What’s New in the 2025 California Mechanical Code (CMC)?

The 2025 CMC cycle brings targeted but important changes that directly affect how you design, review, and inspect systems in California. This overview focuses on three high‑impact areas that are likely to appear on the ICC I4 California Mechanical Inspector exam.

1. MERV 13 Filtration Requirements

California continues to tighten filtration requirements for many occupancies. The 2025 CMC, coordinated with Title 24, increases the use of MERV 13 filters or better in locations where occupant exposure and contaminant generation are high.

For exam purposes, you need to:

• Know which occupancies explicitly require MERV 13.
• Pay attention to exceptions (for example, when lower MERV ratings are allowed because of equipment limitations).
• Understand how filter efficiency interacts with ventilation and recirculation requirements in Chapter 4.

2. A2L Refrigerants

The 2025 CMC acknowledges the growing adoption of A2L refrigerants (lower flammability). That does not mean the rules are relaxed; instead, there are refined requirements for:

• Maximum refrigerant quantities by occupancy and room type.
• Ventilation and leak detection provisions.
• Construction features that mitigate the risks associated with mildly flammable refrigerants.

Expect exam questions that:

• Ask when A2L systems are permitted in specific occupancies.
• Test the boundaries where an installation crosses a quantity or location threshold.\n+

3. Alignment with Title 24

Because California’s energy code (Title 24) and the CMC are developed together, many mechanical requirements are now framed with both safety and energy performance in mind.

Key themes:

• Ventilation requirements that tie into energy recovery or demand-control ventilation.
• Filtration and fan power limitations aligned with energy budgets.
• Exceptions where the energy code drives a different default than the underlying model code.

For inspectors, this means:

• You must read the CMC text and any referenced Title 24 provisions together.
• Exam questions may present scenarios where energy‑driven exceptions change the mechanical requirement.

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Study Strategy for the I4 Exam

• Focus your reading on the sections of the 2025 CMC that mention MERV 13, A2L, or explicit coordination with Title 24.\n+
• When you see “except,” “unless,” or “provided that,” highlight the entire sentence—these phrases often change the base requirement and are prime exam fodder.