Demand-controlled ventilation
Demand-controlled ventilation adjusts outdoor-air ventilation based on changing need, often using carbon dioxide, occupancy, or air-quality sensors. Instead of ventilating every zone at a fixed maximum rate all day, a DCV system can reduce airflow when spaces are lightly occupied and increase it when people or pollutants rise.
What demand-controlled ventilation is
Demand-controlled ventilation, or DCV, is a control strategy for mechanical ventilation. It changes outdoor-air airflow in response to actual or estimated need. A classroom, conference room, theater, gym, laboratory, or office zone may need high ventilation when crowded and much less when empty. DCV tries to match airflow to that changing demand while still meeting minimum air-quality and safety requirements.
How CO2-based DCV works
People exhale carbon dioxide, so indoor CO2 levels can indicate occupancy when the main pollutant source is people. A CO2 sensor sends readings to a building control system, which adjusts dampers, fans, or variable-air-volume boxes. The system may compare indoor CO2 to outdoor CO2 or use a setpoint that represents acceptable ventilation for expected occupants.
Other control signals
DCV does not have to rely only on CO2. Occupancy counters, motion sensors, booking schedules, badge data, humidity sensors, volatile organic compound sensors, particulate sensors, or contaminant-specific sensors can inform ventilation. The right signal depends on the space. A gym, restroom, laboratory, kitchen, and lecture hall can have very different ventilation drivers.
Energy and comfort
Ventilation air usually must be heated, cooled, filtered, humidified, or dehumidified. Reducing unnecessary outdoor air can lower fan energy and heating or cooling loads, especially in climates with extreme temperatures or humidity. DCV can also improve comfort by preventing rooms from being overcooled, overheated, or over-humidified because of constant high outdoor-air rates.
Sensor placement and calibration
DCV depends on trustworthy measurements. A CO2 sensor near a supply diffuser, open window, door, return grille, or occupant breathing directly into the device may misrepresent the zone. Sensors can drift and may need calibration or replacement. Control systems should document sensor location, calibration interval, outdoor-air assumptions, and what happens when a sensor fails.
Limits of CO2
CO2 is useful, but it is not a complete indoor-air-quality measurement. It does not directly measure particulates, formaldehyde, ozone, combustion products, radon, moisture damage, infectious aerosols, or chemicals from materials and activities. CO2-based DCV works best in spaces where people are the dominant source and where minimum ventilation is still maintained for building emissions and background pollutants.
Control logic
A DCV system needs more than a sensor and a damper. It needs a sequence of operation: minimum airflow, maximum airflow, response speed, averaging, alarm limits, outdoor-air measurement, economizer interaction, heating and cooling coordination, and overrides for schedules or special events. Poor logic can save energy on paper while creating stale rooms or unstable HVAC operation.
Where DCV fits
DCV is most useful where occupancy changes a lot and ventilation loads are significant. Examples include classrooms, auditoriums, conference rooms, worship spaces, restaurants, gyms, libraries, and some office zones. It may be less useful where occupancy is steady, pollutant sources are not people, or codes require fixed exhaust or high air-change rates for safety.
Why it matters
Demand-controlled ventilation matters because ventilation is both a health tool and an energy load. Buildings need enough outdoor air, but not every zone needs the same amount at every hour. Well-designed DCV can make ventilation more responsive. Poorly designed DCV can hide problems behind a green dashboard, so sensors, minimums, commissioning, and maintenance are the heart of the strategy.