701.7 Combination Indoor and Outdoor Combustion Air — 2025 California Mechanical Code

Air. The use of a combination of indoor and outdoor com bustion air shall be in accordance with Section 701.7.1 through Section 701.7.3. [NFPA 54:9.3.4] (see Appendix F for example calculations)

FIGURE 701.6.1(1) ALL COMBUSTION AIR FROM OUTDOORS – INLET

AIR FROM VENTILATED CRAWL SPACE AND OUTLET AIR TO

VENTILATED ATTIC

[NFPA 54: FIGURE A.9.3.3.1(1)(a)]

FIGURE 701.6.1(2)

ALL COMBUSTION AIR FROM OUTDOORS

THROUGH VENTILATED ATTIC

[NFPA 54: FIGURE A.9.3.3.1(1)(b)]

148 2025 CALIFORNIA MECHANICAL CODE

COMBUSTION AIR

FIGURE 701.6.1(3) ALL COMBUSTION AIR FROM OUTDOORS

THROUGH HORIZONTAL DUCTS

[NFPA 54: FIGURE A.9.3.3.1(2)]

FIGURE 701.6.2

ALL COMBUSTION AIR FROM OUTDOORS THROUGH

SINGLE COMBUSTION AIR OPENING

[NFPA 54: FIGURE A.9.3.3.2]

701.7.1 Indoor Openings. Where used, openings connecting the interior spaces shall comply with Section 701.5. [NFPA 54:9.3.4(1)]

701.7.2 Outdoor Opening(s) Location. Outdoor opening(s) shall be located in accordance with Section 701.6. [NFPA 54:9.3.4(2)]

701.7.3 Outdoor Opening(s) Size. The outdoor opening(s) size shall be calculated in accordance with the following:

(1) The ratio of the interior spaces shall be the available volume of all communicating spaces divided by the required volume.

(2) The outdoor size reduction factor shall be 1 minus the ratio of interior spaces.

(3) The minimum size of outdoor opening(s) shall be the full size of outdoor opening(s) calculated in accordance with Section 701.6, multiplied by the reduction factor. The minimum dimension of air openings shall not be less than 3 inches (76 mm).

[NFPA 54:9.3.4(3)]