Sequence of operations review
A checklist for reviewing HVAC sequence of operations narratives in construction documents
What a Sequence of Operations Is
A sequence of operations is the written narrative that describes how every controlled HVAC system operates in every mode. The sequence drives controls programming, commissioning test procedures, and operator training. A vague or missing sequence is the single largest source of late-stage rework on commissioning projects.
What Every Sequence Should Cover
- Occupied mode setpoints and operation
- Unoccupied mode setpoints and operation
- Warm-up and cool-down mode transition
- Outside air and economizer operation including high-limit lockout
- Demand-controlled ventilation (where applicable)
- Heating and cooling staging or modulation
- Reset schedules for supply air temperature, water temperature, static pressure
- Smoke control or smoke purge mode (where applicable)
- Safeties and shutdown conditions
- Alarm conditions and operator notifications
- Manual override conditions and timeout
- Response to power loss and recovery
Systems That Need a Written Sequence
- Air handling units
- VAV boxes and terminal units
- Rooftop units and packaged equipment
- Chillers and chilled water plants
- Boilers and heating water plants
- Cooling towers and condenser water systems
- Heat recovery systems
- Exhaust fans (toilet, kitchen, lab, parking)
- Dedicated outside air systems (DOAS)
- VRF systems
- Domestic hot water plants and recirculation
- Smoke control systems
The Sequence Reviewer's Checklist
- Every controlled device referenced in the sequence appears in the points list
- Every point in the points list appears in the sequence
- Setpoints are quantitative, not qualitative
- All transitions between modes are defined
- All failure modes and safeties are defined
- All adjustable parameters listed in a single table
- Alarm priorities defined
- Graphics and trending requirements specified
- Manual override behavior and timeout specified
- BAS network architecture documented
Red Flags in a Sequence
- "System shall maintain comfort conditions", not measurable
- "Coordinate with [other party]", not specified
- Missing handoff between adjacent systems (AHU vs. VAVs, chiller vs. AHU)
- Setpoints referenced but not defined
- Reset schedules referenced but not specified
- Smoke control mentioned without a fan matrix or detail sequence
- Demand-controlled ventilation specified without CO2 setpoints
- Hand-off between local controllers and BAS not defined
- Alarm conditions defined without operator notification path
Try to imagine a controls technician building this system from scratch using only the sequence narrative. If they would have to ask three questions to program any one device, the sequence needs more detail. Vague sequences become RFIs, change orders, and missed commissioning deadlines.
Where Sequences Should Live in the Document Set
- Mechanical drawings sheet series (M-500 commonly)
- Controls specifications (Division 23 09 or Division 25)
- Each system narrative on its own sheet or section
- Common conventions and abbreviations defined once at the start
- Cross-references between drawings and specs consistent
How the Sequence Connects to Commissioning
Commissioning test procedures are derived directly from the sequence. Every controlled response in the sequence becomes one or more functional test scripts. A complete sequence enables the CxA to write thorough scripts before construction is finished, accelerating functional testing and reducing schedule risk at project handoff.
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