California Duct Leakage Testing: Why Ducts Are Just as Important as HVAC Systems

A new air conditioner can carry a top efficiency rating and still perform poorly. The cause is often hidden above the ceiling or under the floor. Ducts carry conditioned air to every room, and when they leak, a large share of that air never arrives. California’s Title 24 Energy Code takes this problem seriously, which is why duct leakage testing is a common part of HVAC compliance. This guide explains what the test measures, when it applies, and how to prepare so your project moves forward without delays.

What Duct Leakage Testing Actually Measures

Consider your duct system to be similar to a garden hose. If the hose has small holes along its length, water sprays out before it reaches the plants, and you have to run the tap longer to get the same result. Leaky ducts work the same way. The blower pushes air into the system, but some of it escapes into the attic, crawlspace, or garage instead of reaching the living space.

Duct leakage testing puts a number on that loss. A certified technician seals off the registers, connects a calibrated fan to the duct system, and pressurizes the ducts to a set level. The equipment then records how much air escapes through gaps, joints, and connections. The result is expressed as a percentage of the system’s total airflow, so a system that moves 1,000 cubic feet of air per minute and loses 50 of it has 5 percent leakage.

This is different from a standard airflow check. Airflow testing asks whether enough air moves across the equipment. Duct leakage testing asks whether the air that does move actually reaches the rooms it was meant to serve. Both tests support the same goal, but they reveal different problems.

Why California Cares About Leaky Ducts

Most homes in the Sacramento Valley and other inland areas have ducts running through hot attics. On a summer afternoon, that attic can be far hotter than the living space below. Air escaping from a leaky supply duct is cooled air wasted into a space nobody uses. Air pulled into a leaky return duct can bring in dust, heat, and unfiltered attic air.

Leakage can also leave far rooms uncomfortable and add wear to compressors and blowers. Because the Energy Code aims to cut energy use statewide, it requires proof that ducts are sealed properly.

When Duct Leakage Testing Is Required

The exact requirement depends on the type of project, the compliance approach, and the code cycle that applies. The 2025 Energy Code took effect on January 1, 2026, and it applies to permits submitted on or after that date. Several situations commonly trigger duct testing.

New Single-Family Construction

New homes with ducts in unconditioned space usually need a leakage test as part of the field verification process. Under the 2025 code, several industry resources describe the limit for new single-family homes as 5 percent of total system airflow. Since limits can differ by compliance path, the Certificate of Compliance for your project is the document that lists the exact requirement.

HVAC Replacements and Duct Alterations

Replacing major HVAC components can bring duct requirements into play. If a contractor installs new equipment and also alters, extends, or replaces part of the duct system, testing may be required. For existing duct systems that are altered, the allowed leakage level is generally more forgiving than for new construction, because older ducts are harder to seal perfectly. Your contractor should confirm the applicable limit before work begins.

Additions and Remodels

When a project adds conditioned space and a new duct run serves it, the new ducts have to meet the space conditioning requirements. Homeowners are often surprised by this, since the mechanical side of an addition is part of the permit.

Exceptions Worth Knowing

Duct systems located entirely within conditioned space can be exempt from leakage testing, because any air that leaks stays inside the home. Some builders choose this design on purpose, but it requires planning early.

How a Duct Leakage Test Works Step by Step

Knowing the sequence makes the visit less mysterious for homeowners and builders alike.

Step 1: Preparation

The technician reviews the compliance documents and inspects the equipment location. Clear access to the air handler and attic saves time.

Step 2: Sealing and Setup

Registers and grilles are covered so air cannot escape through the openings meant for normal use. The testing fan is connected to the duct system, usually at the air handler or a return opening.

Step 3: Pressurization and Measurement

The fan pressurizes the ducts to the reference level, and gauges show how much airflow is needed to hold that pressure. The airflow the fan must supply equals the leakage.

Step 4: Results and Documentation

The technician compares the result with the project requirement. Passing results are recorded through the compliance system. A failed result means the installer must locate and seal the leaks, then schedule a retest.

Common Causes of Duct Leakage

Most failures come from a short list of installation problems.

• Connections at the air handler and plenum that are not fully sealed
• Boots where the duct meets the register that pull away from the ceiling or floor
• Tape used in place of proper mastic or approved sealing products
• Sharp bends, kinks, or crushed sections that stress joints
• Panels, cabinet seams, and return platforms left unsealed
• Ducts damaged after installation by other trades working in the attic

How to Prepare for a Successful Test

Preparation should start during installation, not on test day. Use approved sealing materials such as mastic and listed tapes instead of ordinary cloth tape, which tends to dry out and peel in hot attics. Support ducts so that weight does not pull joints apart. Seal the seams around the air handler, return box, and supply plenum.

Before scheduling the technician, have the installer perform a quick self check. A visual inspection with a flashlight often reveals gaps that a simple dab of mastic can close. Some contractors also run their own preliminary leakage test so they can fix problems before the official visit. This small extra step can prevent the cost and delay of a failed test.

Also keep the site ready. Clear anything blocking the attic hatch or equipment closet, and make sure the power is on.

Duct Leakage and the ECC Program

Since January 1, 2026, the Energy Code Compliance (ECC) program has been the framework for third-party verification of Title 24 field testing. Under the 2025 code, providers and raters that were once described with HERS terminology now work under ECC language. You may still hear people use the older term in conversation, especially when discussing earlier projects, but current paperwork follows the ECC structure.

For builders and homeowners, the practical takeaway is simple. Use a qualified rater who is authorized within the current system, and ask how results will be registered so that the building department can approve the final inspection.

Final Thoughts

Duct leakage testing turns an invisible problem into a measurable one. It protects homeowners from paying to cool an attic, it helps HVAC systems deliver the comfort they were designed for, and it gives builders documented proof that the work meets Title 24. The best results come from planning early: confirm the requirement, seal ducts carefully, check your own work, and schedule testing with a qualified professional. When those steps happen in order, the test becomes a routine checkpoint instead of a last minute obstacle.