ADU Airflow Verification: When Is It Required?
A new ducted ADU system must show at least 350 cfm of airflow per ton, and an altered system with replaced refrigerant parts 300. Small systems can instead pass a return-duct sizing inspection. Ductless units and existing equipment that is only extended follow different rules.
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Which ADU projects measure airflow
- New ducted system, in a detached ADU or an addition: Section 150.0(m)13 requires 350 cfm per ton through the return grilles in every control mode, with fan efficacy of 0.58 W/cfm or less (0.45 for a gas furnace). Small duct high velocity systems use 250 cfm per ton and 0.62.
- Altered system with new refrigerant parts: Section 150.2(b)1F sets 300 cfm per ton wherever charge is verified (heat pumps all zones, air conditioners zones 2 and 8 to 15).
- Addition served by existing equipment: no measurement is required. Separately, an addition's system capacity is capped by Tables 150.2-A and 150.2-B unless 350 cfm per ton is field verified; ductless systems are excepted.
Small systems can skip the measurement
A standard ducted system without zoning dampers may instead build its return to Table 150.0-B and let the ECC-Rater confirm it by inspection. For a single return, 1.5 tons needs a 16-inch duct and 500 square inches of filter grille, 2 tons needs 18 inches and 600, and 2.5 tons needs 20 inches and 800. The return run is limited to 30 feet and 180 degrees of bend; systems under 1.5 or over 2.5 tons are outside the table.
What the rater measures
With the filter in and registers open, airflow is read at the return grilles with a flow hood, or by plenum pressure matching with a flow meter. The cfm is divided by the outdoor unit's nominal tons, shown in the model number (024 means 2 tons), so a 2-ton ADU system needs 700 cfm if new and 600 if altered.
When the number falls short
A new system is corrected and retested. An altered system has a fallback in Section RA3.3.3.1.5: the installer replaces a dirty filter, opens dampers, replaces crushed duct, cleans the coil, sets the fan to high and enlarges an undersized return where possible, certifying each step. The rater reviews that certificate, and charge verification proceeds at the highest airflow reached. The form is CF2R-MCH-23-H.
Related: charge verification, duct leakage, MERV 13 filters and load calculations. Order from the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
The ADU FAQ lists minimum airflow among the checks mini-splits may face, depending on project scope, and marks only duct leakage and fan efficacy as ducted-only.
Section 150.2(b)1F sets 300 cfm per ton for altered systems, while new systems under Section 150.0(m)13 carry 350. An altered system that cannot reach 300 may use the remedial route.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide