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Earlier editions: 20222019

Chapter 4 — VENTILATION AIR

403.6 Design for Varying Operating Conditions.

California Mechanical Code (Title 24, Part 4) · 2025 edition · updated 2026-09-10 · California

Italicized text is a California amendment to the model code, as printed in the official publication.

Ventilation systems shall be designed to be capable of providing not less than the minimum ventilation rates required in the breathing zone where the zones served by the system are occupied, including all full- and part-load conditions. The minimum outdoor air intake flow shall be permitted to be less than the design value at part-load conditions. [ASHRAE 62.1:6.2.5 – 6.2.5.1]

403.6.1 Short-Term Conditions. Where it is known that peak occupancy will be of short duration, ventilation will be varied or interrupted for a short period of time, or both, the design shall be permitted to be based on the average conditions over a time period ( T ) determined by Equation 403.6.1.

T = 3 v/Vbz (Equation 403.6.1)

Where:

T = averaging time period, minutes.

v = the volume of the ventilation zone where averaging is being applied, ft [3] (m [3] ).

Vbz = the breathing zone outdoor airflow calculated in accordance with Equation 403.2.1 and design value of the zone population ( Pz ), cubic foot per minute (CFM).

For SI Units: 1 cubic foot = 0.0283 m [3], 1 cubic foot per minute =

0.0283 m [3] /min, 1 cubic foot per minute = 0.4719 L/s

Acceptable design adjustments based on this optional provision including the following:

(1) Zones with fluctuating occupancy: The zone population ( Pz ) shall be permitted to be averaged over time ( T ).

(2) Zones with intermittent interruption of supply air: The average outdoor airflow supplied to the breathing zone over time ( T ) shall be not less than the breathing zone outdoor airflow ( Vbz ) calculated using Equation 403.2.1.

(3) Systems with intermittent closure of the outdoor air intake: The average outdoor air intake over time ( T ) shall be not less than the minimum outdoor air intake ( Vot ) calculated using Equation 403.3, Equation 403.4, or Equation 403.5.1 as applicable.

[ASHRAE 62.1:6.2.5.2]

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▸Contents — California Mechanical Code (Title 24, Part 4)
California Mechanical Code (Title 24, Part 4)
  1. Chapter 1 — GENERAL CODE PROVISIONS
  2. Chapter 2 — DEFINITIONS
  3. Chapter 3 — GENERAL REGULATIONS
  4. ▸Chapter 4 — VENTILATION AIR
    Overview
    1. 401.1 Applicability. This chapter contains requirements for
    2. 401.2 Indoor Swimming Pools. The design of ventilation
    3. 401.3 Filters. [BSC-CG & DSA-SS] In mechanically ven-
    4. 402.1 Occupiable Spaces. Occupiable spaces listed in
    5. 402.2 Natural Ventilation Procedure. [Not permitted
    6. 402.3 Mechanical Ventilation. [Not permitted for
    7. 402.4 Outdoor Air Intakes. Ventilation system outdoor
    8. 403.1 General. The design outdoor air intake flow rate for
    9. 403.2 Zone Calculations. Ventilation zone parameters
    10. 403.3 Single-Zone Systems. For ventilation systems
    11. 403.4 One Hundred Percent Outdoor Air Systems.
    12. 403.5 Multiple-Zone Recirculating Systems. For ventilation sys…
    13. 403.6 Design for Varying Operating Conditions. Ventilation sys…
    14. 403.7 Exhaust Ventilation. Exhaust airflow shall be provided i…
    15. 403.8 Dynamic Reset. The system shall be permitted to be
    16. 403.9 Air Classification and Recirculation. Air shall be
    17. 403.10 Air Balance. All mechanical ventilation systems
    18. 404.1 System Ventilation Efficiency. The system ventilation ef…
    19. 404.2 Average Outdoor Air Fraction. The average outdoor air fr…
    20. 404.3 Zone Ventilation Efficiency. The zone ventilation
    21. 405.1 General. Rooms or occupied spaces within residential occ…
    22. 405.2 Ventilation Air Rate. The required mechanical ventilatio…
    23. 405.3 Bathroom Exhaust. A mechanical exhaust directly
    24. 405.4 Kitchen Exhaust. A mechanical exhaust system that
    25. 405.5 Ventilation Openings. Occupiable spaces shall be
    26. 406.0 Evaporative Cooling System for Health Care
    27. 407.1 General.
    28. 407.2 Outdoor Air Intakes and Exhaust Outlets.
    29. 407.3 Air Balance.
    30. 407.4 Air Circulation.
    31. 407.5 Variable Air Volume.
    32. 407.6 Economizers.
    33. 407.7 Unoccupied Turndown.
    34. 408.3 Filters for Skilled Nursing Facilities, Intermedi-
    35. 408.4 Filters for Outpatient Facilities.
    36. 408.5 Filters for Recirculating Room Units.
    37. 409.1 Ducts which penetrate construction, intended for X-
    38. 409.2 Duct linings and their use shall meet the requirements
    39. 409.3 The anchorage and supporting structural elements for
    40. 410.1 Laboratory Ventilating Systems. Laboratory ven-
    41. 410.2 Exhaust Hoods and Safety Cabinets. Hoods and
    42. 410.3 Laboratory Fume Hoods. Laboratory fume hoods
    43. 407.8 Building Pressurization.
    44. 408.1 General. Filter efficiencies shall be certified by the
    45. 408.2 Filters for Hospitals.
    46. 411.1 The air from dining areas may be used to ventilate the
    47. 412.1 Boiler, heater and electrical equipment rooms shall be
    48. 412.2 Floor surfaces in occupied spaces above such rooms
    49. 413.1 Rooms in areas where excessive heat or moisture is
    50. 413.2 Kitchen, morgues and laundries located inside a hos-
    51. 414.1 Exhaust Systems. A separate, dedicated exhaust
    52. 414.2 Air Distribution. The supply outlets and exhaust
    53. 415.1 Air Distribution. The supply outlets and exhaust and
    54. 416.1 An alarm system which is based on static pressure con-
    55. 416.2 Other acceptable alarm systems will be allowed when
    56. 416.3 [OSHPD 4] For correctional treatment centers, the
    57. 416.4 Prior to acceptance of the rooms, the alarm system
    58. 417.0 Testing and Balancing Airborne Infection Isola-
    59. 418.1 Air Changes. The ETO sterilization equipment room
    60. 418.2 Exhaust Requirements.
    61. 418.3 Ventilation Requirements.
    62. 418.4 Gas Valves. Installation of gas line hand valves at the
    63. 418.5 Alarm Systems. An Audible and visual alarm system
    64. 419.1 Formula Preparation Area. Air shall be supplied
    65. 419.2 Treatment Area/Room. Air shall be supplied over
  5. Chapter 5 — EXHAUST SYSTEMS
  6. Chapter 6 — DUCT SYSTEMS
  7. Chapter 7 — COMBUSTION AIR
  8. Chapter 8 — CHIMNEYS AND VENTS
  9. Chapter 9 — INSTALLATION OF SPECIFIC APPLIANCES
  10. Chapter 10 — BOILERS AND PRESSURE VESSELS
  11. Chapter 11 — REFRIGERATION
  12. Chapter 12 — HYDRONICS
  13. Chapter 13 — FUEL GAS PIPING
  14. Chapter 14 — PROCESS PIPING
  15. Chapter 15 — SOLAR ENERGY SYSTEMS
  16. Chapter 16 — STATIONARY POWER PLANTS
  17. Chapter 17 — GEOTHERMAL ENERGY SYSTEMS AND AMBIENT TEMPERATURE…
  18. Chapter 18 — REFERENCED STANDARDS
  19. Appendix A — RESIDENTIAL PLAN EXAMINER REVIEW FORM FOR HVAC SY…
  20. Appendix B — PROCEDURES TO BE FOLLOWED TO PLACE GAS EQUIPMENT …
  21. Appendix C — INSTALLATION AND TESTING OF OIL (LIQUID) FUEL-FIR…
  22. Appendix D — FUEL SUPPLY: MANUFACTURED/MOBILE HOME PARKS AND R…
  23. Appendix E — SUSTAINABLE PRACTICES
  24. Appendix F — SIZING OF VENTING SYSTEMS AND OUTDOOR COMBUSTION …
  25. Appendix G — EXAMPLE CALCULATION OF OUTDOOR AIR RATE
  26. Appendix H — PROFESSIONAL QUALIFICATIONS
  27. Appendix I — INDOOR HORTICULTURAL FACILITIES
  28. Appendix J — CLEAN AIR DELIVERY

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