Can my ADU heat pump use backup electric strip heat under the 2025 code?

Yes, but the 2025 Energy Code limits how large the strip heat can be and when it may run. The heat pump itself must meet the design heating load, backup heat generally has to lock out above 35°F outdoors except during defrost or emergency mode, and the installer must certify the control setup.

Size limits

• Section 150.0(h)5 requires heat pump heating capacity to meet the building code minimum without counting supplementary heat. Strip heat cannot make up for an undersized heat pump.
• Section 150.0(h)8 caps electric resistance supplementary heat at the heat pump's nominal cooling capacity at 95°F multiplied by 2.7 kW per ton. In the CEC's Compliance Manual example, a 3-ton heat pump works out to 8.1 kW, which the manual rounds to an 8 kW limit.

Control rules

• 35°F lockout. Section 150.0(h)7 requires supplementary heat to lock out at outdoor temperatures above 35°F. Above that, it may run only during defrost or when the user selects emergency operation.
• Thermostat. Section 150.0(i)2 requires the thermostat to get outdoor temperature from a sensor or internet weather service, display it, lock out backup heat above 35°F and show when supplementary or emergency heat is on, unless another control device handles the lockout.
• Certification. The installer certifies on the Certificate of Installation (CF2R) that the controls were tested.

The small-building exception

Exception 1 to Section 150.0(h)7 covers buildings with less than 500 square feet of conditioned floor area in any climate zone, and buildings of any size in climate zones 7 and 15. Instead of the 35°F lockout, the controls can either keep strip heat off whenever the heat pump alone can meet the load, with the heat pump cutting on before and staying on after the backup heat, or allow backup heat only during defrost and transient periods such as start-up and after a setpoint increase.

Small supplemental heaters

On the prescriptive path, the primary heating system must be a heat pump, and Table 150.1-A does not allow electric resistance space heating. The exception to Section 150.1(c)6 does allow a supplemental heating unit, such as a bathroom wall heater, of no more than 2 kW or 7,000 Btu/h, controlled by a timer of 30 minutes or less, in a space also served by the primary system. Anything beyond that would have to work through the performance path.

Do mini-splits have strip heat?

Most ductless mini-split heads do not include electric strip heat, so these rules mainly affect ducted heat pumps with heat strips in the air handler. Cold-climate mini-splits instead rely on holding more of their capacity at low outdoor temperatures.

What it means for your mechanical plan

• List the strip heat size in kW on the equipment schedule and show that it stays within the 2.7 kW per ton cap.
• Note the lockout setting, or the exception being used, and specify a compliant thermostat.
• Size the heat strip circuit in the electrical load calculation.

See also California's heating requirements for ADUs and mini-splits across climate zones.

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