Duct leakage testing procedures

Leaky ductwork wastes energy twice. The fan must move extra air to make up for what escapes, and the conditioned air that leaks into ceiling spaces, shafts or unconditioned areas is lost. Severe leakage can also prevent outlets from reaching design airflow, undermine pressurization in critical spaces and pull contaminated air into return systems.

Duct leakage testing measures how much air a section of ductwork leaks at a known pressure and compares it with an allowable value. It is commonly specified for medium and high pressure supply ductwork, exhaust systems in laboratories and health care facilities, and any project with demanding energy targets. This guide describes how commercial duct leakage tests are performed and how results are evaluated.

Standards and requirements

The main reference in North America is the SMACNA HVAC Air Duct Leakage Test Manual, which defines leakage classes, test methods and apparatus. Related SMACNA construction standards define duct pressure classes and seal classes. See duct sealing classes and SMACNA leakage standards for background on how sealing relates to leakage.

ASHRAE Standard 90.1 requires leakage testing of a representative portion of ductwork designed for static pressures above 750 Pa (3 in. wg), with a maximum leakage class of 4. In Canada, the NECB includes duct sealing requirements based on duct pressure, and leakage testing requirements are commonly set by the project specification, owner standards or green building program. Health care projects following CSA Z317.2 and laboratory projects often have stricter requirements. Check the governing documents for each project.

Leakage class and allowable leakage

SMACNA expresses leakage using a leakage class (CL), which relates the leakage rate per unit of duct surface area to the test pressure:

• In imperial units, CL = F / P^0.65, where F is leakage in cfm per 100 ft² of duct surface and P is test pressure in inches of water.
• SMACNA also publishes metric equivalents, with leakage in L/s per m² of duct surface and pressure in pascals. The metric CL values are numerically different, so always confirm which unit system a specification uses.

Rearranged, the allowable leakage for a test section is the leakage class multiplied by the test pressure raised to the 0.65 power, multiplied by the duct surface area.

Worked example

A supply duct section has a surface area of about 93 m² (1,000 ft²). It is tested at 500 Pa (2 in. wg) and the specified leakage class is 4 (imperial).

1. Pressure factor: 2^0.65 is about 1.57.
2. Allowable leakage per 100 ft²: 4 times 1.57 is about 6.3 cfm.
3. Allowable leakage for 1,000 ft²: about 63 cfm, or roughly 30 L/s.

If the measured leakage is below this value, the section passes.

Some specifications instead state allowable leakage as a percentage of system design airflow, such as 1% or 2%. This is simpler but less consistent because it does not account for duct size or pressure. Where both approaches are used, the specification should state which governs.

Test apparatus

A commercial duct leakage test rig typically includes:

Instruments must be calibrated, and the orifice or flow element must be used within its rated range. Digital gauges that calculate flow directly simplify the test but still depend on correct setup.

Step-by-step procedure

1. Plan test sections. Divide the system into sections that can be isolated, typically by floor, riser or branch. Sections should be small enough for the rig to pressurize and large enough to be efficient. Testing before ductwork is insulated and concealed is essential.
2. Calculate allowable leakage for each section from its surface area, test pressure and specified leakage class.
3. Seal all openings. Cap open ends and cover outlets, terminal unit inlets and branch takeoffs not included in the section. Temporary seals must be tight, or they will be counted as duct leakage.
4. Connect the test rig to the section through a test port or temporary end cap.
5. Pressurize gradually to the test pressure, which is commonly the duct's pressure class or design operating pressure as stated in the specification. Do not exceed the duct construction pressure rating.
6. Hold and stabilize. Maintain test pressure long enough for readings to stabilize.
7. Measure leakage flow through the orifice or flow meter.
8. Compare with allowable leakage. Record the section, area, test pressure, measured leakage, allowable leakage and result.
9. If the section fails, locate leaks while the duct is pressurized, by listening, feeling, using smoke or soap solution, then repair and retest.
10. Document all tests, including failures and retests, with dates, instruments and witnesses.

Negative pressure ductwork, such as exhaust and return, may be tested under negative pressure to reflect how joints actually behave in service, if the specification requires it.

Planning testing on a construction project

• Sequence with installation. Testing must happen after the ductwork is complete and sealants have cured, but before insulation, ceilings and fireproofing conceal it.
• Coordinate witnesses. The engineer or commissioning provider often witnesses a sample of tests. Schedule these in advance.
• Allow for retests. Some sections will fail on the first attempt. Build time into the schedule.
• Test early sections first. Testing the first installed section shows whether workmanship meets the requirement before the whole system is built.

Why leakage matters for energy and performance

Fan power rises steeply with airflow, so even modest leakage in a high pressure system has a measurable effect on fan energy. See reducing fan energy. Leakage downstream of VAV boxes also affects airflow to rooms and can make air balancing impossible to complete within tolerance. In laboratories and health care spaces, leakage in exhaust ductwork can create contaminant pathways, and leakage in supply ductwork can undermine pressure relationships.

Frequently asked questions

Is all ductwork tested?

Not usually. Many specifications require testing of medium and high pressure ductwork and a percentage of low pressure ductwork, with additional testing if sections fail. Critical exhaust systems are often tested in full.

Can existing ductwork be tested?

Yes, although sealing all openings in an occupied building is more difficult. Testing existing ductwork is useful when investigating airflow shortfalls or planning energy upgrades.

Key takeaways

• Duct leakage testing measures leakage at a known pressure and compares it with an allowable value based on a SMACNA leakage class.
• Allowable leakage depends on duct surface area, test pressure and leakage class.
• Seal all openings carefully, test at the specified pressure and never exceed the duct's construction rating.
• Test sections before insulation and ceilings conceal the ductwork, and allow time for retests.
• Requirements vary by project, code and program, so confirm them in the specification. Our duct leakage testing service covers supply, return and exhaust systems.