Duct Leakage Testing vs. Air Balancing: What’s the Difference?
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Duct leakage testing and air balancing are both used to verify HVAC system performance, but they measure different things.
Duct leakage testing determines whether air is escaping from the ductwork.
Air balancing determines whether the correct amount of air is reaching each part of the building.
A duct system can pass a leakage test and still be improperly balanced. It can also deliver airflow close to the design values while leaking more air than the project specification allows. For that reason, the two processes should be viewed as complementary rather than interchangeable.
What Is Duct Leakage Testing?
Duct leakage testing evaluates the airtightness of a duct system.
During testing, duct openings are temporarily sealed, and the ductwork is pressurized or depressurized to a specified pressure. The airflow required to maintain that pressure represents the amount of air passing through joints, seams, access doors, connections, and other unintended openings (Kanomax, 2025a).
The measured leakage is then compared with the allowable limit established by the project specification or applicable testing standard.
Duct leakage testing helps HVAC professionals:
- Verify duct sealing quality
- Measure the leakage rate
- Meet project requirements
- Support commissioning and quality assurance
- Document whether the ductwork passes or fails
The applicable standard, test pressure, leakage class, duct surface area, and allowable leakage should be confirmed before testing begins (Kanomax USA, 2026a).
What Is Air Balancing?
Air balancing evaluates how air is distributed through an operating HVAC system.
It is part of the broader testing, adjusting, and balancing process, commonly known as TAB.
According to the National Environmental Balancing Bureau, testing involves measuring and documenting parameters such as airflow, pressure, and temperature. Adjusting involves fine-tuning components such as dampers, valves, and fans. Balancing involves distributing airflow throughout the system so the building receives the heating, cooling, and ventilation required by the design (NEBB, n.d.).
During air balancing, technicians may measure airflow at:
- Supply diffusers
- Return grilles
- Exhaust openings
- Main and branch ducts
- Air-handling units
- VAV terminal boxes
The results are compared with the values shown on project drawings or airflow schedules. When readings do not match the design requirements, adjustments may be made to dampers, fan speeds, controls, terminal devices, or other system components.
The AABC National Standards for Total System Balance establish minimum standards intended to help systems achieve their design intent and ensure that proper TAB procedures are followed (AABC, n.d.).
The Main Difference
The simplest way to separate the two processes is:
Duct leakage testing evaluates the duct enclosure. Air balancing evaluates airflow distribution.
Duct Leakage Testing Air Balancing
Measures unwanted air leakage Measures delivered system airflow
Tests sealed ductwork under pressure Tests the HVAC system while operating
Compares leakage with an allowable limit Compares airflow with design values
Evaluates duct sealing quality Evaluates system distribution
Uses a duct leakage tester Uses capture hoods, anemometers, micromanometers, and Pitot tubes
Although the two processes are related, they provide different information about HVAC system performance.
Why One Test Does Not Replace the Other
A duct system may be properly sealed but still distribute air incorrectly.
For example, one room may receive too much airflow while another receives too little because of:
- Incorrect damper positions
- Fan-speed settings
- Control-system issues
- VAV box operation
- Differences in system resistance
In this situation, the ductwork may pass its leakage test while the operating system still requires balancing.
The opposite can also occur.
A technician may adjust the system until airflow at the terminals is close to the design values, but the ductwork could still exceed its allowable leakage limit. The fan may be compensating for some of the air lost before it reaches the occupied areas.
Air balancing does not prove that the ductwork meets a specified leakage limit. Duct leakage testing does not prove that each room or terminal receives the correct amount of airflow.
When Is Each Test Performed?
Duct Leakage Testing
Duct leakage testing is commonly performed after a duct section has been assembled and sealed. Testing before ceilings, insulation, or other finishes restrict access makes it easier to:
- Isolate duct sections
- Locate leaking joints
- Complete repairs
- Retest failed sections
- Avoid more expensive rework later
The DALT 6905 manual recommends confirming the applicable standard, leakage or airtightness class, testing pressure, and duct surface area before beginning the test. It also recommends testing long duct systems in sections when necessary to keep the airflow and surface area within the tester’s operating range (Kanomax, 2025a).
Air Balancing
Air balancing requires the HVAC equipment, controls, dampers, terminal boxes, and distribution system to be operational.
The system must be running so technicians can measure actual airflow, compare it with the design requirements, and adjust components as needed. This generally places duct leakage testing earlier in the construction process and air balancing closer to final system verification or commissioning.
NEBB identifies TAB as applicable to both new construction and existing buildings where technicians need to verify and fine-tune system performance (NEBB, n.d.).
What Equipment Is Used?
Duct Leakage Testing Equipment
A typical duct leakage test may require:
- A duct air leakage tester
- Flexible connection hose
- Static-pressure tubing
- A test flange or connection point
- Temporary seals for duct openings
- High- or low-flow nozzles
- Leak-location tools when necessary
The Kanomax DALT 6905 Duct Air Leakage Tester performs positive and negative pressure testing. Its guided workflow allows users to select the testing standard, nozzle, duct surface area, test duration, and required pressure.
The instrument includes built-in methods for several U.S. and international testing frameworks, including SMACNA, AABC, EN 12237, EN 1507, DW/143, Eurovent 2/2, and GB 50243 (Kanomax, 2025a).
Air-Balancing Equipment
Air-balancing technicians use different instruments depending on the measurement location and application.
Common instruments include:
- Capture hoods
- Micromanometers
- Pitot tubes
- Velocity grids
- Hot-wire anemometers
- Rotating-vane anemometers
Capture hoods are commonly used to measure supply or return airflow directly at diffusers and grilles.
A micromanometer connected to a Pitot tube can measure pressure inside a duct and calculate air velocity and airflow when the duct dimensions are known. The Kanomax Model 6850, for example, measures differential pressure and calculates velocity and airflow when used with a Pitot tube (Kanomax, 2018).
Kanomax’s HVAC testing product line also includes capture hoods, airflow grids, micromanometers, Pitot tubes, and handheld anemometers for testing and balancing applications (Kanomax USA, 2014).
How the DALT 6905 Supports Leakage Testing
The Kanomax DALT 6905 is designed specifically for standards-based duct leakage testing.
Its primary capabilities include:
- Positive and negative pressure testing
- Built-in U.S. and international testing standards
- Large and small nozzle options
- Real-time leakage and static-pressure measurements
- Temperature and atmospheric-pressure measurements
- Storage for up to 1,000 test records
- USB data export
The product supports airflow measurements from 41 to 483 CFM with the large nozzle and 2.4 to 41 CFM with the small nozzle. Its static-pressure range is ±10 inches of water gauge (Kanomax, 2025a).
These capabilities help reduce manual calculations and create a more consistent field-testing process. However, the governing project specification, engineer, commissioning authority, or applicable code should determine the required testing method and acceptance criteria.
Final Thoughts
Duct leakage testing and air balancing are both important parts of HVAC performance verification.
Leakage testing confirms that air remains inside the duct system.
Air balancing confirms that the air reaches the correct locations in the correct quantities.
A complete HVAC system must do both.
By testing duct airtightness and then balancing the operating system, contractors, TAB professionals, and commissioning teams can verify that the installation is properly sealed, correctly adjusted, and capable of performing as intended.
