CO2 Sensors for HVAC Demand Control Ventilation

Carbon dioxide sensors serve as the primary feedback mechanism for demand control ventilation (DCV) systems, enabling ventilation rates to adjust dynamically based on occupancy levels. ASHRAE Standard 62.1 permits CO2-based DCV as a ventilation rate procedure alternative when properly implemented with calibrated sensors meeting specific accuracy requirements.

NDIR Sensor Operating Principles

Nondispersive infrared (NDIR) sensors measure CO2 concentration by exploiting the molecule’s characteristic absorption of infrared radiation at 4.26 μm wavelength. The sensor operates on the Beer-Lambert law, which relates light absorption to gas concentration:

$$I = I_0 \cdot e^{-\alpha \cdot L \cdot C}$$

Where:

• $I$ = transmitted light intensity (W)
• $I_0$ = incident light intensity (W)
• $\alpha$ = absorption coefficient (m²/mol)
• $L$ = optical path length (m)
• $C$ = gas concentration (ppm)

The measured CO2 concentration is calculated from the transmittance ratio:

$$C = \frac{1}{\alpha \cdot L} \ln\left(\frac{I_0}{I}\right)$$

Dual-wavelength NDIR sensors employ a reference wavelength (typically 3.9 μm) insensitive to CO2 to compensate for optical path contamination, light source aging, and detector drift. The compensated measurement uses the ratio:

$$C_{compensated} = K \cdot \ln\left(\frac{I_{ref}/I_{CO2,sample}}{I_{ref,0}/I_{CO2,0}}\right)$$

This approach provides superior long-term stability compared to single-wavelength designs.

CO2 Sensor Integration in DCV Systems

graph TD
    A[CO2 Sensor<br/>Wall/Duct Mounted] --> B[BAS Controller<br/>CO2 Setpoint: 1000-1200 ppm]
    B --> C{CO2 > Setpoint?}
    C -->|Yes| D[Increase OA Damper Position]
    C -->|No| E[Decrease to Minimum OA]
    D --> F[Monitor CO2 Response<br/>Time Constant: 5-15 min]
    E --> F
    F --> G[Verify Min Ventilation Rate<br/>Per ASHRAE 62.1]
    G --> A
    H[Occupancy Schedule] --> B
    I[Outdoor Air CO2<br/>Reference: 400 ppm] --> B

Sensor Accuracy Classes and Applications

ASHRAE 62.1 DCV Requirements:

• Sensor accuracy: ±75 ppm or 5% of reading (whichever is greater)
• Measurement range: 0-2000 ppm minimum
• Calibration: Manufacturer-specified intervals or every 5 years maximum
• Outdoor air CO2 measurement or fixed reference value (typically 400 ppm)

Calibration Methods

1. Fresh Air Calibration (Single-Point ABC)

Automatic baseline calibration assumes the sensor periodically experiences fresh outdoor air (~400 ppm). The algorithm automatically adjusts the baseline over 7-14 days. This method works for spaces with regular unoccupied periods but fails in continuously occupied or contaminated environments.

2. Two-Point Span Calibration

Requires exposure to known reference gases:

• Zero point: Pure nitrogen (0 ppm CO2)
• Span point: Certified calibration gas (typically 1000 ppm)

Calibration factor:

$$K_{span} = \frac{C_{ref}}{C_{measured}}$$

3. Field Verification

Compare sensor readings against calibrated portable reference instrument:

• Allow 15-minute stabilization period
• Record ambient temperature and pressure
• Acceptable deviation: within stated accuracy specification

Mounting Location Requirements

Proper sensor placement directly affects DCV system performance and energy efficiency.

Space Sensors (Return Air Monitoring):

• Height: 3-6 ft above floor (breathing zone)
• Distance: >3 ft from doors, >5 ft from supply diffusers
• Avoid: Direct sunlight, heat sources, dead air pockets
• Density: 1 sensor per 10,000 ft² or per air handler zone

Duct Sensors:

• Location: Return air duct before mixing with outdoor air
• Mounting: Sensor probe extends into airstream centerline
• Airflow velocity: 400-800 fpm minimum for adequate response
• Protection: Shield from condensation and particulate

Outdoor Air Reference:

• Location: Fresh air intake, upstream of economizer
• Height: >8 ft above grade to avoid vehicle exhaust
• Protection: Weather-resistant enclosure with particulate filter

Response Time Characteristics

Sensor response time affects DCV control loop stability and occupant comfort.

The overall system response time to occupancy changes typically ranges from 8-20 minutes. Control algorithms must account for this lag to prevent oscillation while maintaining adequate ventilation during rapid occupancy increases.

Optimal Control Parameters:

• Proportional band: 200-300 ppm above setpoint
• Integral time: 10-20 minutes
• Setpoint: 1000-1200 ppm (600-800 ppm above outdoor baseline)
• Deadband: 50-100 ppm to prevent hunting

Maintenance and Troubleshooting

Routine Maintenance:

• Visual inspection: Quarterly for dust accumulation, physical damage
• Operational verification: Semi-annually against portable reference
• Cleaning: Annual optical chamber cleaning per manufacturer procedure
• Calibration: Per manufacturer specification or every 2-5 years

Common Issues:

• Drift below baseline: Failed ABC algorithm, requires manual calibration
• Erratic readings: Contaminated optics, poor mounting location
• No response to occupancy: Insufficient airflow (duct sensors), sensor failure
• Constant high readings: Calibration drift, insufficient outdoor air introduction

Proper CO2 sensor selection, installation, and maintenance directly determines DCV system effectiveness. Grade B accuracy sensors with dual-wavelength NDIR technology and ABC capability provide the optimal balance of performance, reliability, and lifecycle cost for commercial HVAC applications.