Occupancy Sensor Control for Lighting and HVAC
What You Need to Know
Occupancy sensors stop lights and air conditioning from running in empty rooms. The NCC requires them in most commercial buildings over 250 m². In hotels and motels, a key card or motion detector must cut power to lights, HVAC, and exhaust fans when no one is in the room. Get the sensors wrong and you fail Section J. Get them right and you cut energy use by 20–30%.
The Rules
- In buildings over 250 m² (excluding Class 3), 95% of light fittings must connect to a time switch or occupancy sensor (NCC 2025, J7D4(4))
- In Class 3 sole-occupancy units (hotels, motels), an occupant-activated device must disconnect lighting, air conditioning, exhaust fans, and bathroom heaters when the room is unoccupied. A key card slot, motion detector, or any occupancy-monitoring system meets this rule (NCC 2025, J7D4(2))
- Motion detectors must sense a person before they walk 1 m into the space and turn lights off within 15 minutes of the space becoming empty (NCC 2025, Specification 40, S40C4)
- In fire-isolated stairways, passageways, and ramps, motion detectors must dim lights to 30% or less of peak power after 15 minutes. Full switch-off is not permitted (NCC 2025, Specification 40, S40C4(4))
- Air conditioning systems over 2 kWr and heaters over 1 kW must have time switches with programmable schedules for different times and days (NCC 2025, J6D3(3))
- Mechanical ventilation systems over 1000 L/s must have time switches and either demand control ventilation per AS 1668.2 or an energy recovery system with at least 60% sensible heat effectiveness (NCC 2025, J6D4)
- Lighting near windows must be switched separately from interior zones in Class 5, 6, or 8 buildings over 250 m² (NCC 2025, J7D4(5))
- Air conditioning systems must be capable of full deactivation when the building or zone they serve is unoccupied (NCC 2025, J6D3(1)(a))
What This Means in Practice
Take a 2,000 m² office building with 10 meeting rooms, an open-plan floor, corridors, and a stairwell. Every one of those spaces needs occupancy-based control. The open-plan area needs time switches or sensors covering 95% of fittings. Each meeting room needs a sensor that turns off lights within 15 minutes of the last person leaving. The stairwell needs sensors that dim lights to 30% rather than switching them fully off.
For lighting, the most common approach is ceiling-mounted PIR (passive infrared) sensors. A single ceiling-mounted PIR sensor covers about 80–150 m² with a clear line of sight. For a 100 m² meeting room, one sensor centered on the room works if there are no obstructions. For L-shaped or partitioned rooms, add a second sensor with overlapping coverage. In open-plan areas over 250 m², sensors connect to a central lighting control panel or BMS (building management system) that handles scheduling and zone control together.
For HVAC, occupancy sensors feed signals to the BMS to adjust setpoints or shut down zones. When a meeting room empties, the sensor tells the BMS to widen the temperature dead band or close the VAV (variable air volume) box serving that zone. CO2 sensors take this further. They measure carbon dioxide levels as a proxy for how many people are in a space, then modulate outdoor air supply to match actual demand. This demand control ventilation (DCV) approach is referenced in AS 1668.2 and rewarded under NCC J6D4.
In hotels, key card systems are the simplest solution. The guest inserts the room card at the door. Power flows to lights, HVAC, and exhaust fans. When the guest takes the card and leaves, all systems shut down within minutes. This meets J7D4(2) without any additional sensors.
Key Design Decisions
Sensor Type: PIR vs. Ultrasonic vs. Dual-Technology
PIR sensors detect body heat and work well in rooms with clear sight lines. Ultrasonic sensors detect movement through sound waves and cover larger areas, but they can false-trigger from HVAC airflow near diffusers. Dual-technology sensors combine both methods: they require both PIR and ultrasonic to activate, but only one to stay on. This cuts false triggers by more than half.
Lighting-Only vs. Integrated Lighting and HVAC Control
You can wire sensors to control lights alone, or integrate them with the BMS to also adjust HVAC. Lighting-only is simpler and cheaper. Integrated control saves more energy because HVAC uses two to three times more energy than lighting in most commercial buildings.
CO2 Sensors for Demand Control Ventilation
For spaces with variable occupancy (conference rooms, lecture halls, retail floors), CO2 sensors modulate outdoor air supply to match the actual number of people. The NCC requires DCV for ventilation systems over 1000 L/s where appropriate per AS 1668.2. CO2 sensors mount in the return air path or within the occupied zone at seated head height (about 1.1 m).
Zone Sizing and Switch Layout
The NCC caps lighting switch zones at 250 m² for Class 5 buildings and Class 8 labs. Smaller zones give finer control and better energy savings, but need more sensors and wiring. For most offices, 50–100 m² zones match the building's functional layout (one zone per meeting room, one per open-plan cluster).
Who Needs to Know What
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References
- National Construction Code 2022, Volume One, Part J7 — Artificial lighting and power (J7D4)
- National Construction Code 2022, Volume One, Specification 40 — Lighting and power control devices (S40C2, S40C3, S40C4, S40C5)
- National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation (J6D3, J6D4)
- AS 1668.2-2012, The use of ventilation and airconditioning in buildings — Ventilation design for indoor air contaminant control
- AS/NZS 1680.1:2006, Interior and workplace lighting — General principles and recommendations
- ASHRAE Standard 90.1-2019, Energy Standard for Buildings Except Low-Rise Residential Buildings (international reference)