Can Existing HVAC Serve a New Home Addition?
Often yes. The existing equipment does not have to meet Part 6 and its charge and airflow are not retested, but it must have the capacity, and any duct extended to the addition, of any length, is insulated, sealed and tested. A separate dwelling unit cannot share the same ducts.
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Check one: is the addition a separate dwelling?
The Mechanical Code does not allow return air from one dwelling unit to discharge into another through a shared system, so a furnace with a central return cannot serve both a house and an attached ADU (Example 9-7). A ductless system is the usual answer. Everything below assumes one dwelling.
Check two: does it have the capacity?
Exception 3 to Section 150.2(a) excuses the existing equipment from Part 6 but requires heating capacity that meets CRC 303.10; the manual describes that test as 68 degrees three feet above the floor in habitable rooms. A load calculation under Section 150.2(a)1E must show it. Infiltration in that calculation cannot exceed Table 150.2-C unless a blower door test supplies a real figure, and a block load is allowed for sizing.
Check three: the extended ducts
Exception 4 pulls in the duct rules the moment any length is extended. New ducts in unconditioned space take R-6 in zones 3 and 5 to 7 or R-8 elsewhere. The combined duct system then tests at 10 percent total leakage, or 7 percent to outside, or all accessible leaks are sealed and checked by smoke test, performed by an ECC-Rater. Asbestos-wrapped systems are excepted.
What stays off the list
A 75 square foot porch enclosure with under 25 feet of new duct still needs the sealing test, yet the old air conditioner needs no refrigerant charge check, airflow test or fan watt-draw test (Examples 9-9 and 9-11). An existing electric furnace may be extended (Example 9-36). Electric baseboard heat in place of ducts is possible only through performance modeling (Example 9-37).
Which form
The compliance manual's Table 9-6 pairs extended ducts with sealing and ECC verification, which points away from the CF1R-ADD-02-E for work needing no field verification and toward a registered CF1R-ADD-01-E. Confirm with the department.
Related: HVAC replacement, load calculations, duct leakage testing, a mini-split design guide, room additions and the residential report.
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. Exception 3 to Section 150.2(a) says existing heating and cooling equipment extended to an addition need not comply with Part 6, though its heating capacity must meet CRC 303.10.
Yes. Exception 4 applies when any length of duct is extended to serve the addition, so there is no minimum length.
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