Duct Leakage Testing 101

Ductwork is supposed to be a sealed path from your HVAC equipment to the room being conditioned. In practice, seams, joints, and connections leak — sometimes by design tolerance, sometimes because installation wasn't done correctly. Duct leakage testing is how you find out which one happened.

What Duct Leakage Testing Is

Duct leakage testing pressurizes a section of ductwork with a calibrated fan and measures how much air escapes at a given pressure, compared to how much air the system is supposed to be delivering. The result is a leakage rate — typically expressed as a percentage of total system airflow, or as cubic feet per minute per 100 square feet of duct surface — that can be compared directly against code-required maximums.

How the Test Actually Works

In practice, this means sealing off the section of duct being tested, connecting a calibrated blower door or duct-pressurization fan, and pressurizing the ductwork to a standard test pressure while measuring the airflow required to hold that pressure. If the ductwork were perfectly sealed, holding pressure would require almost no continuous airflow. The more air the fan has to keep pushing in to maintain pressure, the more the ductwork is leaking.

Why Energy Code Requires It

Modern energy codes, including the versions of the IECC Arizona jurisdictions have adopted, set maximum allowable duct leakage rates because leaky ductwork is one of the most common and most expensive ways a building wastes conditioned energy. Air that leaks into an unconditioned attic, crawlspace, or wall cavity is energy the building paid to heat or cool that never does its job — and it does this continuously, for as long as the building operates, until someone finds and fixes it.

What a Failing Result Looks Like

A duct system that fails leakage testing typically shows up in a building two ways before anyone tests for it: rooms that are harder to keep at the right temperature than the equipment sizing would suggest, and utility bills that run higher than a comparable, properly sealed building. Testing turns those vague symptoms into a specific, documented leakage rate — and pinpoints whether the problem is in a specific run of ductwork or systemic throughout the installation, which determines whether the fix is targeted or extensive.

Commercial Systems Analysis performs duct leak testing as part of its commercial test and balance scope, documenting results in the same certified reports used for the rest of a building's air and water balance — so a duct leakage finding isn't an isolated data point, but part of a complete picture of how the building's mechanical systems are actually performing.