402.3 Mechanical Ventilation — 2025 Mechanical Code (CMC)

[Not permitted for OSHPD 1, 2, 3, 4 & 5] Where natural ventilation is not permitted by this section or the California Building Standards Code, mechanical ventilation systems shall be designed, constructed, and installed to provide a method of supply air and either return air or exhaust air, or both, as applicable. Mechan

	TABLE 402.2.1.6(A)(2) MINIMUM OPENABLE AREAS: TWO VERTICALLY SPACED OPENINGS* [ASHRAE 62.1: TABLE 6-6]
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	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)	Hvs ≤ 8.2 ft	Hvs ≤ 8.2 ft	8.2 ft <**Hvs ≤ 16.4 ft**	8.2 ft <**Hvs ≤ 16.4 ft**	16.4 ft <****Hvs	16.4 ft <****Hvs
Vbz/Az ≤ (CFM/ft2)	As/Al ≤ 0.5	As/Al > 0.5	As/Al ≤ 0.5	As/Al > 0.5	As/Al ≤ 0.5	As/Al > 0.5
0.2	2.0	1.3	1.3	0.8	0.9	0.6
0.4	4.0	2.6	2.5	1.6	1.8	1.2
0.6	6.0	3.9	3.8	2.5	2.7	1.7
0.8	8.0	5.2	5.0	3.3	3.6	2.3
1.1	11.0	7.1	6.9	4.5	4.9	3.2

For SI unit: 1 cubic foot per minute = 0.0283 m [3] /min, 1 cubic foot per minute per square foot = 5.08 [(L/s)/m [2] ], 1 square foot = 0.0929 m [2], 1 foot = 304.8 mm,

Where:

Vbz = breathing zone outdoor airflow, per Table 402.1. Az = zone floor area, the net occupiable floor area of the ventilation zone. Hvs = vertical separation between the center of the top and bottom openings’ free operable area; in case of multiple horizontally spaced pairs of openings, use shortest distance encountered. As = openable area of smallest opening (top or bottom); in case of multiple horizontally spaced pairs of top-and-bottom openings, use aggregated areas. Al = openable area of largest opening (top or bottom); in case of multiple horizontally spaced pairs of top-and-bottom openings, use aggregated areas.

Volumetric airflow rates used to estimate required operable 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

ical ventilation systems shall include controls, manual or automatic, that enable the fan system to operate wherever the spaces served are occupied. The system shall be designed to maintain minimum outdoor airflow as required by Section 403.0 under any load conditions.