California Mechanical Code (CMC)

The California Mechanical Code (CMC) is Part 4 of the California Building Standards Code (Title 24, California Code of Regulations). It sets the minimum standards for the design, construction, installation, operation and maintenance of heating, ventilation, air-conditioning, exhaust, refrigeration and other heat-producing mechanical systems and appliances. The CMC is based on the Uniform Mechanical Code (UMC) published by IAPMO, reprinted with California-specific amendments adopted by state agencies.

The 2025 California Mechanical Code is the current edition, effective January 1, 2026 and based on the 2024 UMC, replacing the 2022 CMC. Because Title 24 is updated on a three-year cycle and local jurisdictions may add amendments, the requirements that apply to your HVAC or ventilation project depend on the edition your city or county has adopted and any local changes — which is exactly what GoCodebook reconciles for you.

The CMC covers the full mechanical system: HVAC equipment, ventilation air (Chapter 4), exhaust systems such as kitchen hoods, clothes dryers and product-conveying ducts (Chapter 5), duct construction and installation, combustion air and the venting of water heaters, furnaces and other appliances (flues, vents and chimney connectors). It also addresses refrigeration systems, hydronic and boiler piping, and clearances to combustibles for heat-producing equipment.

A mechanical project rarely stands alone. Fuel-gas piping to furnaces and water heaters is shared with the California Plumbing Code (CPC, Part 5), and equipment efficiency, duct sealing and HVAC sizing are driven by the California Energy Code (Title 24, Part 6). GoCodebook lets you ask one question and see how these parts interact for your scope.

Ventilation rules set how much fresh air a space requires, while exhaust provisions govern kitchen range hoods, bathroom fans, clothes-dryer ducts and commercial Type I/II hoods. Common plan-check issues include dryer-duct length and termination, exhaust-duct material and clearances, and makeup-air requirements for large hoods. The CMC also dictates appliance venting — flue sizing, vent connector slope, and termination clearances for gas water heaters and furnaces — to prevent flue-gas spillage.

Whether a fixture or system needs a permit and inspection is a frequent question. Most installations, replacements and alterations of HVAC and exhaust equipment require a mechanical permit from the local building department. Ask GoCodebook for the controlling CMC section and we return the language plus citation, instead of you paging through the UMC. See where coverage is deepest.

The Building Standards Commission publishes Title 24, and the CMC reprints the UMC with amendments from California state agencies. Cities and counties then adopt the code and may add local amendments — for example tighter exhaust, seismic bracing or all-electric reach codes that affect gas appliances. That means the controlling text for a duct, flue or ventilation question can differ between two neighboring jurisdictions.

GoCodebook identifies the adopted edition and local amendments for your address and returns the governing provision with a citation, so you can verify the original language quickly. You can also cross-check the California Building Code (Part 2) for shaft, rating and occupancy issues that affect mechanical routing.

California's adopted codes — including the California Mechanical Code (CMC) — are published under Title 24 and hosted on code libraries such as UpCodes (up.codes) and ICC Digital Codes from the International Code Council (ICC). Those let you read the text section by section.

GoCodebook goes further: instead of searching a code library, you ask a question and get the controlling provision for the edition and local amendments your jurisdiction adopted, with a citation to verify. See how GoCodebook compares to UpCodes and ICC.