/ Field knowledge · IMC 405
Occupied means outdoor air is on.
IMC 405.1: when the space is occupied, ventilation must operate. If the AC unit supplies the required outdoor air, its controls must hold that OA rate throughout occupancy — including VAV turndown and fan-speed changes, not only at full flow.
Educational 2024 IMC 405.1. Confirm adopted edition. This is not a controls sequence for your BAS.
- Occupied: hold design OA (cfm or % of supply) across turndown.
- Standby: DCV allowed but never below the Ra floor.
- Unoccupied: OA closed unless purge/flush is scheduled.
- Fault: OA sensor/actuator/BAS failure → fail safe to design OA and alarm.
- Show it on M-603 / M-002 and on the controls drawings — not only on the duct plan.
Fan off at setpoint
Ventilation tied only to a cooling/heating call leaves occupied rooms with no OA. OA verified only at design flow, DCV with no Ra floor, and “maintain OA” with no measurable points are the next comments.
Source: DCT 405 · OA method: Chapter 4 · tool: OA control check.
What to send
Whether OA is on the RTU, VAV or CAV, and any sequence of operation. A duct plan without a sequence is the comment.
If OA only exists at full cooling
Send the sequence (or say there isn’t one). Masterbuild will say whether this is 405.1, DCV without a floor, or a fan that shuts off while occupied.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
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