What outdoor design temperatures are used to size ADU heating and cooling?

Your ADU's HVAC is sized at fixed indoor temperatures of 68°F for heating and 75°F for cooling, and at outdoor design conditions taken from Reference Joint Appendix JA2, the ASHRAE Handbook (Fundamentals volume) or ACCA Manual J. Section 150.0(h)2 of the 2025 Energy Code also sets limits so the outdoor values cannot be pushed to extremes that inflate equipment size.

The rules in Section 150.0(h)2

• Indoor design temperatures: 68°F for heating, 75°F for cooling.
• Heating outdoor design temperature: no lower than the 99.0 percent heating dry bulb or the heating winter median of extremes for the location.
• Cooling outdoor design conditions: no greater than the 1.0 percent cooling dry bulb and mean coincident wet bulb values.

In plain terms, you design for a cold day and a hot day that represent the site's normal extremes, not the single worst day on record.

What JA2 contains

JA2 is part of the CEC's 2025 Reference Appendices. Its Table 2-3, "Design Day Data for California Cities," lists cities with their latitude, elevation, cooling dry-bulb and mean coincident wet-bulb values at several percentiles, outdoor daily range, heating winter median of extremes and heating design dry-bulb values. JA2 also names the reference city for each of the 16 climate zones, from Arcata in zone 1 to Blue Canyon in zone 16.

Climate zone and design city are different things

Your climate zone comes from the parcel's physical location, which the CEC's online climate zone tool helps determine. The design temperatures come from the most representative city in the design data. A canyon, a coastal bluff and a valley floor in the same zone can call for different values. The CEC's 2025 Single-Family Compliance Manual says that if your city is not listed in JA2, or another nearby city better matches the site, you should consult the local building department.

Why it matters for heat pumps

• A heat pump's heating output drops as the outdoor temperature drops, so the designer checks capacity at your heating design temperature, not at the standard rating point.
• Inland and desert sites tend to be cooling-driven; mountain sites tend to be heating-driven. The design data shows which load controls the equipment choice.
• Using the wrong city can push the selection a size up or down, which affects comfort, operating efficiency and the electrical circuit size.

What it means for your mechanical plan

• The load calculation should state the design city, the indoor and outdoor design temperatures used and the data source.
• Those values should be consistent with the climate zone and address on the Title 24 report.
• If the site differs from the listed city, note the basis for the chosen data so the plan checker can follow it.

Related reading: which climate zone your ADU is in, what a Manual J calculation is and whether a mini-split works in every climate zone.

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