402.2 Natural Ventilation Procedure — 2025 California Mechanical Code

[Not permitted for OSHPD 1, 2, 3, 4 & 5] Natural ventilation systems shall comply with the requirements of either Section 402.2.1 through Section 402.2.1.6(A) or Section 402.2.2. Designers shall provide interior air barriers, insulation, or other means that separate naturally ventilated spaces from mechanically cooled spaces to prevent high-dew-point outdoor air from coming into contact with mechanically cooled surfaces.

[ASHRAE 62.1:6.4]

402.2.1 Prescriptive Compliance Path. Any zone designed for natural ventilation shall include a mechanical ventilation system designed in accordance with Section 403.0.

Exceptions:

(1) Zones in buildings that have all of the following:

(a) Natural ventilation openings that comply with the requirements of Section 402.2.1.

(b) Controls that prevent the natural ventilation openings from being closed during periods of expected occupancy, or natural ventilation openings that are permanently open.

(2) Zones that are not served by heating or cooling equipment. {ASHRAE 62.1:6.4.1}

402.2.1.1 Ceiling Height. For ceilings that are parallel to the floor, the ceiling height ( H ) to be used in Section 402.2.1.3 through Section 402.2.1.5 shall be the minimum ceiling height in the zone.

For zones where ceiling height increases as distance from the openings is increased, the ceiling height shall be determined as the average height of the ceiling within 20 feet (6096 mm) from the operable openings. [ASHRAE 62.1:6.4.1.1]

402.2.1.2 Floor Area to be Ventilated. The nat urally ventilated area in zones or portions of zones shall extend from the openings to a distance determined in accordance with Section 402.2.1.3, Section 402.2.1.4, or Section 402.2.1.5. Openings shall be in accordance with the requirements of Section 402.2.1.6.

For zones where ceilings are not parallel to the floor, the ceiling height shall be determined in accordance with Section 402.2.1.1. [ASHRAE 62.1:6.4.1.2]

402.2.1.3 Single Side Opening. For zones with openings on only one side of the zone, the naturally ventilated area shall extend to a distance not greater than 2 times the height of the ceiling from the openings. [ASHRAE 62.1:6.4.1.3]

402.2.1.4 Double Side Opening. For zones with openings on two opposite sides of the zone, the naturally ventilated area shall extend between the openings separated by a distance not greater than 5 times the height of the ceiling. [ASHRAE 62.1:6.4.1.4] 402.2.1.5 Corner Openings. For zones with operable openings on two adjacent sides of a zone, the distance from the operable openings shall be not more than 5 H along a line drawn between the two openings that are farthest apart. Floor area outside that line shall comply with Section 402.2.1.1 as a zone having openings on only one side of the zone.

[ASHRAE 62.1:6.4.1.5]

402.2.1.6 Location and Size of Openings. Zones or portions of zones to be naturally ventilated shall have a permanently open airflow path to openings directly connected to the outdoors. The minimum flow rate to the zone shall be determined in accordance with

Section 403.2.1. This flow rate shall be used to deter mine the required openable area of openings, accounting only for buoyancy-driven flow. Wind-driven flow shall be used only where it can be demonstrated that the minimum flow rate is provided during all occupied hours. Openings shall be sized in accordance with Section 402.2.1.6(A). Permanently open airflow path shall include, but not be limited to, pathways that would allow airflow unimpeded by partitions, walls, and furnishings. {ASHRAE 62.1:6.4.1.6}

402.2.1.6(A) Sizing Openings. Where the zone is ventilated using a single opening or multiple single openings located at the same elevation, the openable area as a percent of the net occupiable floor area shall be greater than or equal to the value indicated in Table 402.2.1.6(A)(1). Where the zone is ventilated using two openings located at different elevations or multiple pairs of such openings, the openable area as a percent of the net occupiable floor area shall be greater than or equal to the value indicated in Table 402.2.1.6(A)(2).

Where openings are obstructed by louvers or screens, the openable area shall be based on the net free area of the opening. Where interior zones, or portions of zones, without direct openings to the outdoors are ventilated through adjoining zones, the opening between zones shall be permanently unobstructed and have a free area of not less than twice the percent of occupiable floor area used to determine the opening size of adjacent exterior zones, or 25 square feet (2.3 m [2] ), whichever is greater. Table 402.2.1.6(A)(1) and Table 402.2.1.6(A)(2) are based on buoyancy-driven flow and shall not address thermal comfort. [ASHRAE 62.1:6.4.1.6.1]

402.2.2 Engineered System Compliance Path. For an engineered natural ventilation system, the following shall be included:

(1) Determine hourly environmental conditions, including outdoor air dry-bulb temperature; dew-point temperature; outdoor concentration of contaminants, including PM2.5, PM10, and ozone where data are available; wind speed and direction; and internal heat gains during expected hours of natural ventilation operation.

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TABLE 402.2.1.6(A)(1) MINIMUM OPENABLE AREAS: SINGLE OPENINGS*

For SI units: 1 cubic foot per minute = 0.0283 m [3] /min, 1 cubic foot per minute = 0.4719 L/s, 1 cubic foot per minute per square foot = 5.08

[(L/s)/m [2] ], 1 square foot = 0.0929 m [2]

Where:

Vbz = breathing zone outdoor airflow, per Table 402.1. Az = zone floor area, the net occupiable floor area of the ventilation zone. W S = aggregated width of all single outdoor openings located at the same elevation.

H S = vertical dimension of the single opening or the least vertical dimension of the openings where there are multiple openings.

• Volumetric airflow rates used to estimate required openable area are based on the following:

• Dry-air density of 0.075 lbda/ft [3] (1.2 kgda/m [3] ) at a barometric pressure of 1 atm (101.3 kPa) and an air temperature of 70°F (21°C)

• Temperature difference between indoors and outdoors of 1.8°F (1°C)

• Gravity constant of 32.2 ft/s [2] (9.81m/s [2] )

• Window discharge coefficient of 0.6

(2) Determine the effect of pressure losses along natural ventilation airflow paths on the resulting flow rates, including inlet openings, air transfer grills, ventilation stacks, and outlet openings during representative conditions of expected natural ventilation system use.

(3) Quantify natural ventilation airflow rates of identified airflow paths accounting for wind induced and thermally induced driving pressures during representative conditions of expected natural ventilation system use.

(4) Design to provide outdoor air in quantities sufficient to result in acceptable IAQ as established under Section 403.2.1 or ASHRAE 62.1 during representative conditions of expected natural ventilation system use. {ASHRAE 62.1:6.4.2}

402.2.3 Control and Accessibility. The means to open required operable openings shall be readily accessible to building occupants whenever the space is occupied. Controls shall be designed to coordinate operation of the natural and mechanical ventilation systems.

[ASHRAE 62.1:6.4.3]