Plain English:
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.
Calculation Check: 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.
Code Path: 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).
Coordination Check: 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.
Check: Controls Sequence Documentation
Drawings must show the ventilation system control sequence: what triggers activation, how the outdoor air damper position is controlled, and what happens during unoccupied mode and fire alarm events.
Each operating mode must be addressed explicitly - partial sequences that omit unoccupied or emergency modes generate plan review comments.
Confirm the sequence of operation is included in both the mechanical and the controls contract documents with consistent language.
Review Risk:
Ventilation controls shown on mechanical drawings but not on controls drawings - or the reverse - are a common coordination gap causing conflicting assumptions between the mechanical and controls contractors.
Systems that reference a sequence of operation but do not show OA damper position, setpoints, or occupancy schedule are consistently flagged as incomplete.
A ventilation system shown on a duct plan without a controls sequence is a first-cycle plan review
Field Tip:
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.
Masterbuild QA Lens
The natural ventilation rate alternative is rarely used on commercial projects, but when it is, the compliance path requires the same documentation rigor as the mechanical rate procedure. Confirm the calculation is included in the permit set and the openable area is dimensioned on the drawings with a cross-reference to the calculation.
Drawing / Submittal Check
For IMC 405 (systems control, 405.1): the drawings must show the ventilation system control sequence - specifically, what triggers the system to activate, what controls the outdoor air damper position, and what happens during unoccupied periods and fire alarm events. The controls sequence must address each operating mode explicitly. A ventilation system shown on a duct plan without a controls sequence is a first-cycle plan review comment.
Common Review Risk
Ventilation system controls that are shown on the mechanical drawings but not on the controls drawings - or that appear on the controls drawings without referencing the mechanical drawings - are a common coordination gap. The controls contractor and the mechanical contractor each assume the other has covered the ventilation control sequence. Confirm the sequence of operation is complete and is included in both the mechanical and the controls contract documents.
When To Escalate
Escalate when the ventilation system control sequence requires coordination with the building fire alarm system - when the ventilation must shut down, switch to smoke control mode, or pressurize during a fire event. Ventilation controls that interface with the fire alarm must be reviewed by the fire alarm engineer and approved by the AHJ before the controls contractor programs the sequence.
Ventilation Coordination
For ventilation systems control (IMC 405): the control sequence must show how the outdoor air damper responds to each operating mode - occupied, unoccupied, fire alarm, smoke control, and economizer. A control sequence that only addresses the occupied mode leaves the reviewer unable to verify code compliance for the other modes.