What does ASHRAE 90.1 require for HVAC controls?

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## Mandatory Controls Under ASHRAE 90.1-2019

ASHRAE Standard 90.1, 'Energy Standard for Buildings Except Low-Rise Residential Buildings', is the dominant energy code referenced by the Building Code of Australia (NCC Section J) for commercial HVAC compliance. The 2019 edition significantly expanded mandatory control requirements.

### Direct Digital Control (DDC)

Section 6.4.3.4 requires DDC for most HVAC systems in buildings exceeding certain size thresholds. DDC controllers must provide networked communication (BACnet or equivalent), central trending capability, and the ability to implement the specified control sequences digitally rather than through pneumatic or standalone electronic controllers. The intent is to enable central monitoring, remote adjustment, and automated fault detection.

### Demand-Controlled Ventilation

Spaces with design occupancy exceeding 25 people per 100 m² (and an area >50 m²) must have CO₂-based demand-controlled ventilation. The BMS modulates outdoor air intake based on zone CO₂ concentration, maintaining levels no higher than approximately 700 ppm above outdoor ambient. This can reduce outdoor air heating and cooling loads by 30–60% during periods of low occupancy.

### Economiser Controls

Air-side economisers are mandatory for fan systems ≥16.5 kW cooling capacity in most Australian climate zones (equivalent to ASHRAE Climate Zones 1–6). The economiser must use either differential enthalpy control (comparing outdoor vs. return air enthalpy) or differential dry-bulb control, with a high-limit shutoff that disables the economiser when outdoor conditions exceed the setpoint. Per Guideline 36, the changeover setpoint should include at least 1°C of deadband to prevent short-cycling.

### Variable-Speed Drives and Static Pressure Reset

All VAV supply fans ≥7.5 kW must have variable-speed drives. The duct static pressure setpoint must be reset based on zone damper positions — not held at a fixed value — to minimise fan energy. This trim-and-respond reset strategy can reduce fan power by 30–50% compared to fixed setpoint operation.

### Fault Detection and Diagnostics (FDD)

The 2019 edition added mandatory FDD for air-handling units and zone terminal units in many system types. The BMS must automatically detect and report faults such as stuck dampers, sensor failures, simultaneous heating and cooling, and economiser malfunction — alerting operators before energy waste accumulates into significant cost.

Requirement	Section	Trigger Threshold	Key Specification	Typical Energy Saving
DDC controls	6.4.3.4	Most systems in buildings >5,000 m²	BACnet or equivalent networked control	Enables all other savings measures
Demand-controlled ventilation	6.4.3.9	>25 people/100 m², >50 m² zone	CO₂ sensor, modulate OA to ~700 ppm above ambient	30–60% OA load reduction
Air-side economiser	6.4.3.3	≥16.5 kW cooling capacity	Differential enthalpy or dry-bulb, high-limit shutoff	20–40% cooling energy reduction
VAV fan VSD + static pressure reset	6.4.3.4.2	≥7.5 kW supply fan motor	VSD with trim-and-respond SP reset	30–50% fan energy reduction
Supply air temperature reset	6.4.3.4.3	Multi-zone systems with DDC	Reset SAT upward until one zone nears max cooling	10–25% cooling/compressor energy
Automated FDD	6.4.3.11	AHUs and zone terminals per table	Automatic sensor/actuator fault detection and reporting	5–15% ongoing operational savings
Setback during unoccupied hours	6.4.3.3	All systems with DDC	Auto setback heating/cooling setpoints	10–30% thermal energy reduction

What does ASHRAE 90.1 require for HVAC controls?

Key ASHRAE 90.1-2019 HVAC Control Mandates

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