Heat Pump With a Gas Furnace Backup: What Title 24 Requires
Yes. Title 24 expressly allows a gas furnace as a heat pump's supplementary heat. The furnace must be locked out above 35 F outdoors except during defrost or user-selected emergency heat, the thermostat must show outdoor temperature and flag backup use, and the installer certifies the test on the CF2R.
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The lockout in plain terms
Section 150.0(h)7 covers heat pumps with supplementary heat, "including, but not limited to, electric resistance heaters or gas furnace supplementary heating." The backup is locked out above an outdoor air temperature of 35 F or lower. It may run above that only during defrost or when the occupant selects emergency operation.
What the thermostat must do
- Take outdoor temperature from a sensor or an internet weather service.
- Display that temperature.
- Enforce the 35 F lockout, or defer to another control device that already does.
- Show an indicator whenever supplementary or emergency heat is in use.
That list is Section 150.0(i)2A to F. The installer certifies on the CF2R that the system was tested under the CF2R procedure. The certificate is the CF2R.
No kW cap, but no sizing shortcut
The 2.7 kW per ton ceiling in Section 150.0(h)8 is written for electric resistance backup, so a gas furnace is not a kW-limited element. Sizing is a different matter. A heat pump must meet the California Building Code minimum heating capacity without counting supplementary heat (150.0(h)5B iii), and the manual contrasts this with the older Manual S-2003 habit of sizing to the cooling load and letting backup cover the rest. A backup furnace cannot justify an undersized heat pump.
Alternate controls
Exception 1 to Section 150.0(h)7 lets buildings under 500 square feet, and buildings of any size in zones 7 and 15, use Option A or B controls in place of the 35 F lockout. Option A prevents backup operation when the heat pump alone can meet the load and sets the heat pump cut-on and cut-off temperatures above the backup's. Option B allows backup during defrost and transient periods such as start-ups, provided unnecessary operation is prevented.
Where to read next
Compare the job to a plain swap in swapping central AC for a heat pump and the heat pump checklist. Third-party thermostat questions are on variable-speed heat pumps and smart thermostats. Reports start at residential Title 24.
Common Title 24 Questions
- When is Title 24 required for a home?
- Does a remodel need Title 24?
- Does a room addition need Title 24?
- What plans are needed for a residential Title 24 report?
- How much does a residential Title 24 report cost?
- How long does a residential Title 24 report take?
Frequently Asked Questions
No. Section 150.0(h)8 limits electric resistance supplementary heat. The lockout, thermostat and sizing rules still apply to a gas furnace.
Yes, as supplementary heat under the controls above. The heat pump itself still has to be selected to carry the design heating load.
Explore Heat Pumps
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- Variable-Speed Heat Pump and a Smart Thermostat: Title 24 Rules
- Does California Require a Heat Pump When You Replace an AC?
- What SEER2 Does California Require for a Home AC or Heat Pump?
View all Heat Pumps guides → · HVAC and Water Heating · Title 24 guide