/ Field knowledge · IMC 403.2.1
Return air is not outdoor air.
IMC 403.2.1: recirculated air does not count toward the 403.3 minimum outdoor air. Extra air above the minimum can be recirculated. Note b spaces: all supply becomes exhaust. Note g: do not send more than 10% of another zone’s supply from those spaces. Do not recirculate between dwellings.
Educational 2024 IMC 403.1–403.2.2. Confirm adopted edition. Distinct from 510.1.3 dust recirc and from Table 403.3.1.1. This is not an OA calc.
- 403.1: use supply + return/exhaust. Group R-2/R-3/R-4 may use exhaust, supply, or combo. Supply ≈ return + exhaust overall; slight positive/negative building pressure is not forbidden. Ductwork per Chapter 6.
- 403.2: minimum OA is 403.3. You can reduce OA only with a stamped engineered design showing equal or better contaminant control than 403.3.
- 403.2.1: the 403.3 minimum OA cannot be recirculated. Extra air above the minimum can be.
- No recirculation between different dwellings or dissimilar occupancies.
- Pools/decks: no recirc unless dehumidified to ≤ 60% RH (Manual SPS), and do not send that air to other spaces (the >10% blend rule).
- Table 403.3.1.1 Note b: all supply becomes exhaust (no recirc out).
- Note g: no recirc to other spaces if more than 10% of supply is from those spaces.
- 403.2.2 transfer air: air from occupiable spaces may make up kitchens, baths, toilets, elevators, smoking lounges unless the table prohibits. Meet the exhaust cfm; deliver the required OA either to those rooms or to the source rooms you transfer from (or both).
M-001: “Recirculated air does not count toward the IMC 403.3 minimum OA rate.” Tag Note b/g spaces. Show transfer paths and door undercuts. Occupant load used for OA should be documented — IBC egress load is not automatically the ventilation load.
Return credited as outdoor air
That is the common 403.2.1 comment. Transfer without proving the OA basis is still met is next. Note b/g spaces with no restriction on M-001 generate a first-cycle comment.
Source: DCT 403.1–403.2.2 · tool: recirc screen · table: 403.3.1.1 · dust: 510.1.3.
What to send
Whether any space is Note b or g, whether pool air is shared, and the OA vs return setpoints.
If the RTU is counting return toward OA
Send the schedule. Masterbuild will say whether 403.2.1 even has a recirc path — or this is still minimum outdoor air.
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. Recirculation and mesh sizes below are 2024 IMC education from Daily Code Talk — not a system selection. This page is not legal advice.