When is an economy cycle required under NCC Section J?

NCC 2025 J6D3(1)(c) requires an outdoor air economy cycle when an AHU's total airflow meets or exceeds the threshold in Table J6D3. Thresholds range from 2,000 L/s in climate zone 6 to 9,000 L/s in climate zone 2. Climate zone 1 and humidity-controlled spaces are exempt.

What are the economy cycle airflow thresholds by climate zone in Australia?

NCC 2025 Table J6D3 sets these thresholds: Climate zone 2 is 9,000 L/s, zone 3 is 7,500 L/s, zone 4 is 3,500 L/s, zone 5 is 3,000 L/s, zone 6 is 2,000 L/s, zone 7 is 2,500 L/s, and zone 8 is 4,000 L/s. Climate zone 1 is exempt.

What components does an economy cycle need?

An economy cycle requires three motorised dampers: an outside air damper, a return air damper, and a relief air damper. It also needs temperature sensors (or enthalpy sensors in humid climates) and BMS controls to modulate damper positions based on outdoor conditions.

How much does an economy cycle cost per AHU?

Economy cycles typically add $3,000 to $8,000 per AHU for dampers, actuators, and controls. In climate zones 4 to 7, the payback is often under 2 years through reduced chiller runtime and lower energy costs.

Should I use temperature or enthalpy control for an economy cycle?

Temperature-based control suits dry climates like zones 5 to 7. Enthalpy-based control measures both temperature and humidity, making it better for humid climates like zones 2 and 3. Enthalpy sensors cost more and need regular calibration.

Economy Cycle Design for NCC Section J Compliance

Section 1

What You Need to Know

An economy cycle uses cool outdoor air to reduce cooling load. NCC 2025 Part J6 requires one on any AHU (air handling unit) above a set airflow threshold. The threshold depends on your climate zone, and ranges from 2,000 L/s to 9,000 L/s.

Section 2

The Rules

An economy cycle is required when an AHU's total airflow meets or exceeds the threshold in Table J6D3 (NCC 2025, J6D3(1)(c))
Climate zone 1 is exempt (NCC 2025, J6D3)
Spaces needing humidity control are exempt (e.g., labs, cold stores) (NCC 2025, J6D3)
Thresholds by climate zone: CZ2: 9,000 L/s, CZ3: 7,500 L/s, CZ4: 3,500 L/s, CZ5: 3,000 L/s, CZ6: 2,000 L/s, CZ7: 2,500 L/s, CZ8: 4,000 L/s (NCC 2025, Table J6D3)
When the system shuts down, all motorised outdoor air and return air dampers must close (NCC 2025, J6D3(1)(l))
The outdoor air system must still meet minimum ventilation rates per AS 1668.2 (NCC 2025, F6D6)

Section 3

What This Means in Practice

Take an office building in Sydney (climate zone 5). Your AHU serves 1,500 m² of floor area at 6 air changes per hour. That gives a supply airflow of about 7,500 L/s. The Table J6D3 threshold for climate zone 5 is 3,000 L/s. Since 7,500 L/s exceeds 3,000 L/s, this AHU needs an economy cycle.

The economy cycle needs three motorised dampers: outside air, return air, and relief air. When outdoor air is cooler than return air (say 18°C outside vs. 24°C return), the BMS (building management system) opens the outside air damper and closes the return air damper. The relief damper opens to let excess air escape and prevent duct pressure buildup. The chiller stays off or runs at part load.

In a mild climate like Sydney or Melbourne, this can reduce chiller runtime by weeks or months each year. One Sydney CBD operator reported eliminating chiller use for 60 to 90 days annually. That cuts energy costs and extends chiller life.

Section 4

Key Design Decisions

1

Temperature Control vs. Enthalpy Control

Temperature-based control compares outside and return air temperatures. It is simple and suits dry climates. Enthalpy-based control also measures humidity. It prevents bringing in hot, humid air that adds latent load.

Trade-off: Enthalpy sensors cost more and need regular calibration. In climate zones 2 and 3, enthalpy control avoids moisture problems. In zones 5–7, temperature control works well.

2

Damper Sizing and Selection

Size outside air dampers to handle 100% of the AHU's supply airflow. Use opposed-blade dampers for better modulation. Match the relief air damper to the outside air damper size.

Trade-off: Larger dampers typically cost $2,000–5,000 per AHU including actuators. Undersized dampers restrict airflow and reduce free cooling hours.

3

Minimum Outside Air During Economy Mode

The economy cycle must not drop below the minimum outdoor air rate set by AS 1668.2 (10 L/s per person for offices). The BMS needs a low-limit setpoint to prevent overcooling in winter.

Trade-off: A mixed air temperature sensor (around $200–400) prevents supply air dropping below 13–15°C and causing cold complaints.

4

Relief Air Path

Relief air must have a clear path out of the building. This usually means a relief fan or a barometric relief damper on the building exterior.

Trade-off: A relief fan adds $1,500–3,000 and uses energy. A barometric damper is passive but needs a large opening in the facade or roof.

Section 5

Who Needs to Know What

Section 6

References

National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation (J6D3, Table J6D3)
National Construction Code 2022, Volume One, Part F6 — Ventilation (F6D6)
AS 1668.2-2012, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings
AS 1668.2:2024, The use of ventilation and airconditioning in buildings — Part 2: Mechanical ventilation in buildings (current edition)
AS 1668.1, The use of ventilation and airconditioning in buildings — Part 1: Fire and smoke control in buildings

Economy Cycle Design for NCC Section J Compliance

What You Need to Know

The Rules

What This Means in Practice

Key Design Decisions

Temperature Control vs. Enthalpy Control

Damper Sizing and Selection

Minimum Outside Air During Economy Mode

Relief Air Path

Who Needs to Know What

Need this engineered for your project?

References

Related design memos

Energy Efficiency Requirements for HVAC Under NCC Section J

NABERS Energy Ratings - How HVAC Design Impacts the Score

Green Star Requirements for Building Services