701.4 Indoor Combustion Air — 2025 California Mechanical Code

The required volume of indoor air shall be determined in accordance with the method

in Section 701.4.1 or Section 701.4.2 except that where the air infiltration rate is known to be less than 0.40 ACH (air change per hour), the method in Section 701.4.2 shall be used. The total required volume shall be the sum of the required volume calculated for appliances located within the space. Rooms communicating directly with the space in which the appliances are installed through openings not furnished with doors, and through combustion air openings sized and located in accordance with Section 701.5, are considered a part of the required volume. [NFPA 54:9.3.2]

701.4.1 Standard Method. The minimum required volume shall be 50 cubic feet per 1000 British thermal units per hour (Btu/h) (4.83 m [3] /kW). [NFPA 54:9.3.2.1]

701.4.2 Known Air Infiltration Rate Method. Where

the air infiltration rate of a structure is known, the minimum required volume shall be determined as follows

[NFPA 54:9.3.2.2]:

(1) For appliances other than fan-assisted, calculate using the following Equation 701.4.2(1). [NFPA 54:9.3.2.2(1)]

[Equation 701.4.2(1)]

I other

Required Volume other ≥ [21 ft] ACH [3] 1000 Btu/h

( )

(2) For fan-assisted appliances, calculate using the following Equation 701.4.2(2). [NFPA 54:9.3.2.2(2)]

[Equation 701.4.2(2)]

15 ft [3] I

fan

Required Volume fan ≥ ACH 1000 Btu/h

( )

Where:

I other = All appliances other than fan-assisted input (Btu/h) (kW) I = Fan-assisted appliance input (Btu/h) (kW) fan ACH = Air change per hour (percent of volume of space exchanged per hour, expressed as a decimal)

Required Volume other ≥ [21 ft] [3]

ACH

(

I other

[21 ft] [3]

ACH 1000 Btu/h

( )

15 ft [3] Required Volume fan ≥ ACH

(

15 ft [3] I

fan ACH 1000 Btu/h

( )

For SI units: 1 cubic foot = 0.0283 m [3], 1000 British thermal units per

hour = 0.293 kW

(3) For purposes of these calculations, an infiltration rate greater than 0.60 ACH shall not be used in Equation 701.4.2(1) and Equation 701.4.2(2). [NFPA 54:9.3.2.2(3)]