Sequence of operations review

What is a sequence of operations?

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 should every sequence of operations cover?

Every sequence should cover occupied, unoccupied, warmup, safety interlocks, and alarm responses.

• 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

Which systems need a written sequence of operations?

Air handlers, plants, exhaust, and any controlled terminal unit need a written sequence of operations.

• 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

What belongs on a sequence-of-operations review checklist?

A sequence review checks modes, setpoints, safeties, and that every controlled point is on the points list.

• 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

What red flags show up in a sequence of operations?

Red flags include missing safeties, modes that contradict the schedules, and points that are not on the drawings.

• "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

Where should sequences live in the document set?

Sequences should live in the mechanical specs or a controls package that commissioning can test against.

• 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 does the sequence connect 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.