Replacing Ducted Central Air With Mini-Splits: What Title 24 Asks
Taking out the ducted system takes the duct test with it, because the sealing rule attaches to ducts connected to the altered equipment. The mini-splits still need a load calculation, setback controls, heating that is not primarily electric resistance, and, for heat pumps in any zone or air conditioners in zones 2 and 8 to 15, a verified refrigerant charge. Any ducts left serving other equipment keep their own rules.
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The order the work runs in
- Size it room by room. Loads come from Manual J, SMACNA or ASHRAE methods (Section 150.0(h)1). Block loads are allowed only for additions, so a whole-house swap needs a load for each room.
- Recover the old refrigerant. The compliance manual says removing the old system requires recovery by a technician holding an EPA 608 Type II or Universal certificate.
- Decide what happens to the ducts. Section 150.2(b)1E ties duct sealing to the duct system connected to the altered equipment. With every duct removed, nothing connected remains to test. Ducts left in place for other equipment are tested when that equipment is altered, and a leftover system under 40 linear feet by visual inspection is excepted (Exception 2). How your department reads a partly abandoned system is its call.
- Install the heads and verify the charge. Heat pump mini-splits need charge verification in every climate zone, with the weigh-in route for units the standard test cannot handle (Section 150.2(b)1Fii and 1Fiii).
- Replace the thermostats. Controls on an altered system become setback thermostats (Section 150.2(b)1Fi).
The heating rule works in your favor
Altered or replacement heating may not use electric resistance as the primary source (Section 150.2(b)1G). Heat pump heads meet that in every zone. The exceptions let an owner who already has electric resistance heat keep it in certain cases, such as climate zones 7 and 15.
One question to put to the plan checker
The manual defines an entirely new or complete replacement system as new equipment plus an entirely new duct system. A ductless job has no duct system, so ask whether the department treats it as a complete replacement or as an altered system. The items above apply either way.
Compare the new-construction view on ducted versus ductless in an ADU, then see the installation checklist, duct leakage testing and how many heads one outdoor unit can serve. Reports are on the residential page.
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
Possibly. Duct sealing follows the equipment that is altered. Ducts that still serve other equipment are tested when that equipment is altered, so ask the building department about the ducts you keep.
That is a different job: the furnace is not altered, but the new cooling heads are a new system with its own charge check. Describe the full scope to the plan checker.
Explore Mini-Splits
- Cooling-Only Mini-Split or Heat Pump Mini-Split: What Triggers More?
- Home Addition With a Mini-Split: Title 24 Design Guide
- How Many Heads Can a Mini-Split Have, and Does Adding One Trigger Title 24?
- Installing a Mini-Split Yourself: What Title 24 Still Requires
- Mini-Split in a Garage, Shed or Workshop: Does It Trigger Title 24?
View all Mini-Splits guides → · HVAC and Water Heating · Title 24 guide