Does my ADU heat pump need a special thermostat under the 2025 Energy Code?

Only if the heat pump has supplementary heat, such as electric resistance backup strips or a gas furnace. In that case, the 2025 Energy Code requires a thermostat that reads and displays the outdoor temperature, locks out the backup heat above 35°F except during defrost or emergency mode, and shows when backup heat is running. A typical ductless mini-split with no backup heater doesn't trigger this rule, though other new heat pump control rules may still apply.

The supplementary heat rules

• Lockout (Section 150.0(h)7): heat pumps with supplementary heat must lock it out when the outdoor air is above 35°F. Above that, backup heat may run only during defrost or when the user selects emergency heat.
• Thermostat (Section 150.0(i)2): the thermostat must get the outdoor temperature from a sensor or an internet weather service, display it, enforce the lockout, and indicate when supplementary or emergency heat is on. This doesn't apply if another control device handles the lockout.
• Backup sizing (Section 150.0(h)8): electric resistance backup can't exceed the heat pump's nominal cooling capacity times 2.7 kW per ton, rounded up to the nearest kW.

The small-ADU exception

Exception 1 to Section 150.0(h)7 covers buildings with less than 500 square feet of conditioned floor area, and buildings of any size in climate zones 7 and 15. These can use simpler controls instead. One option keeps the backup heat off whenever the heat pump alone can meet the load. The other allows backup heat only during defrost and short transient periods. Systems using this exception are also exempt from the outdoor-temperature thermostat rules.

Other 2025 heat pump control rules

• Defrost (Section 150.0(h)6): if the heat pump has an installer-adjustable defrost delay timer, it must be set to at least 90 minutes. Exempt: dwellings in climate zones 6 and 7, and dwellings of 500 square feet or less in climate zones 3, 5 to 10, and 15.
• Variable-speed systems (Section 150.0(h)9): when a third-party thermostat is used, the system and thermostat together must be able to modulate compressor speed to match the load.
• Installer certification: for each of these, the installer certifies on the CF2R that the control setup was tested.

Do these apply to garage conversions and attached ADUs?

Yes. Additions must meet the mandatory measures in Sections 150.0(a) through (n), and the HVAC and thermostat rules are in that range.

Practical tips

• If you want backup heat for cold snaps, choose a thermostat or control package listed by the manufacturer as able to meet these requirements.
• Tell your HVAC installer about the CF2R control tests up front so the paperwork is ready before the ECC-Rater's visit.

What this means for your MEP plans and Title 24 report

The mechanical plan should state whether the heat pump has supplementary heat, its type and size, and the control method. The CF1R models the same equipment. We note the applicable control requirements on the plans so your installer knows what to configure. See mechanical plans, should I use a heat pump, and mini-splits in every climate zone.

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