Section 403.3/Outdoor Air Rate
IMC 403.3 and Table 403.3 specify minimum outdoor air ventilation rates by occupancy type for HVAC system design.
Mechanical ventilation systems must provide outdoor air at the rates specified in Table 403.3. Rates are given per person and per unit area depending on the occupancy type. For offices, the rate is 5 cfm per person plus 0.06 cfm per square foot. For classrooms, 10 cfm per person plus 0.12 cfm per square foot. For retail, 7.5 cfm per person plus 0.12 cfm per square foot. The total outdoor air for each zone is the sum of the people-based and area-based components.
Why this section exists
The outdoor air rates in Table 403.3 are based on research into the ventilation levels needed to dilute occupant-generated contaminants (CO2, bioeffluents) and building-generated contaminants (off-gassing from materials, cleaning products). The two-component approach (per person plus per area) accounts for both sources. Insufficient outdoor air leads to sick building syndrome, elevated CO2, and poor indoor air quality complaints.
What plan reviewers look for
Plan reviewers check the ventilation schedule or outdoor air calculation on the mechanical drawings. They verify that the correct occupancy category from Table 403.3 is used for each space, that the occupant count matches the architectural occupant load, and that the total outdoor air rate is reflected in the air handling unit specifications.
Common violations
Related IMC requirements
Section 401.2 establishes when mechanical ventilation is required.Section 403.3.1 covers the ventilation rate procedure for multi-zone systems. ASHRAE 62.1 provides the complete ventilation rate procedure that is the basis for Table 403.3.