/ 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 every bedroom plus living/dining/kitchen — and a balanced system you can prove.
Educational 2024 IMC 403.3.2. Confirm adopted edition. Distinct from 403.3.1 Vbz/Ez. This is not a fan selection.
- 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) = 0.01 × 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 the living/dining/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 25 cfm continuous; baths/toilets 50 cfm intermittent or 20 cfm continuous.
- 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.
Balanced means commissioned supply vs exhaust within a tight tolerance — not a note. Design for delivered cfm: short, smooth ducts; verify at the terminal. 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 4-hour average must still meet QOA — document duty cycle and fan cfm.
Source: DCT 403.3.2 · tool: Eq. 4-9 screen · path: 401.2 · dryer: 504.
What to send
Afloor, bedroom count, whether OA is ducted to bedrooms, and whether the system is balanced.
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.