CMC · California Mechanical Code

How does the CMC Ventilation Air interact with the California Energy and Building Codes?

Short answer for a homeowner: The mechanical code (CMC) sets ventilation requirements, but when the Energy Code applies to your building type, the Energy Code’s ventilation and energy rules take precedence — the Building Code simply says ventilation must be either natural or mechanical (and points to the CMC for mechanical details). Designers must record outdoor‑air rates on the plans, follow CEC energy rules for systems like DOAS/ERV, and meet CMC construction and ductwork requirements.

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Last reviewed: July 6, 2026

What the code requires — plain English with the controlling §

The California Mechanical Code (CMC) requires occupiable spaces to be provided with ventilation (outdoor) air, but it explicitly states that when an occupancy is regulated by the California Energy Code (CEC), the Energy Code’s ventilation requirements supersede the CMC. See § 402.1 of the CMC.

The California Building Code (CBC) requires buildings to be ventilated either naturally or mechanically (per the CMC) and points designers to the CMC for mechanical ventilation details (CBC § 1201–1202).

The single most important rule: when the Energy Code covers an occupancy, the CEC’s ventilation rules take precedence over the CMC (see § 402.1).

Requirements in detail

Which code controls ventilation rates?

  • If the occupancy is subject to the California Energy Code, the CEC controls ventilation rates and system energy features (supersedes CMC) — CMC § 402.1.
  • If not covered by the Energy Code, the CMC Chapter 4 (Ventilation Air) supplies the ventilation rates, methods, and special provisions (health care, natatoria, residential).
  • The CBC § 1202.1 says buildings must have natural ventilation (per CBC §1202.5) or mechanical ventilation in accordance with the CMC—so the CBC defers to the CMC for mechanical ventilation details.

Design documentation and what must appear on plans

  • The CMC requires that the outdoor air ventilation rate and air distribution assumptions used in design be clearly identified on the construction documents (CMC § 402.1.1). This means specifying the design outdoor-air cfm (or L/s), the standard used (ASHRAE 62.1/62.2 or CEC method), and any DOAS/energy-recovery equipment.

Energy Code interactions (what the Energy Code adds)

  • When the CEC covers the occupancy, it adds energy-oriented requirements that affect ventilation systems: required energy recovery in many DOAS/large outdoor-air systems, DOAS fan power limits, bypass and economizer controls, and acceptance testing. Examples from the Energy Code:
    • DOAS / controlled ventilation rules and fan-power limits for DOAS are set in the CEC (DOAS supply fan W/cfm limits and control requirements).
    • Exhaust-air energy recovery requirements (minimum sensible or enthalpy recovery ratios, bypass controls, exceptions) are in the Energy Code (see CEC energy recovery / Table/§ references).
    • The CEC requires acceptance testing for outdoor-air ventilation systems, DOAS, HRV/ERV and air economizers; test procedures are defined in the CEC appendices (NA7.*).

Ducts, sealing, insulation, and acceptance

  • The CEC explicitly requires that air-distribution ducts and plenums meet CMC construction and sealing requirements (CEC § 120.4 → “CMC compliance”) and it sets duct insulation/sealing thresholds and acceptance testing triggers. That means CMC duct construction, sealing, and testing rules apply even when the Energy Code controls ventilation rates.

Health-care and special occupancies

  • Health-care ventilation is handled by CMC Table 4‑A and ASHRAE/ASHE 170; where conflicts exist the more restrictive requirement applies. The CMC also notes that some occupancies may be regulated by the Energy Code instead — check § 402.1 and Figure/Table references.

Decision-relevant table

Question Decision point / value Which code controls? Code Reference
Is the occupancy regulated by the Energy Code? Yes → use Energy Code ventilation rules; No → use CMC Chapter 4 Energy Code (when applicable) supersedes CMC CMC § 402.1
Mechanical vs natural ventilation allowed? Natural per CBC §1202.5 or mechanical per CMC CBC points to CMC for mechanical CBC § 1202.1
DOAS fan power / DOAS controls Fan power limits, min speeds, cycling & shutoff rules CEC / Energy Code CEC DOAS sections (see DOAS/energy recovery provisions)
Energy recovery required? Climate-zone/outdoor-air fraction thresholds; minimum recovery ratios CEC / Energy Code (table-driven) CEC energy-recovery provisions/Table (see § 140.4(q) & Table 140.4‑J)
Duct construction & sealing Construction & sealing per CMC; acceptance testing per CEC thresholds Both — CEC defers to CMC for duct construction; CEC sets when testing is required CEC § 120.4 and CMC duct sections

Exceptions & special cases

  • CEC coverage exception: If the Energy Code regulates the occupancy, the CEC’s ventilation requirements supersede the CMC; always confirm which code covers the occupancy before applying CMC rates. CMC § 402.1.
  • Healthcare: CMC requires health‑care ventilation to follow Table 4‑A and ASHRAE/ASHE 170; where those conflict, use the more restrictive requirement.
  • DOAS exceptions: The Energy Code includes several exceptions to DOAS/energy‑recovery requirements (for example where other site-recovered energy supplies most heating or for systems operating <20 hours/week); see the CEC exceptions in the energy‑recovery section.
  • Duct insulation/sealing exceptions: The CEC contains exceptions for ducts located entirely in conditioned space (no insulation required) and specific field-inspection verification pathways; CMC construction and acceptance details still apply where referenced.

If you need the exact text of a particular CEC provision (for example § 160.2), note: I did not find a retrieval that reproduces CEC § 160.2 verbatim in the supplied files. I can fetch and cite it specifically if you want — say so and I’ll retrieve the exact section. (Do not rely on my memory here; I will fetch and cite the exact text on request.)

Common mistakes

  • Assuming the CMC always sets ventilation rates. (Wrong: if CEC covers the occupancy, the Energy Code controls — CMC § 402.1.)
  • Designing DOAS/ERV/HRV equipment to meet CMC ventilation rates but ignoring CEC energy recovery, fan‑power, or bypass requirements. The CEC can impose energy‑related constraints even when the CMC provides the ventilation rates.
  • Omitting construction document callouts for outdoor-air cfm and distribution assumptions — required by the CMC (§ 402.1.1).
  • Forgetting CEC acceptance testing for outdoor‑air systems (NA7 test procedures) — the owner/contractor must submit Certificates of Acceptance where required.
  • Treating ductwork like “secondary” and ignoring CMC/CEC duct construction, sealing, and insulation requirements — the CEC expressly references CMC duct requirements.

Worked example — office DOAS scenario (concrete)

Scenario: 10,000 ft² open‑plan office served by a central air system. The project is nonresidential and the Energy Code applies.

  1. Which code sets the ventilation rate?

    • Because the occupancy is regulated by the Energy Code, you start with the CEC ventilation requirements — CMC § 402.1 reminds you the Energy Code supersedes the CMC.

  2. Energy Code implications (examples drawn from the CEC provisions):

    • If the design outdoor‑air fraction at full airflow is high enough to trigger energy‑recovery requirements (see the CEC energy recovery table/§), the system may be required to include an ERV/heat recovery with a minimum sensible / enthalpy recovery ratio or meet one of the CEC exceptions. Check the CEC Table 140.4‑J and § 140.4(q).
    • The DOAS fan power and controls must meet CEC DOAS limits (fan W/cfm or the § 140.4(c) requirements depending on input power), and DOAS controls must permit the DOAS to cycle off zone equipment when no heating/cooling call exists (CEC DOAS rules).

  3. CMC / CBC documentation & ductwork:

    • On the construction documents, clearly record the outdoor air cfm used in calculations, the method/standard (CEC method or ASHRAE), and any DOAS/ERV equipment specs per CMC § 402.1.1.
    • Duct construction, sealing and insulation must meet CMC standards and the CEC’s duct‑testing thresholds (CEC § 120.4 / acceptance testing references). Expect NA7 acceptance testing where the CEC requires it.

Net effect: you design to the CEC ventilation rates and energy rules, use the CMC for mechanical installation details and plan notes, and prepare to perform the CEC‑required acceptance tests and CMC‑required ductwork/installation practices.

Related provisions (CMC / CBC / CEC pointers)

  • CMC § 401.1 – Applicability of ventilation chapter and MERV‑13 requirement for mechanical systems.
  • CMC § 402.1 – Occupiable spaces & the supersession note (CEC supersedes CMC for CEC‑regulated occupancies).
  • CMC § 402.1.1 – Construction documents: identify outdoor‑air ventilation rate and assumptions.
  • CBC § 1201 – General interior environment scope (ventilation, lighting, etc.).
  • CBC § 1202 – Ventilation: natural vs mechanical; mechanical ventilation per CMC. CBC § 1202.1 specifically points to the CMC for mechanical ventilation.
  • CEC sections (examples) — DOAS, energy recovery and acceptance testing: see CEC DOAS provisions and exhaust air heat recovery sections (examples: CEC § 140.4(q), Table 140.4‑J, DOAS rules).

Code references

Grounded in the retrieved California Mechanical Code — click a citation to read the verbatim passage:

  • CMC § 140.4 High relevance — show source text

    Exception 2 to Section 140.4(q)1: Compliance is not required for sensible recovery ratio at cooling design conditions for Climate Zone 1.

    1. Energy recovery bypass or control to disable energy recovery and to directly economize with ventilation air based on outdoor air temperature limits specified in Table 140.4-G. For energy recovery systems where the transfer of energy cannot be stopped, bypass shall prevent the total airflow rate of either outdoor air or exhaust air through the energy recovery exchanger from exceeding 10 percent of the full design airflow rate. Exception to Section 140.4(q)2: For DOAS units with the capability to shut off when a separate space-conditioning system serving the same space meets the economizer requirements in Section 140.4(e)1A.

    Exception 1 to Section 140.4(q): Systems meeting Section 140.9(c), Prescriptive requirements for laboratory and factory exhaust systems.

    Exception 2 to Section 140.4(q): Systems serving spaces that are not cooled and that are heated to less than 60°F.

    Exception 3 to Section 140.4(q): Where more than 60 percent of the outdoor air heating energy is provided from site-recovered energy in Climate Zone 16.

    Exception 4 to Section 140.4(q): Where the sum of the airflow rates exhausted and relieved within 20 feet of each other is less than 75 percent of the design outdoor airflow rate, excluding exhaust air that is either:

    1. Used for another energy recovery system,

    2025 CALIFORNIA ENERGY CODE 127

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    NONRESIDENTIAL AND HOTEL/MOTEL OCCUPANCIES—PERFORMANCE AND PRESCRIPTIVE

    COMPLIANCE APPROACHES FOR ACHIEVING ENERGY EFFICIENCY

    1. Not allowed by the California Mechanical Code (Title 24, Part 4) (CMC) for use in energy recovery systems with leakage potential, or
    2. Of Class 4 as specified in Section 120.1(g).

    Exception 5 to Section 140.4(q): Systems expected to operate less than 20 hours per week.

    Note: Authority: Sections 25213, 25218, 25218.5, 25402 and 25402.1, Public Resources Code. Reference: Sections 25007, 25008, 25218.5, 25310, 25402, 25402.1, 25402.4, 25402.8, and 25943, Public Resources Code.

    |TABLE 140.4-J—ENERGY RECOVERY REQUIREMENTS BY CLIMATE ZONE AND PERCENT
    OUTDOOR AIR AT FULL DESIGN AIRFLOW (< 8,

  • CMC § 503.5.10.1. High relevance — show source text

    (3) Systems in Climate Zones 0A, 1A, 2A, and 3A with at least 80 percent outdoor air and employing exhaust air energy recovery complying with Section E 503.5.10.1.

    (4) Systems that prevent reheating, recooling, or mixing of heated and cooled supply air.

    (5) Systems in which at least 75 percent of the energy for reheating (on an annual basis) is from site recovered energy or on-site renewable energy. [ASHRAE 90.1:6.5.3.5]

    E 503.5.6.5 Fractional Horsepower Fan Motors. Motors for fans that are [1] ⁄ 12 hp (62.1 W) or more and less than 1 hp (0.7 kW) shall be electronically-commutated motors or shall have a motor efficiency of not less than 70 percent where rated in accordance with DOE 10 CFR 431. These motors

    shall also have the means to adjust motor speed for either balancing or remote control. Belt-driven fans shall be permitted to use sheave adjustments for airflow balancing in lieu of a varying motor speed.

    Exceptions:

    (1) Motors in the airstream within fan coils and terminal units that operate when providing heating to the space served.

    (2) Motors installed in space conditioning equipment certified in accordance with Section E

    503.4 through Section E 503.4.4.1.

    (3) Motors shown in Table E 503.5.6.5(1) or Table E 503.5.6.5(2). [ASHRAE 90.1:6.5.3.6]

    E 503.5.6.6 Low Power Fans. Fans that are not

    covered by Section E 503.5.6.5 and having a fan nameplate electrical input power of less than 180 W or having a motor nameplate horsepower less than

    1 / 12 hp (62.1 W) shall meet the fan efficacy requirements specified in ASHRAE 90.1. [ASHRAE 90.1:6.5.3.7]

    E 503.5.6.7 Ventilation Design. The required minimum outdoor air rate is the larger of the minimum outdoor air rate or the minimum exhaust air rate

    required by Chapter 4, ASHRAE 62.1, ASHRAE 62.2, ASHRAE/ASHE 170, or applicable codes or accreditation standards. Outdoor air ventilation systems shall comply with one of the following:

    (1) Design minimum system outdoor air provided shall not exceed 135 percent of the required minimum outdoor air rate.

    (2) Dampers, ductwork, and controls shall be provided that allow the system to supply no more than the required minimum outdoor air rate with a single setpoint adjustment.

    (3) The system includes exhaust air energy recovery complying with Section E 503.5.10.1.

    [ASHRAE 90.1:6.5.3.8]

    E 503.5.6.8 Occupied-Standby Controls. Zones serving only rooms that are required to have automatic partial OFF or automatic full OFF lighting controls in accordance with ASHRAE 90.1, where the Chapter 4 or ASHRAE 62.1 occupancy category permits ventilation air to be reduced to zero when

    «

    438 2025 CALIFORNIA MECHANICAL CODE

  • CMC § 160.2 High relevance — show source text

    The ventilation rate required by the authority having jurisdiction, the facility Environmental Health and Safety Department or Section 160.2(c)3; or

    iii. The mechanical exhaust flow minus the available transfer air. Available transfer air shall be from another conditioned space or return air plenums on the same floor and same smoke or fire compartment, and that at their closest point are within 15 feet of each other. Exception 1 to Section 170.2(c)4M: Spaces that are required by applicable codes and standards to be maintained at a positive pressure differential relative to adjacent spaces. Exception 2 to Section 170.2(c)4M: Spaces where the highest amount of transfer air that could be used for exhaust makeup may exceed the available transfer airflow rate and where the spaces have a required negative pressure relationship. N. Dedicated outdoor air systems (DOAS). HVAC systems that utilize a dedicated outdoor air system (DOAS) such as a DX-DOAS, HRV or ERV unit to condition, temper or filter 100 percent outdoor air separate from local or central spaceconditioning systems serving the same space shall meet the following criteria:

    1. DOAS unit fan systems with input power less than 1 kW shall not exceed a total combined fan power of 1.0 W/cfm. DOAS with fan power greater than or equal to 1 kW shall meet the requirements of Section 140.4(c).
    2. The DOAS supply air shall be delivered directly to the occupied space or at the outlet of any terminal heating or cooling coils and shall cycle off any zone heating and cooling equipment fans, circulation pumps and terminal unit fans when there is no call for heating or cooling in the zone. Exception 1 to Section 170.2(c)4N2: Active chilled beam systems. Exception 2 to Section 170.2(c)4N2: Sensible-only cooling terminal units with pressure-independent variableairflow regulating devices limiting the DOAS supply air to the greater of latent load or minimum ventilation requirements. Exception 3 to Section 170.2(c)4N2: Any configuration where a DOAS unit provides ventilation air to a downstream fan (a terminal box, air handling unit or other space-conditioning equipment) where the total system airflow can be reduced to ventilation minimum or the downstream fan power is no greater than 0.12 watts per cfm when space temperatures are within the thermostat deadband (at low speed per manufacturer’s literature).
    3. DOAS supply and exhaust fans shall have a minimum of three speeds to facilitate system balancing.
    4. DOAS with mechanical cooling providing ventilation to multiple zones and operating in conjunction with zone heating and cooling systems shall not use heating or heat recovery to warm supply air above 60°F when representative building loads or outdoor air temperature indicates that the majority of zones requires cooling.

    2025 CALIFORNIA ENERGY CODE 255

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    MULTIFAMILY BUILDINGS—PERFORMANCE AND PRESCRIPTIVE COMPLIANCE APPROACHES

    O. Exhaust air heat recovery. Fan systems designed to operate to the criteria listed in either Table 170.2-I or Table 170.2-J shall include an exhaust air heat recovery system that meets the following: i. A sensible energy recovery ratio of at least 60 percent or an enthalpy recovery ratio of at least 50 percent for both heating and cooling design conditions. ii. Energy recovery bypass or control to disable energy recovery and to directly economize with ventilation air based on outdoor air temperature limits specified in Table 170.2-G.

  • CMC § 140.4 High relevance — show source text

    Exception 3 to Section 140.4(q): Where more than 60 percent of the outdoor air heating energy is provided from site-recovered energy in Climate Zone 16.

    Exception 4 to Section 140.4(q): Where the sum of the airflow rates exhausted and relieved within 20 feet of each other is less than 75 percent of the design outdoor airflow rate, excluding exhaust air that is either:

    1. Used for another energy recovery system,

    2025 CALIFORNIA ENERGY CODE 127

    on Jul 18, 2025 11:14 AM (CDT) THEREUNDER.

    NONRESIDENTIAL AND HOTEL/MOTEL OCCUPANCIES—PERFORMANCE AND PRESCRIPTIVE

    COMPLIANCE APPROACHES FOR ACHIEVING ENERGY EFFICIENCY

    1. Not allowed by the California Mechanical Code (Title 24, Part 4) (CMC) for use in energy recovery systems with leakage potential, or
    2. Of Class 4 as specified in Section 120.1(g).

    Exception 5 to Section 140.4(q): Systems expected to operate less than 20 hours per week.

    Note: Authority: Sections 25213, 25218, 25218.5, 25402 and 25402.1, Public Resources Code. Reference: Sections 25007, 25008, 25218.5, 25310, 25402, 25402.1, 25402.4, 25402.8, and 25943, Public Resources Code.

    TABLE 140.4-J—ENERGY RECOVERY REQUIREMENTS BY CLIMATE ZONE AND PERCENT
    OUTDOOR AIR AT FULL DESIGN AIRFLOW (< 8,000 HOURS/YEAR)    		 Col2 Col3 Col4 Col5 Col6 Col7 Col8 Col9 Col10 Col11 Col12 Col13 Col14 Col15 Col16 Col17
    % OUTDOOR
    AIR AT FULL
    DESIGN
    AIRFLOW    		 **1 ** **2 ** **3 ** **4 ** **5 ** **6 ** **7 ** **8 ** **9 ** 10 11 12 13 14 15 16
    ≥ 10% and < 20% NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR NR
    ≥ 20% and < 30% ≥ 15,000 ≥ 20,000 NR NR NR NR NR NR NR NR ≥ 18,500 ≥ 18,500 ≥ 18,500 ≥ 18,500 ≥ 18,500 ≥ 18,500
    ≥ 30% and < 40% ≥ 13,000 ≥ 15,000 NR NR NR NR NR NR NR NR ≥ 15,000 ≥ 15,000 ≥ 15,000 ≥ 15,000 ≥ 15,000 ≥ 15,000
    ≥ 40% and < 50% ≥ 10,
  • CMC § 1.0 High relevance — show source text

    D. The outdoor design temperatures for cooling shall be no greater than the 1.0 percent Cooling Dry Bulb and Mean Coincident Wet Bulb values.

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    MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS

    1. Outdoor condensing units . A. Clearances. Installed air conditioner and heat pump outdoor condensing units shall have a clearance of at least 5 feet (1.5 meters) from the outlet of any dryer vent. B. Liquid line drier. Installed air conditioner and heat pump systems shall be equipped with liquid line filter driers if required, as specified by manufacturer’s instructions.
    2. Central forced-air heating furnaces . A. Temperature rise . Central forced-air heating furnace installations shall be configured to operate in conformance with the furnace manufacturer's maximum inlet-to-outlet temperature rise specifications.
    3. Air-distribution and ventilation system ducts, plenums and fans. A. CMC compliance.

    i. All air-distribution system ducts and plenums, including, but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0 and ANSI/SMACNA006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. ii. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection a or b below:

    a. Ducts shall have a minimum installed level of R-6.0, or

    Exception to Section 160.3(b)5Aiia : Portions of the duct system located in conditioned space below the ceiling separating the occupiable space from the attic are not required to be insulated if all of the following conditions are met: i. The noninsulated portion of the duct system is located entirely inside the building’s thermal envelope as confirmed by visual inspection. ii. At all locations where noninsulated portions of the duct system penetrate into unconditioned space, the penetration shall be draft stopped compliant with CFC Sections 703.1 and 704.1 and air-sealed to the construction materials that are penetrated, using materials compliant with CMC Section E502.4.2 to prevent air infiltration into the cavity. All connections in unconditioned space are insulated to a minimum of R-6.0 as confirmed by visual inspection. b. Ducts do not require insulation when the duct system is located entirely in conditioned space. For buildings with three or fewer habitable stories, duct systems located entirely in conditioned space shall be confirmed through field verification and diagnostic testing in accordance with the requirements of Reference Residential Appendix RA3.1.4.3.8. iii. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. iv. Openings shall be sealed with mastic, tape or other duct-closure system that meets the applicable requirements of UL 181, UL 181A or UL 181B or aerosol sealant that meets the requirements of UL 723. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used. v. Building cavities, support platforms for air handlers, and plenums designed or constructed with materials other than sealed sheet metal, duct board or flexible duct shall not be used for conveying conditioned air.

  • CMC § 8.5 High relevance — show source text

    21_–0.27|75|0.75 inch|0.75 inch|1.0 inch|1.0 inch|1.0 inch| |Residential
    40–60|0.21–0.27|75|R-6|R-5|R-7|R-6|R-5| |Nonresidential
    40–60|0.21–0.27|75|0.5 inch|0.5 inch|1.0 inch|1.0 inch|1.0 inch| |Nonresidential
    40–60|0.21–0.27|75|R-3|R-3|R-7|R-6|R-5| |Below 40|0.20–0.26|50|1.0 inch|1.5 inches|1.5 inches|1.5 inches|1.5 inches| |Below 40|0.20–0.26|50|R 8.5|R 14|R 12|R 10|R 9|

    Footnote to Table 120.3-A-1 and Table 120.3-A-2: These thicknesses are based on energy efficiency considerations only. Issues such as water vapor permeability or surface condensation sometimes require vapor retarders or additional insulation.

    SECTION 120.4—REQUIREMENTS FOR AIR DISTRIBUTION SYSTEM DUCTS AND PLENUMS

    Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.4(a) through 120.4(g).

    Exception to Section 120.4: Systems serving healthcare facilities shall comply with the applicable requirements of the California Mechanical Code.

    (a) CMC compliance. All air distribution system ducts and plenums, including but not limited to building cavities, mechanical closets, air-handler boxes and support platforms used as ducts or plenums, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, and 605.0, and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition incorporated herein by reference. Connections of metal ducts and the inner core of flexible ducts shall be mechanically fastened. Openings shall be sealed with mastic, tape, aerosol sealant or other duct-closure system that meets the applicable requirements of UL 181, UL 181A, or UL 181B. If mastic or tape is used to seal openings greater than [1] / 4 inch, the combination of mastic and either mesh or tape shall be used.

    Portions of supply-air and return-air ducts conveying heated or cooled air located in one or more of the following spaces shall be insulated to a minimum installed level of R-8:

    1. Outdoors; or

    2. In a space between the roof and an insulated ceiling; or

    3. In a space directly under a roof with fixed vents or openings to the outside or unconditioned spaces; or

    4. In an unconditioned crawlspace; or

    5. In other unconditioned spaces.

    2025 CALIFORNIA ENERGY CODE 77

  • CMC § 160.3 High relevance — show source text

    Mass radiant floor slab systems shall incorporate floor temperature onto the optimum start algorithm. Exception to Section 160.3(a)2J: Systems that must operate continuously.

    (b) Dwelling unit space-conditioning and air distribution systems.

    1. Building cooling and heating loads. Building heating and cooling loads shall be determined using a method based on any one of the following, using cooling and heating loads as two of the criteria for equipment sizing and selection: A. The ASHRAE Handbook, Equipment Volume, Applications Volume and Fundamentals Volume; or B. The SMACNA Residential Comfort System Installation Standards Manual; or

    C. The ACCA Manual J.

    Exception to Section 160.3(b)1: Block loads, the total load for all rooms combined that are served by the central equipment, may be used for the purpose of system sizing for additions. NOTE: Heating systems are required to have a minimum heating capacity adequate to meet the minimum requirements of the CBC.

    1. Design conditions . Design conditions shall be determined in accordance with the following: A. For the purpose of sizing the space-conditioning (HVAC) system, the indoor design temperatures shall be 68°F for heating and 75°F for cooling. B. Outdoor design conditions shall be selected from one of the following: i. Reference Joint Appendix JA2, which is based on data from the ASHRAE Climatic Data for Region X; or ii. The ASHRAE Handbook Fundamentals Volume; or

    iii. The ACCA Manual J.

    C. The outdoor design temperatures for heating shall be no lower than the 99.0 percent Heating Dry Bulb or the Heating Winter Median of Extremes values.

    D. The outdoor design temperatures for cooling shall be no greater than the 1.0 percent Cooling Dry Bulb and Mean Coincident Wet Bulb values.

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    MULTIFAMILY BUILDINGS—MANDATORY REQUIREMENTS

    1. Outdoor condensing units . A. Clearances. Installed air conditioner and heat pump outdoor condensing units shall have a clearance of at least 5 feet (1.5 meters) from the outlet of any dryer vent. B. Liquid line drier. Installed air conditioner and heat pump systems shall be equipped with liquid line filter driers if required, as specified by manufacturer’s instructions.
    2. Central forced-air heating furnaces . A. Temperature rise . Central forced-air heating furnace installations shall be configured to operate in conformance with the furnace manufacturer's maximum inlet-to-outlet temperature rise specifications.
    3. Air-distribution and ventilation system ducts, plenums and fans. A. CMC compliance.

    i. All air-distribution system ducts and plenums, including, but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0 and 605.0 and ANSI/SMACNA006-2006 HVAC Duct Construction Standards Metal and Flexible 3rd Edition, incorporated herein by reference. ii. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection a or b below:

    a. Ducts shall have a minimum installed level of R-6.0, or

  • CMC § 5.3 High relevance — show source text

    (g) Duct sealing. Duct systems shall comply with Subsection 1 or 2 below:

    1. New duct systems that meet the criteria in Subsections A, B, C and D below shall be sealed to a leakage rate not to exceed 6 percent of the nominal air handler airflow rate as confirmed through acceptance testing, in accordance with Reference Nonresidential Appendix NA7.5.3; A. The duct system does not serve a healthcare facility; and B. The duct system provides conditioned air to an occupiable space for a constant volume, single zone, space-conditioning system; and C. The space-conditioning system serves less than 5,000 square feet of conditioned floor area; and D. The combined surface area of the ducts located outdoors or in unconditioned space is more than 25 percent of the total surface area of the entire duct system.
    2. New duct systems that are not subject to testing under Section 120.4(g)1 shall instead meet the duct leakage testing requirements of CMC Section 603.9.2.

    Note: Authority: Sections 25213, 25218, 25218.5, 25402 and 25402.1, Public Resources Code. Reference: Sections 25007, 25008, 25218.5, 25310, 25402, 25402.1, 25402.4, 25402.8, and 25943, Public Resources Code.

    SECTION 120.5—REQUIRED NONRESIDENTIAL MECHANICAL SYSTEM ACCEPTANCE

    Nonresidential and hotel/motel buildings shall comply with the applicable requirements of Sections 120.5(a) through 120.5(b).

    Exception to Section 120.5: Systems serving healthcare facilities.

    (a) Before an occupancy permit is granted, the following equipment and systems shall be certified as meeting the Acceptance Requirements for Code Compliance, as specified by the Reference Nonresidential Appendix NA7. A Certificate of Acceptance shall be submitted to the enforcement agency that certifies that the equipment and systems meet the acceptance requirements:

    1. Outdoor air ventilation systems shall be tested in accordance with NA7.5.1.

    2. Constant volume, single zone unitary air conditioning and heat pump unit controls shall be tested in accordance with NA7.5.2.

    3. Duct systems that are subject to testing under Section 120.4(g)1, Section 141.0(b)2Di or Section 141.0(b)2Dii shall be tested in accordance with NA7.5.3.

    4. Air economizers, DOAS, HRV or ERV systems shall be tested in accordance with NA7.5.4. Exception 1 to Section 120.5(a)4: Air economizers installed by the HVAC system manufacturer and certified to the Commission as being factory calibrated and tested are not required to comply with the Functional Testing section of the air economizer controls acceptance test as described in NA7.5.4.2. Exception 2 to Section 120.5(a)4: The DOAS, HRV, or ERV unit that does not meet the exhaust air heat recovery ratio as specified in Section 140.4(q)1 or does not include bypass or control to disable energy recovery as specified in Section 140.4(q)2.

    5. Demand control ventilation systems required by Section 120.1(c)3 shall be tested in accordance with NA7.5.5.

    6. Supply fan variable flow controls shall be tested in accordance with NA7.5.6.

  • CMC § 140.8 Medium relevance — show source text

    C. An energy management control system (EMCS) or other controls that provides the specified lighting control functionality and complies with all requirements applicable to the specified controls may be used to meet these requirements. 4. Internally illuminated address signs. Internally illuminated address signs shall either: A. Comply with Section 140.8; or B. Consume no more than 5 watts of power. 5. Residential garages for eight or more vehicles. Lighting for residential parking garages for eight or more vehicles shall comply with the applicable requirements for nonresidential garages in Sections 110.9, 130.0, 130.1, 130.4, 140.6 and 141.0.

    (l) Reserved.

    (m) Air-distribution and ventilation system ducts, plenums and fans.

    1. CMC compliance.

    A. All air-distribution system ducts and plenums, including but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, 605.0 and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition, incorporated herein by reference.

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    SINGLE-FAMILY RESIDENTIAL BUILDINGS— MANDATORY FEATURES AND DEVICES

    B. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection i or ii below:

    i. Ducts shall have a minimum installed level of R-6.0, or Exception 1 to Section 150.0(m)1Bi: Portions of the duct system located in conditioned space below the ceiling separating the occupiable space from the attic are not required to be insulated if all of the following conditions are met:

    a. The noninsulated portion of the duct system is located entirely inside the building’s thermal envelope as confirmed by visual inspection. b. At all locations where noninsulated portions of the duct system penetrate into unconditioned space, the penetration shall be draft stopped compliant with CFC Sections 703.1 and 704.1 and air-sealed to the construction materials that are penetrated, using materials compliant with CMC Section E502.4.2 to prevent air infiltration into the cavity. All connections in unconditioned space are insulated to a minimum of R-6.0 as confirmed by visual inspection. Exception 2 to Section 150.0(m)1Bi : Ducts located in an unvented attic shall have a minimum insulation value of R-4.2, verified by visual inspection where:

    a. The attic has at least R-30 insulation between the roof rafters in contact with the roof deck.

    b. The gable ends meet the wall insulation requirements of Section 150.1(c)1B. c. The dwelling unit achieves a whole building leakage rate of 3.0 ACH50 or less, as confirmed by field verification and diagnostic testing in accordance with Reference Residential Appendix RA3.8. ii. Ducts do not require insulation when the duct system is located entirely within conditioned space, as confirmed through field verification and diagnostic testing in accordance with the requirements of Reference Residential Appendix RA3.1.4.3.8. For dwelling units with attics, the duct system shall be located below the ceiling separating the occupiable space from the attic.

  • CMC § 150.0 Medium relevance — show source text

    Forward phase cut dimmers controlling LED light sources in these spaces shall comply with NEMA SSL 7A. Exception 1 to Section 150.0(k)2F: Ceiling fans may provide control of integrated lighting via a remote control. Lighting integral to kitchen range hoods and bathroom exhaust fans. Exception 2 to Section 150.0(k)2F: Luminaires connected to a circuit with controlled lighting power less than 20 watts or controlled by an occupancy or vacancy sensor providing automatic-off functionality. Exception 3 to Section 150.0(k)2F: Navigation lighting rated less than 5 watts, such as night lights, step lights, and path lights. Lighting controlled by automatic-off controls and located internal to drawers, cabinetry with opaque fronts, or cabinetry with doors. G. Independent controls. Lighting integrated with the exhaust fans shall be controlled independently from the fans. The following shall be controlled separately from ceiling-installed lighting such that one can be turned on without turning on the other: i. Undercabinet lighting. ii. Undershelf lighting. iii. Interior lighting of display cabinets.

    iv. Switched outlets.

    1. Residential outdoor lighting. In addition to meeting the requirements of Section 150.0(k)1A, luminaires providing residential outdoor lighting shall meet the following requirements, as applicable: A. Outdoor lighting permanently mounted to a residential building or to other buildings on the same lot shall meet the following requirements: i. Controlled by a manual ON and OFF control switch that permits the automatic actions of Item ii below; and ii. Controlled by one of the following controls: a. A photocell and a motion sensor; or b. A photocell and an automatic time switch control; or c. Controlled by an astronomical time clock control. B. Controls that override to ON shall not be allowed unless the override automatically returns the automatic control to its normal operation within 6 hours. C. An energy management control system (EMCS) or other controls that provides the specified lighting control functionality and complies with all requirements applicable to the specified controls may be used to meet these requirements.
    2. Internally illuminated address signs. Internally illuminated address signs shall either: A. Comply with Section 140.8; or B. Consume no more than 5 watts of power.
    3. Residential garages for eight or more vehicles. Lighting for residential parking garages for eight or more vehicles shall comply with the applicable requirements for nonresidential garages in Sections 110.9, 130.0, 130.1, 130.4, 140.6 and 141.0.

    (l) Reserved.

    (m) Air-distribution and ventilation system ducts, plenums and fans.

    1. CMC compliance.

    A. All air-distribution system ducts and plenums, including but not limited to, mechanical closets and air-handler boxes, shall meet the requirements of the CMC Sections 601.0, 602.0, 603.0, 604.0, 605.0 and ANSI/SMACNA-006-2006 HVAC Duct Construction Standards Metal and Flexible, 3rd Edition, incorporated herein by reference.

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    SINGLE-FAMILY RESIDENTIAL BUILDINGS— MANDATORY FEATURES AND DEVICES

    B. Portions of supply-air and return-air ducts and plenums of a space heating or cooling system shall be insulated in accordance with either Subsection i or ii below:

  • CMC § 13.3 Medium relevance — show source text

    CERTIFIED TO THE ENERGY COMMISSION means, when used in association with appliances, certified under Section 1606 of Title 20 of the California Code of Regulations; and otherwise means certified by the manufacturer in a declaration, executed under penalty of perjury under the laws of the State of California, that all the information provided pursuant to the certification is true, complete, accurate and in compliance with all applicable provisions of Part 6; and if applicable that the equipment, product or device was tested under the applicable test method specified in Part 6.

    CERTIFYING ORGANIZATION is an independent organization recognized by the Commission to certify manufactured devices for performance values in accordance with procedures adopted by the Commission.

    CIE 13.3 is the International Commission on Illumination (Commission Internationale de l’Eclairage) document titled “Method of Measuring and Specifying Colour Rendering Properties of Light Sources,” 1995 (CIE 13.3-1995).

    CIE 15 is the International Commission on Illumination (Commission Internationale de l’Eclairage) document titled “Technical Report: Colorimetry,” 2018 (CIE 15:2018).

    CIRCULATING FAN means a fan that is not a ceiling fan, but that is used to move air within a space that has no provision for connection to ducting or separation of the fan inlet from its outlet, and designed to be used for the general circulation of air.

    CLIMATE ZONES are the 16 geographic areas of California for which the commission has established typical weather data, prescriptive packages and energy budgets. Climate zones are defined by ZIP code and listed in Reference Joint Appendix JA2. FIGURE 100.1-A is an approximate map of the 16 climate zones.

    CLOSED-CIRCUIT COOLING TOWER is a cooling tower that utilizes indirect contact between a heated fluid, typically water or glycol, and the cooling atmosphere to transfer the source heat load through sensible heat, latent heat and mass transfer indirectly to the air, essentially combining a heat exchanger and cooling tower into an integrated and relatively compact device.

    CODES, CALIFORNIA HISTORICAL BUILDING CODE is the California Historical Building Code, California Code of Regulations, Title 24, Part 8 and Part 2 (Chapter 34).

    CODES, CBC is the 2025 California Building Code.

    CODES, CEC is the 2025 California Electrical Code.

    CODES, CFC is the 2025 California Fire Code.

    CODES, CMC is the 2025 California Mechanical Code.

    CODES, CPC is the 2025 California Plumbing Code.

    COEFFICIENT OF PERFORMANCE (COP), COOLING is the ratio of the rate of net heat removal to the rate of total energy input, calculated under designated operating conditions and expressed in consistent units, as determined using the applicable test method in the Appliance Efficiency Regulations or Section 110.2.

    COEFFICIENT OF PERFORMANCE (COP), HEATING is the ratio of the rate of net heat output to the rate of total energy input, calculated under designated operating conditions and expressed in consistent units, as determined using the applicable test method in the Appliance Efficiency Regulations or Section 110.2.

    COEFFICIENT OF PERFORMANCE (COP), HEAT PUMP is the ratio of the rate of useful heat output delivered by the complete heat pump unit (exclusive of supplementary heating) to the corresponding rate of energy input, in consistent units and as determined using the applicable test method in Appliance Efficiency Regulations or Section 110.2.

  • CMC § 110.2. Medium relevance — show source text

    CODES, CALIFORNIA HISTORICAL BUILDING CODE is the California Historical Building Code, California Code of Regulations, Title 24, Part 8 and Part 2 (Chapter 34).

    CODES, CBC is the 2025 California Building Code.

    CODES, CEC is the 2025 California Electrical Code.

    CODES, CFC is the 2025 California Fire Code.

    CODES, CMC is the 2025 California Mechanical Code.

    CODES, CPC is the 2025 California Plumbing Code.

    COEFFICIENT OF PERFORMANCE (COP), COOLING is the ratio of the rate of net heat removal to the rate of total energy input, calculated under designated operating conditions and expressed in consistent units, as determined using the applicable test method in the Appliance Efficiency Regulations or Section 110.2.

    COEFFICIENT OF PERFORMANCE (COP), HEATING is the ratio of the rate of net heat output to the rate of total energy input, calculated under designated operating conditions and expressed in consistent units, as determined using the applicable test method in the Appliance Efficiency Regulations or Section 110.2.

    COEFFICIENT OF PERFORMANCE (COP), HEAT PUMP is the ratio of the rate of useful heat output delivered by the complete heat pump unit (exclusive of supplementary heating) to the corresponding rate of energy input, in consistent units and as determined using the applicable test method in Appliance Efficiency Regulations or Section 110.2.

    COMBINED ENERGY EFFICIENCY RATIO (CEER) is the ratio of net cooling capacity (in Btu/hr) to total rate of electrical energy input (in watts) of a cooling system under designated operating conditions, including standby mode, as determined using the applicable test method in the Appliance Efficiency Regulations.

    COMBUSTION AIR POSITIVE SHUT-OFF is a means of restricting air flow through a boiler combustion chamber during standby periods, used to reduce standby heat loss. A flue damper and a vent damper are two examples of combustion air positive shut-off devices.

    COMBUSTION EFFICIENCY is a measure of the percentage of heat from the combustion of gas or oil that is transferred to the medium being heated or lost as jacket loss.

    COMMERCIAL BOILER is a type of boiler with a capacity (rated maximum input) of 300,000 Btus per hour (Btu/h) or more and serving a space heating or water heating load in a commercial building.

    COMMISSION is the California State Energy Resources Conservation and Development Commission, which is also referred to as the California Energy Commission.

    COMPARTMENTALIZATION is when a dwelling unit enclosure area, including walls, ceilings, and floors shared with exterior spaces or adjacent spaces in the building, including but not limited to neighboring units, corridors, and elevator shafts, is constructed to prevent air leakage.

    10 2025 CALIFORNIA ENERGY CODE

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    ALL OCCUPANCIES—GENERAL PROVISIONS

  • CMC § 920-2020 Medium relevance — show source text

    of Indoor Pool Dehumidifiers|Dehumidifiers| |AHRI 920-2020|Performance Rating of Direct Expansion-Dedicated Outdoor Air System Units|Air Conditioners| |AHRI 1200-2013|Performance Rating of Commercial Refrigerated Display Merchandisers and
    Storage Cabinets|Commercial| |AHRI 1230-2021|Performance Rating of Variable Refrigerant Flow (VRF) Multi-Split Air-Condi-
    tioning and Heat Pump Equipment|Refrigerants| |AHRI 1250-2020|Performance Rating of Walk-in Coolers and Freezers|Walk-in Coolers and
    Freezers| |AHRI 1360-2017|Performance Rating of Computer and Data Processing Room Air Conditioners|Air Conditioners| |AMCA|AMCA|AMCA| |AMCA 208-2018|Calculation of the Fan Energy Index|Fan Energy Index| |ANSI/AMCA 210-
    2016/ASHRAE 51-2016|Laboratory Methods of Testing Fans for Certified Aerodynamic Performance
    Rating|Low Power Fans| |ANSI/AMCA 220-2021|Laboratory Methods of Testing Air Curtain Units for Aerodynamic Perform-
    ance Rating|Air Curtain Units| |AMCA 500-D-2018|Laboratory Methods of Testing Dampers for Rating|Dampers| |ASHRAE|ASHRAE|ASHRAE| |ASHRAE 52.2-2017|Method of Testing General Ventilation Air-Cleaning Devices for Removal
    Efficiency by Particle Size|Cleaning Devices| |ASHRAE 55-2020|Thermal Environmental Conditions for Human Occupancy|Miscellaneous|

    378 2025 CALIFORNIA MECHANICAL CODE

    ), Copyright © 2025 IAPMO, and may not be used for any other purpose or distributed to any other persons or parties.

    REFERENCED STANDARDS

    TABLE 1801.2 (continued) « STANDARDS, PUBLICATIONS, PRACTICES, AND GUIDES

    DOCUMENT NUMBER DOCUMENT TITLE APPLICATION
    ASHRAE 62.2-2022 Ventilation and Acceptable Indoor Air Quality in Residential Buildings Ventilation
    ANSI/ASHRAE/IES 90.1-
    2019    		 Energy Standard for Buildings Except Low-Rise Residential Buildings (with
    Addenda a and d)    		 Energy
    ASHRAE/IES 90.2-2018 Energy-Efficient Design of Low-Rise Residential Buildings Energy, Dwellings
    ASHRAE 90.4-2019 Energy Standard for Data Centers Data Centers
    ASHRAE 127-2020 Method of Testing for Rating Air-Conditioning Units Serving Data Center
    (DC) and Other Information Technology Equipment (ITE) Spaces    		 Air Conditioners
    ASHRAE 129-1997 (R2002) Measuring Air-Change Effectiveness Air Change Effectiveness
    ASHRAE GRP 158-1979 Cooling and Heating Load Calculation Manual Ventilation
    ASHRAE 169-2021 Climatic Data for Building Design Standards Miscellaneous
    ASHRAE/ACCA 183-
    2007 (R2020)    		 Peak Cooling and Heating Load Calculations in Buildings Except Low-
    Rise Residential Buildings    		 Cooling and Heating
    Load,

Frequently asked questions

Can I use CMC ventilation rates if the Energy Code covers my building?

No. If the Energy Code regulates the occupancy, the CEC controls ventilation rates and energy-related system requirements — the CMC explicitly states that CEC provisions supersede CMC § 402.1.

Where do I show outdoor‑air values on the drawings?

The CMC requires the outdoor‑air ventilation rate and air distribution assumptions to be clearly identified on the construction documents (see § 402.1.1).

If I install an ERV/HRV, which code governs recovery efficiency requirements?

The Energy Code sets energy‑recovery performance requirements (minimum sensible/enthalpy ratios and bypass/control rules). The CMC governs mechanical installation details, but the Energy Code dictates when and what recovery performance is required.

Are duct sealing and testing CMC or CEC issues?

Both: the CEC requires acceptance testing in defined situations and references the CMC for duct construction rules. The CMC contains the duct construction and sealing requirements; the CEC determines when testing is required and the acceptance procedures.

What about ventilating healthcare spaces?

Healthcare ventilation is addressed in CMC Table 4‑A and ASHRAE/ASHE 170; where those differ, the more restrictive requirement applies. Check the CMC health‑care provisions carefully.

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