What is Manual S, and how does it set the heat pump size for my ADU?

Manual S is ACCA's procedure for choosing equipment that actually matches the heating and cooling loads from your Manual J calculation. The 2025 Energy Code makes it mandatory: Section 150.0(h)5 requires systems to be sized based on ACCA Manual S-2023, and a heat pump's heating capacity has to cover the required heating load without counting any backup heat.

How Manual S works

Manual J tells you how much heat your ADU loses in winter and gains in summer at design conditions. Manual S takes those numbers and checks them against the manufacturer's performance data for a specific indoor and outdoor unit pairing. A heat pump's output changes with outdoor temperature, so the designer looks at what the unit delivers at your design temperature, not just its nameplate rating.

What the 2025 Energy Code adds

• Heating comes first for heat pumps. Under Section 150.0(h)5, heat pump heating capacity must meet the minimum requirements of the California Building Code without supplementary heating, and there is no maximum limit on heating capacity.
• No minimum cooling size. The code sets no lower limit on cooling capacity, because the building code requires heat, not cooling.
• Cold sites may end up larger for cooling. The CEC's 2025 Single-Family Compliance Manual explains that where the heating load is much larger than the cooling load, you select a system that meets the heating load, even if it is oversized for cooling, instead of relying on strip heat to cover the gap.

Extra limits for ADU additions

If your ADU is an addition, such as an attached unit or a garage conversion, Section 150.2(a)1E adds maximum-capacity limits. Where airflow is not field verified at 350 cfm per ton or more, ducted systems must stay within the capacity caps in Table 150.2-A (heating) and Table 150.2-B (cooling). Ductless systems are exempt from those caps, and variable-speed or multi-speed systems may step up to the next available size when the cap falls between two models. The envelope leakage assumed in an addition's load calculation is also capped by Table 150.2-C unless it is tested.

Why it matters in a small ADU

ADU loads are often small, while equipment comes in fixed sizes. An oversized unit short-cycles and controls humidity poorly; an undersized heat pump may not hold temperature on the coldest design day. Manual S is the documented step that shows the plan checker your selection is neither.

What it means for your mechanical plan

• The plan should carry the Manual J design loads next to the selected model's capacity at design conditions.
• The equipment schedule must list the exact indoor and outdoor model numbers that were checked, and they must match the Title 24 report.
• If the installer later swaps models, the sizing check has to be redone; see what happens if HVAC is installed differently than approved.

For the load side of the process, read what a Manual J calculation is, and for the full design overview, see the ADU mechanical guide.

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