Does a concealed-duct mini-split count as ducted for Title 24 testing?

Yes. A ceiling-concealed or slim-duct "mini-split" moves air through ductwork, so the Energy Code treats it as a ducted system rather than a ductless wall unit. That changes what the ECC rater (formerly HERS rater) verifies and what your plans must show, so decide early whether your ADU will use wall-mounted heads, concealed-duct units or a mix.

What changes when ducts are involved

The CEC's 2025 ADU FAQs list the common ECC verifications for mini-split and multi-split systems. Several apply only to ducted equipment:

• Duct leakage testing under Section 150.0(m)11, with a limit of 5 percent of air handler airflow for a new single-family system.
• Airflow and fan efficacy under Section 150.0(m)13: at least 350 cfm per ton and no more than 0.58 W/cfm for air handlers that are not gas furnaces, checked at maximum compressor and fan speed for variable-speed equipment.
• MERV 13 filtration under Section 150.0(m)12 when the system supplies air through more than 10 feet of ductwork, including an accessible, labeled and gasketed filter location.
• Duct insulation and location, including the prescriptive choice between a high-performance attic and ducts entirely in conditioned space.

Items that apply to both types include refrigerant charge verification for heat pumps in all climate zones and, depending on the compliance approach, verified SEER2, HSPF2 and rated heating capacity.

The variable-capacity heat pump option

The CEC's 2025 Single-Family Compliance Manual describes a variable-capacity heat pump (VCHP) compliance option. Ducted VCHP systems with indoor units certified to the CEC as low-static do not need the duct leakage and fan efficacy tests of Sections 150.0(m)11 and 150.0(m)13. Instead, the rater verifies items such as ducts and indoor units located entirely in conditioned space, supply to all habitable spaces, wall-mounted thermostats, airflow and filter sizing. Ductless VCHP systems also require refrigerant charge verification and wall-mounted thermostats in zones over 150 square feet. Whether this option fits depends on the exact model and your Title 24 approach.

Mixed systems

Many multi-zone outdoor units can serve a wall head in one room and a concealed-duct unit in another. The ducted portion still brings the ducted requirements with it, so the plans and the Title 24 report need to describe each indoor unit accurately.

What it means for your mechanical plan

• Identify every indoor unit by type and model in the equipment schedule.
• For concealed-duct units, show supply and return duct runs, filter location and size, and where the ducts sit relative to the insulated ceiling.
• Make sure the energy model describes each system as ducted or ductless exactly as drawn; a mismatch invites corrections.

Background: what HERS (now ECC) testing involves and whether a mini-split suits your ADU.

Sources: energy.ca.gov, energy.ca.gov, energy.ca.gov.