Quick answer
- A mechanical permit is required for AC, heat pump, and furnace changeouts
- Title 24 typically requires HERS testing on AC changeouts (charge + duct leakage)
- Proper sizing (Manual J/S/D) matters — bigger is not better
- Gas furnaces are checked for venting and combustion safety
- Rules and HERS thresholds vary locally — verify with your building department
When this applies
Replacing the condenser, furnace, or air handler
Swapping out major equipment is a 'changeout' and needs a permit. A repair (capacitor, contactor, motor) generally does not.
How it works
What Title 24 / HERS adds
For AC and heat-pump changeouts, an independent HERS rater commonly verifies refrigerant charge and duct leakage so the new system actually delivers its rated efficiency. This is separate from the city inspection and is part of a code-compliant install.
Why sizing is on the line
A permitted install is the moment to confirm the system is sized to the home (a Manual J load calculation), not just matched to the old unit. Oversized systems short-cycle and waste energy; the permit process encourages doing it right.
What to watch out for
Skipping HERS verification
Unpermitted changeouts skip HERS testing, so charge and duct leakage go unverified — a frequent cause of high bills and poor cooling on an otherwise new system.
Reusing undersized or leaky ductwork
Bolting new equipment onto old, leaky ducts undermines efficiency. Permitted work surfaces these issues instead of burying them.
Replacing your AC or furnace? We handle the permit and HERS testing.
Frequently asked questions
What is HERS testing and do I need it?
HERS (Home Energy Rating System) testing is independent verification — typically of refrigerant charge and duct leakage — required by Title 24 on most AC changeouts in California. It confirms your new system performs as rated.
Does replacing just the furnace need a permit?
Yes. A furnace replacement needs a mechanical permit and is inspected for venting and combustion safety, even if the AC stays.
Will a permit make my install take longer?
Pulling the permit and scheduling HERS testing is routine for us and rarely slows the project meaningfully — and it protects your efficiency, safety, and resale paperwork.
Keep exploring
Reviewed by John Stevenson Plumbing, Heating & Air. Based on California Title 24 energy code and mechanical permit requirements for HVAC changeouts (HERS verification, sizing, combustion safety) and our local install experience. HERS thresholds and local amendments vary — confirm with your city or county building department. Last updated 2026-06-25.