CO₂ Monitoring and Demand-Controlled Ventilation

Fundamental Principles

Demand-controlled ventilation (DCV) modulates outdoor air ventilation rates based on actual occupancy rather than design occupancy. Carbon dioxide (CO₂) serves as an effective proxy for occupant-generated bioeffluents and metabolic contaminants. The mass balance equation governing CO₂ concentration in a ventilated space:

$$\frac{dC}{dt} = \frac{N \cdot G - Q(C - C_o)}{V}$$

Where:

• $C$ = indoor CO₂ concentration (ppm)
• $t$ = time (hours)
• $N$ = number of occupants
• $G$ = CO₂ generation rate per person (cfm·ppm or L/s·ppm)
• $Q$ = ventilation rate (cfm or L/s)
• $C_o$ = outdoor CO₂ concentration (typically 400-450 ppm)
• $V$ = space volume (cubic feet or cubic meters)

At steady-state conditions ($dC/dt = 0$), the required ventilation rate to maintain a target CO₂ concentration:

$$Q = \frac{N \cdot G}{C_{target} - C_o}$$

ASHRAE Standard 62.1 recognizes DCV as an acceptable ventilation strategy for spaces with variable and unpredictable occupancy, provided CO₂ sensors maintain calibration and the control system responds appropriately.

CO₂ Generation Rates

Human metabolic CO₂ production varies with activity level and metabolic rate:

For typical office environments, a standard generation rate of 0.3 L/s (0.0127 cfm·ppm) per person is commonly applied.

Sensor Technology and Placement

Non-Dispersive Infrared (NDIR) Sensors

NDIR sensors measure CO₂ concentration based on infrared absorption at 4.26 μm wavelength. CO₂ molecules absorb specific infrared frequencies proportional to concentration. The Beer-Lambert Law describes absorption:

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

Where:

• $I$ = transmitted light intensity
• $I_0$ = incident light intensity
• $\alpha$ = absorption coefficient
• $C$ = CO₂ concentration
• $L$ = optical path length

Sensor specifications:

• Accuracy: ±50 ppm or ±3% of reading
• Range: 0-2000 ppm (typical) or 0-5000 ppm (extended)
• Response time: 60-120 seconds
• Calibration interval: 2-5 years (self-calibrating) or annually (manual)
• Operating temperature: 32-122°F (0-50°C)

Strategic Sensor Placement

1. Return air pathway: Single sensor in return duct measures mixed space conditions
2. Breathing zone: 3-6 feet above floor in occupied zone
3. Away from sources: Minimum 6 feet from exhaust grilles, operable windows, or occupant clusters
4. Representative location: Areas reflecting average space occupancy

graph TD
    A[Outdoor Air<br/>400-450 ppm] -->|Ventilation Rate Q| B[Occupied Space<br/>Volume V]
    B -->|Return Air| C[CO₂ Sensor<br/>Measures C]
    D[Occupants N] -->|Generation Rate G| B
    C -->|Feedback Signal| E[DCV Controller]
    E -->|Damper Position| F[OA Damper]
    F -->|Modulate Airflow| A
    E -->|Minimum OA<br/>Override| G[Ventilation Standard]

    style C fill:#f9f,stroke:#333,stroke-width:2px
    style E fill:#bbf,stroke:#333,stroke-width:2px

Control Strategies

Proportional Control

The ventilation rate adjusts proportionally to CO₂ concentration above outdoor levels:

$$Q = Q_{min} + K(C - C_o)$$

Where:

• $Q_{min}$ = minimum ventilation rate per code
• $K$ = proportional gain (cfm/ppm or L/s/ppm)
• $C$ = measured indoor CO₂ concentration

Setpoint Control

Outdoor air dampers modulate to maintain indoor CO₂ at or below a setpoint (typically 1000-1200 ppm):

$$Q = Q_{min} \text{ when } C \leq C_{setpoint}$$ $$Q = Q_{design} \text{ when } C > C_{setpoint}$$

Reset Control

The setpoint resets based on outdoor temperature or cooling load to optimize energy consumption while maintaining air quality.

Design Requirements Per ASHRAE 62.1

Minimum ventilation airflow: DCV systems must provide the breathing zone outdoor airflow required by ASHRAE 62.1 at design occupancy conditions:

$$V_{bz} = R_p \cdot P_z + R_a \cdot A_z$$

Where:

• $V_{bz}$ = breathing zone outdoor airflow (cfm)
• $R_p$ = outdoor air rate per person (cfm/person)
• $P_z$ = zone population (design occupancy)
• $R_a$ = outdoor air rate per unit area (cfm/ft²)
• $A_z$ = zone floor area (ft²)

Sensor requirements:

• Located in return air stream or breathing zone
• Accuracy within ±75 ppm of actual concentration
• Maintained and calibrated per manufacturer specifications

Applicable spaces: Spaces with design occupancy ≥25 people/1000 ft² and variable occupancy patterns (classrooms, meeting rooms, theaters, restaurants).

Energy Savings Analysis

Annual energy savings from DCV implementation:

$$E_{savings} = \rho \cdot c_p \cdot \Delta T \cdot (Q_{design} - Q_{avg}) \cdot h_{occupied} \cdot \eta_{system}$$

Where:

• $\rho$ = air density (0.075 lb/ft³ or 1.2 kg/m³)
• $c_p$ = specific heat of air (0.24 Btu/lb·°F or 1.005 kJ/kg·K)
• $\Delta T$ = temperature difference between outdoor and indoor air
• $Q_{design}$ = design ventilation rate
• $Q_{avg}$ = average DCV-controlled ventilation rate
• $h_{occupied}$ = annual occupied hours
• $\eta_{system}$ = system efficiency factor

Typical energy savings range from 20-60% of ventilation-related heating and cooling energy, depending on:

• Climate zone
• Occupancy variability
• Space type
• Operating schedule

Performance Comparison

Implementation Considerations

Sensor calibration: Self-calibrating sensors use ABC (Automatic Background Calibration) logic, assuming periodic exposure to outdoor air concentrations. Manual calibration with reference gas (400 ppm or 1000 ppm) provides higher accuracy.

Minimum ventilation override: Systems must provide code-required minimum ventilation regardless of CO₂ readings to address non-bioeffluent contaminants (materials emissions, cleaning products).

Response time: Controller algorithms must account for sensor lag time (60-120 seconds) and mixing dynamics to prevent oscillation or underdamping.

Multiple zone systems: Each zone requires individual sensors or proportional distribution based on representative sampling locations.

Commissioning verification: Functional testing confirms sensor accuracy, control response, minimum airflow delivery, and alarm functionality at various occupancy levels.

Integration with Building Automation

Modern DCV systems integrate with building automation systems (BAS) to provide:

• Real-time CO₂ trending and alarming
• Automatic sensor drift compensation
• Coordinated control with economizer cycles
• Occupancy-based scheduling optimization
• Energy consumption tracking
• Remote diagnostics and calibration verification

The synergy between CO₂-based DCV and other energy conservation measures (economizers, variable air volume, heat recovery) maximizes both indoor air quality and energy efficiency across diverse building types and climates.