Daily Code Talk — Chapter 4 Recap (IMC 2024)

🧠 Plain English Highlights• 401 (General): Chapter 4 governs occupied spaces. Ventilate when occupied (401.3). Choose NAT-402 or MECH-403. Keep OA intakes separated/protected (401.4–401.5). Capture point sources with exhaust to outdoors (401.6).• 402 (Nat): Openings controllable by occupants (402.1). Openable area ≥ 4% of floor area (402.2). Vent via adjoining spaces only with ≥ 8% transfer (min 25 sf) (402.3). Below-grade needs 1.5× horizontal clear space (402.4).• 403 (Mech): Provide supply and return/exhaust (403.1). Determine OA via 403.3; follow recirculation limits (403.2.1) and transfer-air rules (403.2.2).• 403.3.1: Table 403.3.1.1 → Vbz = Rp·Pz + Ra·Az → Ez (Tbl 403.3.1.1.1.2) → Voz = Vbz/Ez → system OA via Eq 4-3/4-4 or Zp/Ev/D (Eq 4-5/4-6/4-8). Provide table exhaust (403.3.1.2). DCV allowed but never below Ra floor (403.3.1.3). VAV must hold OA at turndown (403.3.1.4).• 403.3.2 (R-2/R-3/R-4 ≤3 stories): Each dwelling gets OA by exhaust/supply/combo; continuous or ≥1 hr/4 hr with average meeting Eq 4-9 (403.3.2.1). 30% reduction only if ducted to bedrooms + living/dining/kitchen and whole-house is balanced. Corridors/common areas ≥ 0.06 cfm/ft² (403.3.2.2). Local exhaust: kitchens 100 int / 25 cont; baths 50 int / 20 cont (403.3.2.3). Label OA controls (403.3.2.4). Fans listed to ANSI/AMCA 210–ASHRAE 51 (403.3.2.5).• 404 (Garages): Run continuously or automate with CO+NO₂ detectors listed to UL 2075. Modes: Full-On ≥ 0.75 cfm/ft²; Standby ≥ 0.05 cfm/ft² (404.1). Accessory rooms: positive pressure; ventilate per 403.3.1 (404.2).• 405 (Controls): If occupied, ventilation runs. If the AC unit provides required OA, controls must automatically maintain required OA during occupancy (405.1).• 406 (Uninhabited): Crawl/attic: natural per IBC or mechanical ≥ 0.02 cfm/ft²; auto-enable when RH > 60% (406.1).• 407 (Healthcare): Ambulatory care & Group I-2 must meet IMC 407 + ASHRAE 170 + NFPA 99; design to the most restrictive (407.1).🚧 Frequent Redlines• Vbz shown but Ez/Voz or Zp/Ev/D missing.• DCV below Ra floor; VAV not holding OA at low flow.• Intake clearances/screens not per 401.4–401.5.• R projects claiming 30% reduction without ducted, balanced design.• Garages missing NO₂ or UL 2075 citation.• Natural ventilation claimed without 4%/8%/below-grade proof.🔧 Quick Field Tips• Tag diffuser types; when unsure, use Ez = 0.8.• Show transfer-air math for kitchens/toilets used as makeup.• For garages, trend ppm/speed/cfm; fail-safe to Full-On on detector/BAS fault.• In dwellings, show Eq 4-9 duty-cycle calc and label OA controls.If you want a one-page Chapter 4 calculator (Vbz/Voz/Zp/Ev/Vou/Vot + Eq 4-9) and paste-ready sequences for DCV/VAV/garage, comment IMC4KIT.