HVAC Controls Sequences of Operation

What You Need to Know

A sequence of operation tells the BMS (building management system) how to run your HVAC plant. It covers every mode: startup, normal running, economy cycle, fault response, and shutdown. Without a clear sequence, the controls contractor programs their own logic. That leads to wasted energy, comfort complaints, and finger-pointing at commissioning. This memo covers how to write good sequences and what the standards require.

• Air-conditioning systems above 2 kWr must have time switches or BMS scheduling to stop after-hours operation (NCC 2025, J6D3(3))
• Fans with supply airflow above 1,000 L/s must run at variable speed where the air quantity varies (NCC 2025, J6D3(1)(e))
• Economy cycle dampers need BMS control with outside air temperature sensing (NCC 2025, J6D3(1)(c))
• Outdoor air rates must meet AS 1668.2 at all times during occupied hours. Sequences must keep outdoor air dampers at the correct minimum position (AS 1668.2:2024)
• ASHRAE Guideline 36-2024 provides standardised high-performance sequences for VAV AHUs, single-zone AHUs, and dual-duct AHUs (international reference, not code in Australia)
• AIRAH DA28 covers BMS design, specification, and commissioning practices for Australian buildings (AIRAH DA28:2011)

A sequence of operation is a step-by-step instruction set for one piece of plant. For a typical VAV (variable air volume) AHU (air handling unit), the sequence covers startup, supply air temperature control, duct static pressure control, economy cycle changeover, and shutdown. Each step lists the trigger condition, the control action, and the expected result. For example: “When the time schedule calls for start, the supply fan starts at minimum speed. The exhaust and fresh air dampers open to 30%. The return air damper opens to 70%. The fan stays at minimum speed until the lead chiller has run for 10 minutes.”

Good sequences use PID (proportional-integral-derivative) control loops for analogue outputs like valve positions and fan speeds. The supply air temperature resets based on zone demand. Chilled water temperature resets from 6°C to 9°C during part load. A duct static pressure sensor sits at two-thirds of the way along the main supply duct and sends a 0–10 VDC signal to the DDC (direct digital controller). The DDC adjusts the fan VFD (variable frequency drive) to hold the setpoint.

The most common failure is vague sequences that leave decisions to the controls contractor. If the sequence says “maintain comfortable conditions” instead of “hold supply air at 13°C when cooling demand exceeds 70%,” every installer will do something different. Write every setpoint, every deadband, every time delay, and every failure response. ASHRAE Guideline 36-2024 provides copy-ready sequences for common system types. Australian projects can use it as a starting template, then adapt for local codes and conditions.