Application of CO₂ Detectors in Building Ventilation Control

Modern high-rise offices, shopping malls, hospitals, schools, and hotels are densely populated with frequent activity, and indoor air quality directly affects health, productivity, and comfort. Prolonged closed or poorly ventilated spaces can quickly elevate CO₂ levels, causing headaches, drowsiness, reduced concentration, and, in severe cases, affecting decision-making and safety.

Ventilation is the most direct and effective means to improve indoor air quality. Traditional constant air volume systems maintain fixed airflow regardless of occupancy, either wasting energy or underperforming during peak periods. Demand-controlled ventilation (DCV) systems based on CO₂ concentration emerged to dynamically adjust fresh air in real time, implementing “more ventilation when occupied, less or none when unoccupied,” becoming a mainstream technology for green and smart buildings.

Relationship between CO₂ Concentration, Indoor Air Quality, and Ventilation Demand

According to ASHRAE 62.1 and GB/T 18883 "Indoor Air Quality Standards," indoor CO₂ concentration is an important proxy for evaluating ventilation adequacy:

• Outdoor fresh air CO₂ concentration: ~400–450 ppm;
• Good indoor air: typically below 800–1000 ppm;
• Above 1500 ppm: discomfort begins for most people;
• 2000–3000 ppm: noticeable drowsiness and reduced concentration;
• Higher concentrations: potential health risks.

CO₂ primarily originates from human respiration, correlating closely with occupancy and duration. Therefore, CO₂ concentration directly reflects actual ventilation demand, more precise and energy-efficient than relying solely on schedules or temperature/humidity control.

Basic Working Principle of DCV Systems

Typical DCV system architecture:

1. CO₂ sensors monitor key areas (conference rooms, lobbies, offices, classrooms) in real time;
2. Sensors transmit concentration signals (typically 4-20mA or RS485) to the building automation system (BAS) or fresh air controller;
3. Controller adjusts variable-speed fans or dampers based on set thresholds (e.g., 800 ppm to increase fresh air, 1000 ppm reaches maximum design airflow);
4. When CO₂ falls below the lower limit, fresh air is gradually reduced to save fan energy.

Compared with traditional constant air volume systems, DCV can save 30%–60% of fresh air energy in areas with fluctuating occupancy, while maintaining air quality.

Core Advantages of Nexisense CO₂ Detectors

The Nexisense SGA-501 CO₂ detectors are designed for building environments, balancing precision, stability, and system integration:

• High-performance imported NDIR (non-dispersive infrared) sensors, ranges 0–5000 ppm or 0–10000 ppm;
• 2.4-inch HD display showing concentration, units, and alarm status, supporting multiple languages;
• Three configurable alarm thresholds, ≥85dB audible/visual alarms, one-key mute;
• Built-in relay output for direct linkage to fans, dampers, or alarms;
• 4-20mA analog and RS485 digital output (Modbus compatible), compatible with over 90% of BAS, PLC, DDC;
• Full-range temperature and humidity compensation for varying indoor conditions;
• Industrial-grade aluminum housing, suitable for wall or ceiling installation;
• Data storage and export for analysis and energy audits.

These features allow Nexisense detectors to serve as both standalone monitors and fully integrated DCV system sensors.

Typical Applications and Installation Recommendations

Common deployment locations:

• Large open offices: 1–2 sensors per 500–800㎡, installed at 1.2–1.8 m height (breathing zone);
• Conference/multi-function rooms: near return air or activity center, avoiding direct supply air;
• Classrooms/training rooms: opposite or mid-back wall from teaching area;
• Lobbies/restaurants: multiple points based on occupancy patterns;
• Parking lots and equipment rooms: auxiliary monitoring to prevent CO₂ accumulation.

Installation tips:

• Avoid supply/return vents, windows, or doors;
• Choose representative locations, avoid direct sunlight or heat sources;
• Use shielded cables, keep away from high-power lines;
• Annual calibration with standard gas recommended.

Energy Saving and Comfort Benefits

Buildings using Nexisense CO₂-based DCV systems reported:

• 25%–55% reduction in annual fan energy (depending on usage patterns);
• CO₂ maintained below 800–1000 ppm over 95% of the time;
• Reduced complaints of drowsiness and headaches, improved productivity;
• Improved overall HVAC efficiency with heat recovery systems;
• Complete historical data and logs for green building certification and energy benchmarking.

System Integration and Long-Term Maintenance

RS485 and 4-20mA outputs enable easy integration into existing BAS:

• CO₂ → DDC controller → VFD/damper → fresh air unit;
• Centralized monitoring, trend analysis, fault alarms via building management platform;
• Integration with energy management systems for a closed-loop “CO₂–airflow–energy” data chain.

Maintenance:

• Annual calibration with standard gas;
• Clean housing and sensor optics regularly;
• Check relay and communication status;
• Assess sensor life (typically 5–10 years) during major building maintenance.

FAQ

1. Why use CO₂ instead of schedules? CO₂ reflects real occupancy, enabling demand-controlled ventilation and energy savings.
2. Acceptable indoor CO₂? Below 1000 ppm long-term; optimal<800 ppm; above 1500 ppm increase ventilation.
3. Measurement principle? High-precision NDIR detects specific CO₂ absorption wavelengths, high interference resistance.
4. How does it save energy? When CO₂ is low, fresh air reduces, lowering fan speed and HVAC energy consumption.
5. Supported outputs/protocols? 4-20mA, RS485 (Modbus RTU), compatible with most BAS and fresh air controllers.
6. Installation height? 1.2–1.8 m breathing zone, avoid direct supply/return or high traffic below.
7. Frequent calibration? Factory-calibrated; annual verification recommended.
8. Benefits of DCV? Lower energy consumption, improved air quality, higher comfort and productivity, supports green building certification.

Conclusion

In today’s energy-conscious and health-focused environment, CO₂-based DCV is key for achieving energy efficiency, comfort, and healthy indoor air. Nexisense CO₂ detectors provide reliable measurement, flexible communication, and environmental adaptability, forming a high-value sensing foundation for building automation. They enable ventilation to shift from “fixed schedule” to “smart demand,” reducing operating costs and creating a fresher, more productive indoor experience. As smart building and carbon reduction goals advance, precise CO₂ sensors will play a core role in intelligent and green building management.