Furnace or Air Handler in the Garage: Title 24 Rules for a Swap
A furnace or air handler in a garage carries a stricter duct rule than the same swap indoors. Under Section 150.2(b)1D the system must reach 6 percent leakage or have every accessible garage leak sealed and verified by a smoke test, whatever length of duct is replaced, and a raised platform cannot serve as a return plenum.
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Why the garage changes the rule
Elsewhere in a house, a swap needs 10 percent total or 7 percent to outside, and the full duct provisions start past 25 feet of new duct. Sections 150.2(b)1D and 1E add a garage clause: when altered ducts, air handlers, coils or plenums sit in a garage, Section 150.2(b)1Diic applies regardless of how much duct is touched. It offers two ways through:
- Measured total leakage of 6 percent of air handler airflow or less.
- All accessible leaks in the garage sealed, verified by visual inspection and a smoke test by an ECC-Rater.
The manual adds that sampling is not allowed for the smoke route, and the local agency has the last word on what is accessible.
What the smoke test looks like
Appendix RA3.1.4.3.6 has the rater inject non-toxic smoke into a fan holding the ducts at 25 Pa with grilles and registers sealed, then inspect every accessible portion. The system passes if no visible smoke exits, or if smoke comes only from a furnace cabinet that is gasketed and sealed by the manufacturer. The paired visual check (RA3.1.4.3.7) covers plenum connections, boots and duct joints.
The platform trap
Example 9-40 describes an upflow unit on a platform in a garage, used as a return plenum, being replaced and moved three feet. Section 150.0(m)1E bars platforms and building cavities from conveying conditioned air, and moving the platform alters it, so sheet metal, duct board or flex duct must now carry the return air. Count that duct in the quote.
A heat-only furnace still counts
Example 9-41 defines an air handler to include a gas furnace blower without a cooling coil, so the garage clause applies to a furnace-only swap.
Other furnace checks
Combustion air follows California Mechanical Code Chapter 7 (Section 150.0(o)1K); the furnace runs within the maker's maximum temperature rise (150.0(h)4A).
The general job list is in the furnace checklist, the length trigger in the 25-foot rule, and replace-versus-seal in old duct decisions. 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 150.2(b)1D applies the garage clause regardless of the length of new or replacement duct installed in the garage space.
Not if it is altered. Example 9-40 treats a relocated platform as altered and requires sheet metal, duct board or flex duct for the return air.
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