403.6 Design for Varying Operating Conditions — 2025 California Mechanical Code

Ventilation systems shall be designed to be capable of providing not less than the minimum ventilation rates required in the breathing zone where the zones served by the system are occupied, including all full- and part-load conditions. The minimum outdoor air intake flow shall be permitted to be less than the design value at part-load conditions. [ASHRAE 62.1:6.2.5 – 6.2.5.1]

403.6.1 Short-Term Conditions. Where it is known that peak occupancy will be of short duration, ventilation will be varied or interrupted for a short period of time, or both, the design shall be permitted to be based on the average conditions over a time period ( T ) determined by Equation 403.6.1.

T = 3 v/Vbz (Equation 403.6.1)

Where:

T = averaging time period, minutes.

v = the volume of the ventilation zone where averaging is being applied, ft [3] (m [3] ).

Vbz = the breathing zone outdoor airflow calculated in accordance with Equation 403.2.1 and design value of the zone population ( Pz ), cubic foot per minute (CFM).

For SI Units: 1 cubic foot = 0.0283 m [3], 1 cubic foot per minute =

0.0283 m [3] /min, 1 cubic foot per minute = 0.4719 L/s

Acceptable design adjustments based on this optional provision including the following:

(1) Zones with fluctuating occupancy: The zone population ( Pz ) shall be permitted to be averaged over time ( T ).

(2) Zones with intermittent interruption of supply air: The average outdoor airflow supplied to the breathing zone over time ( T ) shall be not less than the breathing zone outdoor airflow ( Vbz ) calculated using Equation 403.2.1.

(3) Systems with intermittent closure of the outdoor air intake: The average outdoor air intake over time ( T ) shall be not less than the minimum outdoor air intake ( Vot ) calculated using Equation 403.3, Equation 403.4, or Equation 403.5.1 as applicable.

[ASHRAE 62.1:6.2.5.2]