402.2 Natural Ventilation Procedure

[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]

70 2025 CALIFORNIA MECHANICAL CODE

VENTILATION AIR

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

	[ASHRAE 62.1: TABLE 6-5]
Vbz/Az ≤ (CFM/ft2)	TOTAL OPENABLE AREAS IN ZONE AS A PERCENTAGE OFAz	TOTAL OPENABLE AREAS IN ZONE AS A PERCENTAGE OFAz	TOTAL OPENABLE AREAS IN ZONE AS A PERCENTAGE OFAz
Vbz/Az ≤ (CFM/ft2)	HS/WS ≤ 0.1	0.1 <**HS/WS ≤ 1**	HS/WS > 1
0.2	4.0	2.9	2.5
0.4	6.9	5.0	4.4
0.6	9.5	6.9	6.0
0.8	12.0	8.7	7.6
1.1	15.5	11.2	9.8

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]