Carbon dioxide (CO₂) sensors play an increasingly important role in modern HVAC control strategies—especially in environments where occupancy varies throughout the day. When used correctly, these sensors help maintain indoor air quality, optimize ventilation, and significantly reduce energy consumption.
Why CO₂ Matters in HVAC Control
CO₂ is naturally present in the atmosphere. Outdoor concentrations typically range from 300–500 ppm. Indoors, CO₂ levels rise as people exhale. A person at rest produces around 456 liters of CO₂ per day, according to a New York Times estimate. As occupancy increases, CO₂ concentration increases—and that rise can be used as a signal for when to bring in fresh outside air.
This concept forms the foundation of Demand Control Ventilation (DCV).
Meeting Ventilation Requirements
ASHRAE standards require certain levels of fresh outside air during occupied hours to maintain acceptable indoor air quality. During unoccupied hours, conditioning outside air is not necessary—reducing energy demand.
But what about spaces that are not occupied continuously?
That’s where CO₂-driven control strategies become valuable.
What Is On-Demand (CO₂-Based) Ventilation?
Demand Control Ventilation uses CO₂ sensing to introduce fresh air only when occupancy levels increase. Instead of maintaining constant outside air ventilation, the system responds to real-time CO₂ concentrations.
This method is ideal for spaces with unpredictable or intermittent use, such as:
- Conference rooms
- Cafeterias
- Classrooms
- Auditoriums
- Worship centers
- Multi-purpose rooms
When CO₂ levels rise above normal thresholds, actuators open outdoor air dampers, increasing ventilation. When levels drop, dampers close—preserving energy and maintaining comfort.
Understanding CO₂ Concentration
CO₂ is measured in parts per million (ppm), which represents the quantity of CO₂ molecules per million molecules of air.
Here’s a general guide to CO₂ level interpretation:
|
CO₂ Level (ppm) |
Impact |
|
300–500 |
Typical outdoor levels |
|
<600 |
Acceptable indoor level |
|
600–1000 |
Odors noticeable, comfort declines |
|
1000 |
ASHRAE/OSHA threshold |
|
1000–2500 |
Drowsiness, reduced focus |
|
2500–5000 |
Headaches and adverse effects expected |
|
30,000 |
Nausea, increased breathing and pulse rate |
|
50,000 |
Headaches, impaired vision |
|
100,000 |
Unconsciousness, risk of death |
High CO₂ levels are not just about discomfort—they can impair concentration, cause headaches, and indicate poor ventilation.
CO₂ and Indoor Odor Issues
While CO₂ itself is not typically harmful at normal indoor levels, research shows that odors and discomfort become more noticeable at around 600 ppm. That’s because CO₂ levels often correlate with increased concentrations of Volatile Organic Compounds (VOCs).
VOCs come from
- Cleaning chemicals
- Building materials
- Paints
- Furniture
- Carpet adhesives
The EPA notes that VOC concentrations can be up to 10 times higher indoors than outdoors. Elevated CO₂ levels can amplify the perception of these odors.
Energy Savings With CO₂-Based DCV
Demand Control Ventilation can produce significant energy savings, especially in buildings where occupancy varies widely. Instead of conditioning outside air continuously, the system introduces fresh air when needed.
Savings are influenced by:
- Climate conditions
- Occupancy patterns
- Control strategy
- Equipment efficiency
Even without exact percentages, the potential reductions in heating and cooling costs can be substantial.
Real-World Example
Consider a large gymnasium conditioned 24/7 with basic actuator-controlled dampers. If no DCV or economizer strategy is present, the system may:
- Bring in fresh air continuously
- Condition outside air even when the space is empty
- Waste energy due to outdated or uninformed control decisions
Education and communication between contractors and building managers are essential. Properly informing facility operators about code requirements and available technologies can prevent long-term waste and reduce operating costs.
Final Thoughts
CO₂ sensors are a powerful tool in modern HVAC control design. When paired with Demand Control Ventilation strategies, they:
- Maintain acceptable indoor air quality
- Reduce unnecessary outdoor air conditioning
- Improve comfort during peak occupancy
- Lower operational energy costs
For spaces with fluctuating occupancy, CO₂-based control offers a practical, efficient, and code-compliant solution.
