CO2 Sensors for Demand-Controlled Ventilation

CO2 Sensor Technology for DCV Systems

Carbon dioxide sensors serve as the primary measurement device in demand-controlled ventilation (DCV) systems, enabling real-time modulation of outdoor air delivery based on occupancy levels. The effectiveness of DCV strategies depends critically on sensor accuracy, placement, and integration with building automation systems.

NDIR Sensor Technology

Non-dispersive infrared (NDIR) sensors represent the industry standard for HVAC applications due to their stability, accuracy, and cost-effectiveness. These sensors operate by measuring the absorption of infrared radiation at the 4.26 μm wavelength specific to CO2 molecules.

Operating Principle:

The sensor transmits infrared light through a sample chamber. CO2 molecules absorb energy at their characteristic wavelength, and the detector measures the intensity reduction:

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

Where:

• $I$ = transmitted light intensity (W/m²)
• $I_0$ = incident light intensity (W/m²)
• $\alpha$ = absorption coefficient (m²/mol)
• $C$ = CO2 concentration (ppm)
• $L$ = optical path length (m)

The CO2 concentration is calculated from the Beer-Lambert relationship:

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

Sensor Accuracy and Performance Specifications

ASHRAE 62.1 mandates specific accuracy requirements for CO2 sensors used in DCV applications. The standard requires sensors to maintain accuracy within ±75 ppm or 5% of reading at 1000 ppm, whichever is greater.

Temperature and humidity compensation is essential for accurate readings. Sensors must correct for these environmental variables:

$$C_{corrected} = C_{measured} \times K_T(T) \times K_{RH}(RH)$$

Where $K_T$ and $K_{RH}$ are temperature and humidity correction factors provided by the manufacturer.

Sensor Calibration Methods

Three calibration approaches are employed in HVAC applications:

1. Manual Calibration

Technicians expose sensors to known CO2 concentrations (typically 400 ppm outdoor air and 1000 ppm reference gas):

$$Slope = \frac{C_{ref,high} - C_{ref,low}}{V_{high} - V_{low}}$$

$$Offset = C_{ref,low} - Slope \times V_{low}$$

2. Automatic Background Calibration (ABC)

The sensor assumes the lowest sustained reading over 7-14 days represents outdoor air (approximately 400 ppm) and adjusts automatically. This method is unsuitable for continuously occupied spaces or locations lacking periodic exposure to outdoor air.

3. Single-Point Calibration

The sensor is exposed to outdoor air and adjusted to 400-420 ppm. This simplified method provides adequate accuracy for most HVAC applications.

Sensor Placement Strategy

Strategic sensor placement directly impacts DCV system performance and energy efficiency. ASHRAE 62.1 provides guidance for single-zone and multi-zone applications.

Single-Zone Systems:

• One sensor in the return air stream near the air handler
• Minimum 3 feet from return grille to ensure mixed air sample
• Protected from direct airflow impingement

Multi-Zone Systems:

Multiple sensors required when zones have different occupancy patterns:

$$V_{ot} = \sum_{i=1}^{n} V_{oi} \times \frac{CO2_i - CO2_{outdoor}}{CO2_{design} - CO2_{outdoor}}$$

Where:

• $V_{ot}$ = total outdoor air requirement (cfm)
• $V_{oi}$ = design outdoor air for zone $i$ (cfm)
• $CO2_i$ = measured CO2 in zone $i$ (ppm)
• $CO2_{design}$ = design setpoint (typically 1000-1100 ppm)

graph TD
    A[Outdoor Air<br/>400 ppm] --> B[Air Handler]
    B --> C[Zone 1 Sensor<br/>850 ppm]
    B --> D[Zone 2 Sensor<br/>1050 ppm]
    B --> E[Zone 3 Sensor<br/>720 ppm]
    C --> F[Return Air Stream]
    D --> F
    E --> F
    F --> G[Return Air Sensor<br/>Mixed Average]
    G --> H[BAS Controller]
    H --> I{Control Logic}
    I -->|Increase OA| B
    I -->|Decrease OA| B

    style C fill:#90EE90
    style D fill:#FFB6C6
    style E fill:#87CEEB
    style H fill:#FFD700

Space Sensor Placement:

• Mount at breathing zone height (3-6 feet above floor)
• Avoid locations near doors, windows, or ventilation diffusers
• Minimum 1 sensor per 10,000 square feet or per zone
• Additional sensors for high-density areas (conference rooms, classrooms)

graph LR
    subgraph "Sensor Network Architecture"
    A[CO2 Sensor 1] --> E[Zone Controller]
    B[CO2 Sensor 2] --> E
    C[CO2 Sensor 3] --> E
    D[Return Air Sensor] --> E
    E --> F[BACnet/Modbus]
    F --> G[Building Automation System]
    G --> H[Damper Control]
    G --> I[Fan Speed Control]
    G --> J[Data Logging]
    end

    style E fill:#FFD700
    style G fill:#87CEEB

Control Integration and Algorithms

The BAS integrates CO2 sensor readings with ventilation control using proportional or stepped control strategies.

Proportional Control:

$$OA% = OA_{min} + (OA_{max} - OA_{min}) \times \frac{CO2_{measured} - CO2_{setpoint,low}}{CO2_{setpoint,high} - CO2_{setpoint,low}}$$

Typical setpoints:

• $CO2_{setpoint,low}$ = 800 ppm (minimum outdoor air)
• $CO2_{setpoint,high}$ = 1000 ppm (maximum outdoor air)

Stepped Control:

Maintenance and Verification

Regular maintenance ensures sustained accuracy and system performance:

Quarterly Checks:

• Visual inspection for physical damage or obstruction
• Verify sensor readings against portable reference meter
• Clean optical surfaces per manufacturer instructions

Annual Calibration:

• Two-point calibration using outdoor air and reference gas
• Document drift from previous calibration
• Replace sensors exhibiting drift >75 ppm annually

Performance Verification per ASHRAE 90.1:

Verify DCV operation reduces outdoor air intake during low occupancy periods while maintaining minimum ventilation rates per ASHRAE 62.1. Energy savings potential:

$$E_{saved} = \rho \times c_p \times Q_{reduced} \times \Delta T \times h_{operation}$$

Where typical savings range from 20-40% of conditioning energy in variable-occupancy spaces.

Sensor Selection Criteria

Critical Factors:

• Accuracy specification matching ASHRAE 62.1 requirements
• Temperature range compatible with installation location (-10°F to 120°F minimum)
• Output signal compatibility (4-20 mA, 0-10 VDC, BACnet, Modbus)
• Self-diagnostics and fault reporting capability
• ABC logic disable option for 24/7 occupied spaces
• Warranty period and manufacturer support

Proper sensor selection, placement, and maintenance form the foundation of effective demand-controlled ventilation systems that balance indoor air quality with energy efficiency.