, verify the required operating duty and capture performance through the appropriate commissioning process. Check the connected cupboard separately as part of the complete fume hood design.
Our PVC welding guide explains the fabrication questions to resolve before installation. For shape and routing decisions, compare round and rectangular duct.
Questions about duct leakage testing
Is a duct leakage test the same as a fume cupboard containment test?
No. One assesses leakage through a defined duct boundary; the other assesses the cupboard’s containment under its specified test conditions.
What leakage limit should I use?
Use the criterion specified for the project by the responsible designer or testing specialist. It should identify the method, pressure and calculation basis.