Does a ducted ADU heat pump need a Manual D duct design?

In most cases, yes. The 2025 CALGreen environmental comfort measures (Section 4.507) require heating and cooling systems to be sized, designed and selected using ACCA Manual J for loads, Manual D for duct sizing and Manual S for equipment, or equivalent methods. The 2025 Energy Code then tests whether your ducts actually deliver enough air, so a sound duct design is what carries you through ECC verification.

What Manual D does

Manual D sizes each supply and return duct so the air handler can deliver the airflow each room needs at the fan's available static pressure. It accounts for duct length, fittings, filters, grilles and coil resistance. Undersized, crushed or kinked ducts are a common reason a system cannot reach its rated airflow.

The Energy Code tests the result

• Airflow and fan watt draw. Under Section 150.0(m)13, ducted systems that provide cooling must show at least 350 cfm per ton of nominal cooling capacity and an air-handler fan efficacy of no more than 0.58 watts per cfm for air handlers that are not gas furnaces. Small duct high velocity systems use 250 cfm per ton and 0.62 W/cfm. An ECC rater (formerly HERS rater) verifies these.
• Return duct sizing alternative. Standard ducted systems without zoning dampers can instead follow the return duct sizing in Tables 150.0-B and 150.0-C. Table 150.0-B, for single-return systems, covers 1.5 to 2.5 tons, limits the return duct to 30 feet and sets minimum duct diameters and filter grille areas.
• Duct leakage. Section 150.0(m)11 limits total leakage of a new single-family duct system to 5 percent of air handler airflow when the air handler is installed and connected.
• Filter pressure drop. The design has to absorb the MERV 13 filter required by Section 150.0(m)12: either a nominal 2-inch filter sized by the designer, or a 1-inch filter sized for a face velocity of 150 feet per minute or less.
• Static pressure probe. Section 150.0(m)13A requires a test hole or permanent probe in the supply plenum downstream of the coil.

Variable-speed systems

For multispeed or variable-speed compressors, airflow and fan efficacy are verified at maximum compressor speed and maximum fan speed. Your ducts have to be sized for the top of the system's range, not an average.

What it means for your mechanical plan

• The plan should show supply and return duct sizes, register locations, the air handler, filter location and size, and a return path from every closed bedroom.
• Duct lengths and fittings on the drawing should be realistic for the actual framing, soffits and chases, or the installed system will not match the design.
• The duct design, the Manual J loads and the equipment selection need to agree with each other and with the Title 24 report.

If you are still choosing between ducted and ductless equipment, compare options in HVAC for multi-bedroom ADUs over 1,000 SF, and review what a rater checks in HERS (now ECC) testing.

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