/ Field knowledge · IMC 403.3.2
Townhouses do not use the office OA table.
IMC 403.3.2: Group R-2 / R-3 / R-4, three stories or fewer. Whole-house OA is Equation 4-9, not Table 403.3.1.1. The 30% reduction needs OA ducted to each bedroom and at least one of the living room, dining room, or kitchen — and a balanced system.
Educational 2024 IMC 403.3.2. Confirm adopted edition. Distinct from 403.3.1 Vbz/Ez. This is not a fan selection. Packet: 403.3.2 PDF · ES.
- 403.3.2.1 dwelling units: whole-house mechanical OA required (exhaust, supply, or balanced; OA duct to AHU return allowed). Must run continuously when occupied, or at least 1 hour of every 4 hours with equivalent average OA.
- Equation 4-9: QOA (cfm) = 0.03 × Afloor (ft²) + 7.5 × (Nbr + 1). Nbr is not less than 1.
- 30% reduction only if (1) OA is ducted directly to each bedroom and to one or more of the living room, dining room, or kitchen, and (2) the system is balanced.
- 403.3.2.2 common areas: corridors / other common conditioned spaces OA ≥ 0.06 cfm/ft².
- 403.3.2.3 local exhaust: kitchens 100 cfm intermittent or 50 cfm continuous; baths/toilets 50 cfm intermittent or 25 cfm continuous under the 2024 model Table 403.3.2.3.
- 403.3.2.4 controls: unit controls with clear text/symbol indicating the OA system function.
- 403.3.2.5 fans: OA/exhaust fans listed and labeled for airflow per ANSI/AMCA 210 — ANSI/ASHRAE 51.
For the reduction, document that the whole-house outdoor-air system is balanced, with the supply and exhaust airflows and commissioning evidence. The model reduction conditions do not state a generic numeric "tight tolerance"; use the applicable design and verification basis. Design for delivered cfm and verify at the terminals. Label the control "Whole-House Ventilation" so it is not disabled.
30% off with no bedroom OA
Missing corridor 0.06 cfm/ft² is the other common redline. If cycling, the controls must enable at least one hour of operation in each four-hour period and the four-hour average must meet the applicable Equation 4-9 basis. More airflow cannot make a shorter cycle qualify. Document the schedule and delivered cfm; confirm the adopted text before combining separate exceptions.
Source: ICC 2024 IMC §403.3.2 · DCT 403.3.2 · tool: Eq. 4-9 screen · path: 401.2 · dryer: 504.
What to send
Afloor, bedroom count, the OA delivery points (each bedroom plus at least one living/dining/kitchen space), and the supply/exhaust balance evidence.
If the townhouse set used Table 403.3.1.1
Send Afloor and Nbr. MasterBuild will say whether 403.3.2 even has Equation 4-9 — or the 30% cut was assumed.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
MasterBuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. Equation 4-9 and Ez-free rates below are 2024 IMC education from Daily Code Talk — not a sealed outdoor-air design.