In this article, details about Title 24 ventilation calculation and key differences between ASHRAE Standard 62.1 and California Title 24 Commercial code ventilation calculation methods are highlighted. The full article for performing ventilation calculations using ASHRAE standard 62.1 Ventilation Rate Procedure (VRP) can be found here.
California Title 24 2022
Section 120.1 of Title 24 lists the requirements for Ventilation and Indoor Air Quality for different building use types. The outdoor airflow rate to the zone (Vz) is determined according to the following equations:
Vz = Ra * Az (Equation 120.1 - F)
Where:
Ra (Unit: CFM/ft2 or L/s/m2) = outdoor airflow rate required per unit area as determined from Table 120.1-A.
Az (Unit: ft2 or m2) = Net occupiable floor area of the ventilation zone.
Exception to Equation 120.1 - F
For spaces designed for an expected number of occupants per the Exception to Section 1004.5 of the California Building Code (CBC), or spaces with fixed seating per Section 1004.6 of the CBC, the outdoor airflow rate to the zone (Vz) shall be determined in accordance with Equation 120.1 - G
Vz = max of Rp * Pz or Rz * Az (Equation 120.1 - G)
Where:
Rp = 15 CFM (7.07 l/s) of outdoor airflow per person
Pz = Expected number of occupants.
Key Differences between ASHRAE 62.1 VRP and Title 24 Ventilation Calculation
For ASHRAE Standard 62.1, the outdoor airflow required (Vbz) is based on the summation of people-related sources and area-related sources. The equation for the calculation of Vbz can be found in the article referenced at the top. Also, the air distribution effectiveness (Ez) varies based on the system type used (table 6.2.2.2).
For Title 24 ventilation calculations, the outdoor airflow is largely based on area-related sources (equation 120.1 - F), and in exceptional cases the maximum of area-related or people-related sources (equation 120.1 - G). The air distribution effectiveness (Ez) is not part of the calculations and can be set to 1 for Title 24 related calculations.
Performing Title 24 Ventilation Calculation
In order to represent Title 24 Ventilation Calculations on the platform minor modifications can be made to the inputs. First, in each room templates enter the required Rp and Rz into fields 1 and 2 respectively shown below.
Next set bot the Cooling and Heating Air Distribution Effectiveness to 1 in fields 7 and 8 above. This essentially removes the Ez from the calculations.
Finally, the minimum exhaust rate based on the occupancy category (Title 24 Table 120.1 B) can be adjusted either with fields 5 or 6 shown above.