Introduction
Duct leakage testing is one of the most frequently required HERS verification measures in California. If your project involves new construction, an HVAC replacement, or a new duct system, there is a good chance duct leakage testing will appear on your CF1R as a required HERS measure.
This post explains what duct leakage testing is, how it works, when it's required, what the pass/fail thresholds are, and how to set your project up for a clean test.
What Is Duct Leakage Testing?
Duct leakage testing is a diagnostic procedure that measures how much air leaks out of an HVAC duct system through gaps, cracks, and poorly sealed connections. Air that leaks out of the ducts before reaching the living space represents wasted energy — your HVAC system is conditioning air that never arrives where it's needed.
In California, duct leakage is measured and expressed as a percentage of the system's airflow capacity. The goal is to confirm that the duct system meets the maximum allowable leakage threshold specified in the compliance documentation.
How Duct Leakage Testing Works
A certified HERS Rater performs the test using a device called a duct blaster. The duct blaster is a calibrated fan that is temporarily sealed to a duct register or the air handler. The remaining registers and grilles in the system are temporarily sealed with foam or tape.
The duct blaster pressurizes the duct system to a standard test pressure (typically 25 Pascals). The airflow required to maintain that pressure is measured. That airflow measurement represents the leakage — the more air required to maintain pressure, the more the system is leaking.
There are two main test protocols: total leakage testing (measures all leakage in the system including leakage to the interior of the building) and leakage to outside testing (measures only the leakage that escapes to outside the conditioned space — typically a more stringent and meaningful measure for energy compliance).
The specific test protocol required on your project is determined by the CF1R.
When Is Duct Leakage Testing Required in California?
Duct leakage testing is required under Title 24 for new construction with forced-air HVAC systems (virtually all new single-family homes with ducted HVAC require duct leakage testing), HVAC replacements in existing homes when the existing duct system is being retained (in many climate zones, replacing just the air handler or compressor triggers a duct leakage test on the existing ducts), new duct systems installed as part of a building alteration, and when duct leakage compliance credit is claimed on the CF1R (for projects where tight duct systems are part of the energy compliance strategy).
The specific trigger conditions are defined in the Title 24 standards and vary by climate zone and project type. Review the HERS verification measures on your CF1R to confirm whether duct leakage testing is required.
What Are the Pass/Fail Thresholds?
Under the 2022 Title 24 standards, the maximum allowable duct leakage thresholds depend on the scope of work. For new systems with all new ducts, the maximum allowable total leakage is 6% of the system's nominal airflow. For existing duct systems being retained with a new HVAC equipment installation, the threshold depends on whether any duct replacement occurred — existing systems with duct replacement must meet 5% or less, while existing systems with no duct replacement must meet 15% or less. Supply and return ductwork leaking specifically to the outdoors has a maximum acceptable leakage rate of 2%.
These thresholds are more stringent than previous code cycles, reflecting California's ongoing push toward tighter duct systems and reduced energy waste.
How to Prepare for Duct Leakage Testing
Coordinate with your HVAC contractor before the test. The duct system should be fully installed, all connections sealed with mastic or UL-listed tape, and all registers and grilles installed before the HERS Rater arrives. Incomplete duct work is a frequent cause of test delays.
Schedule the test before final close-up where possible. Duct work in unconditioned attics or crawlspaces should be accessible at the time of testing. After drywall is installed it is much harder to access and repair leaking duct connections.
Ask your HVAC contractor about their typical leakage test results. Experienced contractors who work on California projects regularly understand the Title 24 thresholds and should be sealing their work to meet them. If your contractor is unfamiliar with the duct leakage requirements, that is a red flag.
What Happens If the Test Fails?
If the duct system fails the leakage test, the HVAC contractor must identify and seal the leaking connections and the test must be repeated. The HERS Rater cannot sign off on a failed test.
Common causes of test failures include unsealed duct boots at registers, poorly connected flex duct sections, unsecured duct connections at the air handler, missing mastic at duct branch points, and improperly sealed return air plenums.
Finding and sealing leaks after drywall has been installed is time-consuming and expensive. This is why scheduling the duct leakage test before close-up is strongly recommended.
Conclusion
Duct leakage testing is a standard HERS verification requirement for most new construction and many HVAC replacement projects in California. A certified HERS Rater performs the test using a duct blaster, and results must meet the thresholds specified in the CF1R before a CF3R can be issued. Coordinating with your HVAC contractor to seal duct work properly — and scheduling the test before final close-up — are the two most important things you can do to set your project up for a clean test.