Sequence of operations
The written description of how an HVAC control system behaves in every mode: what runs, at which setpoints, how equipment stages and resets, and what happens on each fault, alarm, and restart. It is the document commissioning tests the installed controls against.
Also called SOO, control sequence, sequence of control
What is a sequence of operations in HVAC controls?
A sequence of operations is the written script for an HVAC control system, stating what every piece of equipment does in normal operation and in each failure. In a critical room the sequence of operations also sets restart order, such as proving exhaust airflow before supply starts in a containment room.
Sequence of operations: how it works
The sequence turns design intent into control logic a programmer can build and a tester can check. For each system it lists the inputs read, the outputs commanded, and the conditions that change them, so every behavior has a written reason. During commissioning, testers create each condition and compare what the controls do with what the sequence says.
Where you meet the term
- In the controls section of a mechanical specification or on the controls drawings.
- In a functional performance test script, where each line of the sequence becomes a test step.
- In a critical room design, where failure and restart sequences decide whether containment holds through an outage.
How to check it
- Ask for the as-built sequence. The as-built sequence should match what the building automation system is programmed to do, and it is the first document to read when equipment behaves unexpectedly. Owner check
- Test the controls against it. A commissioning agent forces each mode and fault, trends the response, and records any difference between the programmed behavior and the written sequence. Technician
Sequence of operations: what it is not
- A sequence of operations is not a points list. A points list names the inputs and outputs, and the sequence says how they relate.
- A sequence that describes only normal operation is incomplete for a critical room. The fault and restart behavior is where containment and product are lost.
Sequence of operations questions
Who writes the sequence of operations?
The design engineer writes it as part of the design documents, the controls contractor refines it into programming, and the commissioning agent reviews both. The final as-built version belongs in the operations and maintenance record.
What should a failure sequence include?
For each fault: normal operation, the signal and threshold that detect the fault, the alarm and who receives it, the safe state of each fan and damper, and what must be proven before recovery.
Why do fans need a restart order after a power failure?
Fans on different drives and power sources return at different moments, and a supply fan that restarts before its exhaust can briefly push a negative room positive. A written restart order enforced by the automation system prevents that.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- Guideline 0, The Commissioning ProcessASHRAE guideline defining commissioning as a quality process running from owner requirements through design review, construction checks, functional performance testing, and documented issue resolution, with verification carried out against the documented control sequences rather than against installer judgement.
- NFPA 90A, Standard for the Installation of Air-Conditioning and Ventilating SystemsFire protection standard for air-conditioning and ventilating systems, requiring duct smoke detectors downstream of filters and ahead of branch connections in supply systems above 2,000 cubic feet per minute and at each story in return systems above 15,000 cubic feet per minute serving more than one story, with the detectors stopping the air handling unit on detection; local mechanical codes can place detectors differently.