Does my ADU heat pump need a special thermostat or control setup in 2025?

Possibly. Every heat pump needs a setback thermostat, and the 2025 Energy Code added control rules for heat pumps with backup heat, for variable-speed systems paired with third-party thermostats and for defrost settings. The installer has to test and certify these settings on the Certificate of Installation (CF2R).

The baseline: a setback thermostat

Section 150.0(i)1 requires all heating or cooling systems, including heat pumps, that are not controlled by a central energy management control system to have a setback thermostat as specified in Section 110.2(c). Confirm that the controller you specify for each mini-split zone meets this.

New for 2025

• Heat pumps with supplementary heat (Section 150.0(i)2). The thermostat must receive and display the outdoor air temperature from a sensor or internet weather service, lock out supplementary heat above 35°F except during defrost or emergency heat, and indicate when backup or emergency heat is running. Exceptions apply where another device handles the lockout, for systems using the alternative controls in Exception 1 to Section 150.0(h)7, and for room air-conditioner heat pumps.
• Variable- or multi-speed systems with third-party thermostats (Section 150.0(h)9). The system and thermostat together must be able to respond to loads by modulating compressor speed.
• Defrost (Section 150.0(h)6). If the heat pump has an installer-adjustable defrost delay timer, it must be set to 90 minutes or more. Exceptions cover dwelling units in climate zones 6 and 7, and units of 500 square feet or less in climate zones 3, 5 through 10 and 15.

Why this matters for ADUs

Many owners want a smart thermostat that matches the main house. With a variable-speed heat pump, an incompatible third-party thermostat can run the equipment like a simple on-off unit and waste its modulation. Section 150.0(h)9 is aimed at that problem, so check compatibility with the heat pump manufacturer before choosing a thermostat.

The size-based exceptions also matter for small ADUs. A unit of 500 square feet or less may be exempt from the defrost setting, depending on climate zone, and a building under 500 square feet can use the alternative supplementary heat controls instead of the 35°F lockout.

Wall thermostats for ductless systems

If your Title 24 report uses the CEC's variable-capacity heat pump compliance option for ductless equipment, the Compliance Manual calls for wall-mounted thermostats in zones over 150 square feet. Check the CF1R before relying on handheld remotes.

What it means for your mechanical plan

• Specify the thermostat or controller for each zone and note its compatibility with the equipment.
• Note the defrost delay and supplementary heat lockout settings, or the exception that applies.
• Remind the installer that these items are certified on the CF2R.

See also Title 24 HVAC requirements for new ADUs, what goes in an equipment schedule and what changed for ADUs in 2025.

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