CO Monitoring and Control Systems

Physical Basis of CO Accumulation

Carbon monoxide accumulation in enclosed parking garages results from incomplete combustion in internal combustion engines. The pollutant behaves as a well-mixed gas under typical garage conditions due to:

Molecular diffusion: CO molecular weight (28 g/mol) approximates air (29 g/mol), preventing stratification.

Turbulent mixing: Vehicle movement and thermal plumes from engines create convective currents that rapidly distribute CO throughout the space.

The steady-state CO concentration without ventilation follows:

$$C_{ss} = \frac{G}{Q}$$

where:

• $C_{ss}$ = steady-state concentration (ppm)
• $G$ = CO generation rate (cfm-ppm)
• $Q$ = ventilation airflow rate (cfm)

This relationship demonstrates why demand-controlled ventilation based on actual CO levels provides superior energy efficiency compared to continuous ventilation.

Code-Required CO Limits

The International Mechanical Code (IMC) Section 404 establishes ventilation requirements based on OSHA exposure limits:

Design philosophy: Ventilation systems must maintain concentrations below the 8-hour TWA under normal operations, with alarm and emergency response for excursions toward the 1-hour limit.

CO Sensor Technology and Placement

Electrochemical Sensors

Modern parking garage systems employ electrochemical CO sensors operating on the principle:

$$CO + \frac{1}{2}O_2 \rightarrow CO_2 + 2e^-$$

The oxidation reaction at the sensing electrode produces a current proportional to CO concentration. This current ($I$) relates to concentration:

$$I = nFAk_m C$$

where:

• $n$ = number of electrons (2 for CO)
• $F$ = Faraday constant (96,485 C/mol)
• $A$ = electrode area (cm²)
• $k_m$ = mass transfer coefficient (cm/s)
• $C$ = CO concentration (mol/cm³)

Sensor specifications:

• Range: 0-200 ppm typical
• Accuracy: ±3 ppm or ±5% of reading
• Response time: T90 < 60 seconds
• Operating temperature: -40°F to 122°F
• Expected life: 5-7 years

Strategic Sensor Placement

ASHRAE Standard 62.1 and manufacturer guidelines recommend:

Horizontal spacing: Maximum 2,500 ft² per sensor in areas with good mixing; reduce to 1,000 ft² near ramps, dead-end corners, or low-velocity zones.

Vertical placement:

• 4 to 6 feet above floor level in most applications
• Lower placement (3 feet) near vehicle exhaust pipes captures source emissions
• Avoid ceiling mounting—CO remains well-mixed and ceiling sensors lag concentration changes

Critical locations:

1. Traffic lanes and intersections (highest vehicle density)
2. Ramp approaches (acceleration zones increase emissions)
3. Dead-end parking bays (poor natural circulation)
4. Near garage exits (captures outbound traffic)
5. Multi-level: minimum one sensor per floor level

graph TD
    A[CO Sensor Network] --> B[Zone 1: Entry/Exit Ramps]
    A --> C[Zone 2: Main Parking Areas]
    A --> D[Zone 3: Dead-End Bays]
    A --> E[Zone 4: Upper Levels]

    B --> F[Sensors at 3-4 ft Height]
    C --> G[Sensors Every 2,500 ft²]
    D --> H[Increased Density: 1,000 ft²]
    E --> I[Min 1 Sensor Per Level]

    F --> J[Controller Input]
    G --> J
    H --> J
    I --> J

    J --> K{CO Level Analysis}
    K -->|< 10 ppm| L[Minimum Ventilation]
    K -->|10-25 ppm| M[Stage 1 Fans]
    K -->|25-35 ppm| N[Stage 2 Fans]
    K -->|> 35 ppm| O[All Fans + Alarm]

Multi-Level Monitoring Requirements

For parking structures with multiple levels:

Independent monitoring: Each floor requires separate sensor coverage because vertical air movement between levels is minimal without mechanical ventilation.

Sensor zoning: Divide large floors into control zones of 10,000-20,000 ft², each with 4-8 sensors providing redundant coverage and allowing localized fan control.

Worst-case control: The zone with highest CO reading determines the ventilation rate for that floor, though advanced systems can provide zone-specific control.

Ventilation Control Strategies

Fan Staging Based on CO Levels

Demand-controlled ventilation stages exhaust fans based on measured CO concentrations:

Control logic timing:

• Increase ventilation: Immediate response when threshold exceeded
• Decrease ventilation: 10-15 minute time delay prevents short cycling
• Alarm activation: 2-minute sustained reading above 35 ppm reduces nuisance alarms

Setpoint Selection Rationale

Primary setpoint (15-25 ppm): Activates normal ventilation well below the 35 ppm STEL, accounting for:

• Sensor accuracy (±3-5 ppm)
• Spatial variation between sensor locations
• Response time lag
• Safety factor for occupant protection

Alarm setpoint (35 ppm): Matches OSHA 1-hour STEL. Triggers:

• Maximum ventilation
• Visual/audible alarms at attendant station
• Building automation system notification
• Potential automated emergency response

Advanced Control Algorithms

Proportional control: Fan speed modulates continuously based on CO level:

$$VFD_{speed} = VFD_{min} + (VFD_{max} - VFD_{min}) \times \frac{CO_{measured} - CO_{setpoint}}{CO_{range}}$$

This approach provides smoother operation and better energy efficiency than stepped staging.

Predictive algorithms: Monitor rate of CO increase:

$$\frac{dCO}{dt} = \frac{CO_t - CO_{t-\Delta t}}{\Delta t}$$

Rapid increases trigger preemptive fan activation before setpoints are reached, useful during peak traffic periods.

Energy Savings Through Demand Control

Continuous ventilation at the IMC-prescribed rate of 0.75 cfm/ft² operates fans constantly. CO-based demand control reduces runtime dramatically:

Typical energy reduction: 50-75% compared to continuous operation in facilities with variable occupancy.

Annual operating hours comparison:

Fan energy follows the cube law for speed reduction:

$$\frac{Power_2}{Power_1} = \left(\frac{Speed_2}{Speed_1}\right)^3$$

Operating at 50% speed consumes only 12.5% of full-speed power, making variable-speed control especially advantageous.

System Integration and Commissioning

BAS integration: CO monitoring systems interface with building automation through:

• BACnet or Modbus protocols
• 4-20 mA analog signals (4 mA = 0 ppm, 20 mA = 100 ppm)
• Dry contact relays for alarm conditions

Calibration requirements:

• Initial calibration with certified span gas (50 ppm CO typical)
• Quarterly verification checks recommended
• Annual recalibration per manufacturer specifications
• Zero-point calibration in fresh air

Functional testing: Verify each control stage activates at correct setpoints and confirm alarm notification paths operate properly.

Sensor Maintenance and Reliability

Failure modes:

• Sensor drift (gradual reading increase or decrease)
• Electrolyte depletion (reduced sensitivity)
• Temperature effects (±0.3 ppm/°F deviation from calibration temperature)

Redundancy strategy: Deploy 2-3 sensors per control zone with voting logic:

• Average reading controls ventilation
• Alarm if sensor readings diverge >10 ppm (indicates failure)
• Manual override capability for sensor replacement

Replacement scheduling: Track sensor installation dates and replace at 5-7 year intervals regardless of apparent function to prevent unexpected failures.

References

• IMC Section 404: Enclosed Parking Garages
• ASHRAE Standard 62.1: Ventilation for Acceptable Indoor Air Quality
• OSHA 29 CFR 1910.1000: Air Contaminants (CO limits)