Daily Code Talk #40 — IMC 405: Systems Control (405.1)

🧠 Plain English Highlights• 405.1: When a space is occupied, ventilation must operate (manual or automatic). If the air-conditioning unit provides the required outdoor air (OA), its controls must automatically maintain the required OA rate throughout occupancy—across VAV turndown and fan speed changes, not just at full flow.🧮 Step-by-Step (mirror on M-601/M-603)1. Identify spaces needing OA (401–403).2. Define modes: Occupied, Standby, Unoccupied.3. OA logic for AC units supplying OA: • Occupied: Hold design OA (cfm or % of supply) across turndown. • Standby: Allow DCV but never below the Ra floor. • Unoccupied: OA closed unless purge/flush is scheduled.4. Fault handling: On OA sensor/actuator/BAS failure → fail safe to design OA and alarm.5. Trend & verify: Log OA position/airflow with occupancy status.🗺️ Where to Show It (sheet map)• M-001: “IMC 405 controls: Ventilation operates when occupied; AC units supplying required OA automatically maintain required OA during occupancy.”• M-603 / M-002: Occupancy source, OA enable logic, DCV minimum (Ra floor), purge timing, fault→safe rules, alarms.• M-601: For each AHU/RTU/AC—design OA cfm (or %), minimum OA at VAV low flow, OA device type, sensor types/locations.• Controls submittal: BAS points list (occupancy, OA damper cmd/pos, airflow station or proxy curve, CO₂ if DCV, alarms).🚧 Frequent Redlines• Ventilation tied only to cooling/heating call (fan off at setpoint → no OA while occupied).• OA verified only at design flow—no method at VAV minimums.• DCV with no documented Ra floor.• No fail-safe on OA sensor/actuator fault.• Sequences that say “maintain OA” but lack measurable points.🔧 Field Tips• No airflow station? Use a commissioned damper-position–to–OA-cfm curve and document the test method.• For packaged RTUs, set a mechanical stop and verify spring-return to minimum OA on power loss.• Use reliable occupancy (schedule + local override; add PIR/CO₂ if appropriate).• Add a pre-occupancy purge (e.g., 1–2 air changes) where owner policy or local guidance calls for it.Want a paste-ready IMC 405 control note and a BAS points checklist? Comment “IMC405” and I’ll send it.