Can a New ADU Use a Gas Furnace?
Yes, through the performance path only. Table 150.1-A marks gas space heating NA in every climate zone, and Section 150.1(c)6 makes a heat pump the prescriptive system. A gas furnace in a new detached ADU also needs the 240-volt circuit and panel space of Section 150.0(t), and an attached ADU cannot share the house's furnace ducts.
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The model has to absorb it
The compliance manual explains that the standard design carries the heat pump from Table 150.1-A. Modelling a gas furnace instead raises the heating source energy significantly against that budget, and the design has to make it up with envelope or other measures.
Details that belong to furnaces
- Fan efficacy: 0.45 W/cfm for a gas furnace air handler, tighter than the 0.58 W/cfm other air handlers may use, confirmed with the 350 cfm per ton airflow check.
- Temperature rise within the furnace manufacturer's maximum, and capacity sized by ACCA Manual S-2023 Table N2.5.
- Combustion air under the California Mechanical Code, Chapter 7, when the appliance sits in conditioned space, which matters in a closet in a small unit.
- Electric-ready under Section 150.0(t): a dedicated 240-volt circuit rated 30 amps within 3 feet, a cover marked "240V ready", and a reserved double-pole breaker space marked "For Future 240V use".
Other ADU types
Section 150.0(t) is not among the mandatory measures that Section 150.2(a) carries to additions, so an attached ADU does not owe the circuit. Its heating still has to avoid the house's shared return: the California Mechanical Code bars return air from one dwelling unit entering another, and Example 9-7 rules out extending a central-return furnace to an attached ADU. A new system serving an addition must meet the new-home space-conditioning requirements (Example 9-11).
In a conversion of existing conditioned space, the replacement heating system may not use electric resistance as its primary source (Section 150.2(b)1G). Example 9-38 confirms a gas unit may replace a gas furnace prescriptively; new cooling equipment brings a setback thermostat.
The wider picture is on heat pump requirements, ADU heat pump requirements and electric-ready requirements. See the attached ADU page, the detached ADU report and the ADU page.
Common Title 24 Questions
- Do ADUs need Title 24 in California?
- What plans are needed for an ADU Title 24 report?
- How much does an ADU Title 24 report cost?
- How long does an ADU Title 24 report take?
- Does a garage conversion ADU need Title 24?
- Does a JADU need a Title 24 report?
Frequently Asked Questions
The Energy Code does not force a later switch. Section 150.0(t) only prepares the wiring and panel space so a heat pump can replace the furnace.
Not prescriptively. Table 150.1-A shows electric resistance space heating not allowed in all 16 zones, apart from a supplemental heater of up to 2 kW on a 30-minute timer.
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View all ADU and JADU guides → · Permits and Compliance · Title 24 guide