We're turning Section 403 into a four-part series. This is where plan reviews slow down: misapplied people and area rates, recirculation violations, and undersized multi-zone intakes that surface at TAB and commissioning.
We'll cover 403 in four parts:
Part 1 (403.1-403.2.2): System basics-supply and return or exhaust; Group R-2/R-3/R-4 options; engineered reduction path; "do not recirculate required outdoor air"; special rules for pools and table-flagged spaces; transfer air allowances.
Part 2 (403.3.1.1 and Table 403.3.1.1): Breathing-zone foundation-pick the correct occupancy, set design occupant load, calculate people and area components, deliver to the breathing zone, and handle unlisted spaces.
Part 3 (403.3.1.1.1-403.3.1.1.2, 403.3.1.3-403.3.1.4): Zone effectiveness and system intake-apply Ez, calculate Voz and Vot for single-zone, one hundred percent outdoor air, and multi-zone recirculating systems using Ev and diversity; exhaust tie-ins; DCV floors at Ra and VAV outdoor air tracking across turndown.
Part 4 (403.3.2 for Group R-2/R-3/R-4): Whole-house outdoor air with Equation 4-9, corridor outdoor air, required local exhaust capacities, intermittent versus continuous control allowances, control labeling, and fan listing per AMCA 210-ASHRAE 51.
On Plans: Why It Matters
Common redlines: using occupant density lighter than the table, recirculating required outdoor air or prohibited airstreams (pool, nail salon, note b or note g spaces), skipping Ez/Ev so the multi-zone intake is low, DCV set below the area-rate floor, and misapplied Equation 4-9 or missing local exhaust in Group R.
Part 1 drops tomorrow.
Code Path: IMC Section 403.1 (System Basis) -> Section 403.2 (Return/Exhaust) -> Table 403.3.1.1 (Breathing Zone Rates) -> Section 403.3.1.1.1-403.3.1.1.2 (Zone Effectiveness, System OA) -> Section 403.3.2 (Group R-2/R-3/R-4). For multi-zone systems, apply Eq. 4-3 or 4-4 for system OA.
Section 403.1: Both supply and return or exhaust are required for every mechanical ventilation system.
Table 403.3.1.1: Rates are per person (Rp) and per area (Ra) - both apply simultaneously. Neither alone is sufficient.
Section 403.3.1.1.2: Ez values range from 0.5 to 1.2 depending on diffuser type and throw direction.
Check: Before You Submit
Confirm the ventilation calculation is included in the permit package with occupancy, density, OA cfm, and Ez values shown.
Verify equipment schedules show minimum OA cfm and the basis (table row, density, and floor area used).
Confirm multi-zone calculations use Eq. 4-3 or 4-4 and show the critical zone identification.
Review Risk:
OA cfm on the equipment schedule does not match the ventilation calculation - the most common 403 plan review comment.
Using a lighter occupant density than Table 403.3.1.1 without documented justification.
Recirculating required outdoor air through fan coils or VAV boxes without showing ventilation effectiveness compliance.
Masterbuild QA Lens
Mechanical ventilation compliance starts with occupant load and occupancy classification - not fan cfm. Confirm the ventilation calculations tie back to a table-driven OA rate per Table 403.3.1.1 or an ASHRAE 62.1 calculation, and that the cfm shown on the equipment schedule matches the calculation sheet. Undocumented OA values are the most common ventilation redline.
Drawing / Submittal Check
For the 403 kickoff compliance set: the ventilation calculation must be included in the permit set (not just referenced) showing occupancy type, floor area, design occupant load, Table 403.3.1.1 OA rate, and resulting cfm. Every AHU must show OA cfm on M-601. The calculation must trace from the table to the schedule - a reviewer who cannot connect the two will write a comment.
Common Review Risk
The most common 403 comment: the OA cfm on the equipment schedule does not match the ventilation calculation, or the calculation exists but is not included in the permit set. Either gap generates the same comment: 'provide ventilation calculation demonstrating compliance with Table 403.3.1.1.' Include the calculation in the permit set and confirm the equipment schedule matches it before submittal.
When To Escalate
Escalate when the mechanical ventilation system for a large building has not been balanced - when the test and balance report shows outdoor air rates that do not match the design at multiple zones. Ventilation systems that are not balanced to the design rates are not compliant regardless of whether the equipment is correctly sized. A rebalancing scope is required.
Load Assumption Check
Section 403 ventilation rates are the starting point for OA CFM in every occupied space. Confirm occupancy type, design occupant density, and area-per-person assumptions before the ventilation schedule is built - errors here propagate through the entire load calculation.
Duct System Coordination
The 403 series covers ventilation rates that set the OA fraction for every air-handling system in the project. Duct sizing downstream of the OA damper depends on getting these rates right from the start.