CO2 Sensors: A Crucial Component in HVAC Indoor Air Quality Management

Due to the global pandemic, IAQ (IAQ) has become far more important in daily life. However, the significance of IAQ for HVAC system design and performance has been understood for much longer.

Image Credit: Amphenol Advanced Sensors

CO₂ sensors are a mainstay of HVAC IAQ management and represent one of the most crucial monitoring devices in any system. These sensors monitor the air for CO₂, a natural byproduct of breathing that is harmful in high concentrations, supplying critical data on ventilation effectiveness and occupancy levels.

While CO₂ sensors do not always receive the same level of attention as other IAQ sensors in HVAC monitoring systems, they remain one of the most important components.

Importance of IAQ

Arguably, no element is more important for advanced indoor climate control than HVAC IAQ. While temperature changes are easy to detect, other factors that impact IAQ can be harmful and often go unnoticed.

The health implications of poor IAQ can be extreme. Inadequate ventilation and filtration can result in a build-up of pollutants, including CO₂, volatile organic compounds (VOCs), particulates, and microbial contaminants.

These pollutants can trigger various health issues, from headaches and eye irritation to more severe respiratory diseases.

Poor IAQ can significantly impact cognitive functions, including decision-making and concentration, in settings such as schools and offices.

CO₂ as an HVAC IAQ Indicator 

CO₂ concentration is one of the oldest yet most important indicators HVAC IAQ systems monitor. It has been used for decades to assess IAQ and ventilation effectiveness.

While CO₂ is not considered a pollutant like VOCs, its presence in enclosed spaces poses a threat to health and IAQ in many of the same ways.

At concentrations over 1000 parts per million (ppm), CO₂ can result in discomfort and health issues, including drowsiness and decreased cognitive function.

High CO₂ levels can also correlate with the presence of other pollutants. Increased CO₂ concentrations usually indicate poor ventilation, meaning other indoor air pollutants and contaminants may have also accumulated.

CO₂ Sensors

As with any metric, there is more than one way to measure CO₂ levels in a space. As a result, there are many sensor types available for HVAC systems, including:

• Electrochemical sensors
• Metal oxide semiconductor (MOS) sensors
• Photoacoustic sensors
• Chemical sensors

The most common type of CO₂ sensor employed in HVAC system design is the Non-Dispersive Infrared (NDIR) sensor, favored for its high reliability and accuracy. NDIR sensors operate based on the principle that CO₂ molecules absorb specific light frequencies depending on their structure.

The basic design of an NDIR sensor includes an air sample chamber, an infrared light source, an infrared filter, and an infrared detector. The CO₂ concentration is calculated by measuring the amount of infrared light absorbed by CO₂ in the air flowing through the sample chamber.

NDIR sensors can be split into two categories: nuanced and better suited for specific applications.

Single-channel NDIR sensors employ a single wavelength detection design and sophisticated firmware algorithms to maintain sensor accuracy over their lifetime.

Dual-channel NDIR sensors include two independent wavelength detection measurements to compensate for sensor drift. The second reference channel helps compensate for any variances in the light source or sensor over time, enhancing the accuracy and reliability of the measurements.

CO₂ Sensor Integration

Although advanced sensor integration into new and existing HVAC systems is not impossible, it still requires some forethought and careful consideration.

For effective integration of HVAC CO₂ sensors, several factors must be considered: 

1. Compatibility with existing HVAC systems
2. Sensor accuracy and calibration 
3. Single vs. dual channel technology 
4. Sensor placement and coverage 
5. Communication protocols
6. New IAQ regulations 

Compatibility with Existing HVAC Systems

Adding any advanced sensor technology to an existing system is usually not as simple as plug-and-play, particularly with older systems.

Older HVAC systems were not designed with advanced connectivity and compatibility in mind, features required to interface seamlessly with modern CO₂ sensor modules. Compatibility issues can arise due to differences in communication protocols, such as UART, I2C, and PWM.

This mismatch can result in issues with accurate data transmission and sensor functioning. The technological disparity between new sensors and old systems may require additional interfaces or software for proper integration, and in some cases, means using new hardware, including wireless and battery-operated equipment.

Sensor Accuracy and Calibration

During their lifetime, CO₂ sensors can lose drift, decreasing their ability to measure CO2 levels accurately.

While no sensor is made to last forever, selecting the correct sensor and calibration protocol helps ensure the device remains functional and accurate for as long as possible.

Proper sensor selection and maintenance can also enhance energy savings and improved air quality.

One of the biggest mistakes in HVAC system design is integrating sensors in positions that later become inaccessible. Even wireless sensor technology has range limits.

Single Vs. Dual Channel Technology 

Both types of NDRI sensors have applications that are better suited to their specific functionality.

For single-channel sensors, consideration of the operating environment is important. The sensor will function only with periodic drops to ambient CO₂ levels (no less than 400 ppm). Such sensors are ideally suited to HVAC systems in movie theaters, exhibition halls, or automotive applications. 

Dual-channel sensors contain more technology and are ideal for more demanding situations where CO₂ levels do not vary much, such as greenhouses, hospitals, and continuously occupied buildings. They should never be installed in spaces where ambient CO₂ levels drop below 400 ppm.

CO₂ Sensor Placement and Coverage

Besides accessibility, CO₂ sensor placement is the most important consideration, as sensors in suboptimal positions do not provide the required data.

Some important factors to consider when deciding on sensor placement are: 

• Sensors should generally be positioned within the breathing zone (approximately 4-6 feet from the floor)
• Sensors should be placed away from doors, windows, and direct ventilation sources to avoid skewed readings due to external air.
• Sensors should be placed in different zones in large buildings with varied environments, such as schools, offices, and commercial spaces, to ensure CO₂ levels are accurately monitored in all areas and differences in occupancy and activity levels are accounted for.
• In areas where CO₂ usage is high or where CO₂ is stored (such as grow rooms and storage areas), sensors should be placed lower and closer to potential leak sources, unlike in general office or residential areas.

Communication Protocols

Sensors must be able to take readings and interface with the HVAC’s control system.

The majority of HVAC systems still rely on analog communication protocols. Analog sensors typically provide a linear output, generally with ranges of 0-5 or 0-10 V. This method of communication is widely adopted due to its reliability, simplicity, and ease of integration with various HVAC systems.

However, the market is shifting toward digital communication protocols, which offer several advantages over analog, including higher accuracy, more detailed data, and improved noise immunity. Digital is also essential for modern building automation systems.

New IAQ Regulations 

The regulatory landscape regarding IAQ and CO₂ monitoring systems is changing.

Post-pandemic, governments and industry groups have implemented new standards and guidelines, setting more strict requirements for HVAC system performance. Old regulations, many of which are industry standards, such as the ANSI/ASHRAE Standards 62.1 and 62.2, are being updated.

These new rules and regulations have a significant impact on HVAC system design.

Monitoring CO₂ for Better HVAC IAQ

The importance of IAQ in building management is greater than ever before. Regardless of how HVAC systems or regulations evolve, CO₂ monitoring will remain a major component of keeping indoor environments safe for occupants.

Integrated HVAC systems with advanced sensor technology provide an easier and more efficient way to keep CO₂ levels in check and spaces properly ventilated.

This information has been sourced, reviewed and adapted from materials provided by Amphenol Advanced Sensors.

For more information on this source, please visit Amphenol Advanced Sensors.