Demand Control Ventilation with CO2 Sensors
What You Need to Know
Demand control ventilation (DCV) uses CO2 sensors to match outdoor air supply to real-time occupancy. AS 1668.2 allows CO2 as a control signal to modulate airflow. The NCC 2025 sets a CO2 verification limit of 850 ppm over 8 hours (Table F6V1). DCV saves energy in spaces where the number of people changes throughout the day.
The Rules
- CO2 must stay below 850 ppm averaged over 8 hours (NCC 2025, Table F6V1)
- The 850 ppm limit is based on 450 ppm above ambient outdoor CO2 of 400 ppm (NCC 2025, F6V1)
- All mechanical ventilation must comply with AS 1668.2 (NCC 2025, F6D6)
- Outdoor air must never drop below 0.35 L/s per m² of floor area, even with DCV active (AS 1668.2-2012, Cl 3.3)
- Each occupied zone must receive at least 10 L/s per person of outdoor air at design occupancy (AS 1668.2-2012, Table A1 for offices)
- CO2 is a control input only - it does not replace the outdoor air rate calculation (AS 1668.2-2012)
- Economy cycles are required for systems exceeding airflow thresholds set by climate zone, exempt in climate zone 1 and where dehumidification is needed (NCC 2025, J6D3(1)(c))
What This Means in Practice
Take a 150 m² meeting room rated for 50 people. At full occupancy, you need 500 L/s of outdoor air (50 × 10 L/s per person). But this room sits empty half the day. With a fixed system, you supply 500 L/s all day and waste energy cooling or heating that air.
With DCV, a CO2 sensor tracks real occupancy. When 10 people sit in the room, CO2 stays low. The BMS (building management system) drops outdoor air toward the floor rate: 150 m² × 0.35 L/s/m² = 52.5 L/s. When all 50 people arrive, CO2 rises, and the BMS ramps outdoor air back to 500 L/s.
The control setpoint is typically 800 ppm. This gives a 50 ppm buffer below the 850 ppm limit. The BMS starts increasing outdoor air when CO2 passes 600–700 ppm and reaches full outdoor air by 800 ppm. This proportional control avoids sudden damper swings.
Key Design Decisions
Wall-Mounted vs. Duct-Mounted CO2 Sensors
Wall sensors measure the breathing zone directly. Duct sensors sample return air, which averages the whole zone. For single-zone systems like meeting rooms, wall sensors give faster response. For open-plan areas served by one AHU, a return-air duct sensor works well.
CO2 Setpoint Selection
A setpoint of 800 ppm keeps CO2 below the 850 ppm verification limit (NCC 2025, Table F6V1). Lower setpoints (700 ppm) give better air quality but use more energy. Higher setpoints (850 ppm) risk exceeding the limit during peaks.
Single-Zone vs. Multi-Zone DCV
A single CO2 sensor can control one AHU serving one zone. Multi-zone systems need a sensor in each zone, with the BMS responding to the highest reading. Multi-zone DCV costs more but saves more energy.
Sensor Accuracy and Calibration
NDIR (non-dispersive infrared) sensors are the industry standard. Quality sensors hold ±30 ppm accuracy for 5–7 years. Auto-zero calibration corrects drift over time. Budget for sensor replacement or recalibration every 5 years.
Who Needs to Know What
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References
- 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 (updated edition)
- National Construction Code 2022, Volume One, Part F6 — Light and ventilation, Table F6V1
- National Construction Code 2022, Volume One, Part J6 — Air-conditioning and ventilation
- ASHRAE Standard 62.1-2022, Ventilation and Acceptable Indoor Air Quality (international reference)