Demand-controlled ventilation (DCV) regulates the ventilation airflow rate using signals from air-pollutant sensors or occupancy sensors.
Advantages of DCV
DCV provides two advantages compared to time-scheduled ventilation.
- Better control of indoor pollutant concentrations.
- Lower energy use and peak power demand.
DCV sensors most commonly measure carbon dioxide (CO2) and/or relative humidity. Typically, the ventilation system withdraws air from “damp or humid” rooms (bathrooms, kitchens, laundry, work areas) and supplies air to “dry” rooms (offices, living rooms, bedrooms). Occupancy sensors, like those used for lighting systems, may also control ventilation systems. Occupancy sensors are a common strategy for bathroom ventilation.
Human respiration and combustion appliances generate CO2, which is a minor pollutant. DCV controls use CO2 as a surrogate. The term surrogate means that ventilation controls use CO2 concentration to control the concentration of other occupant-related pollutants. Designers assume that controlling CO2 controls all occupancy-related pollutants. CO2 sensors are relatively precise, reliable, and inexpensive compared to other types of DCV pollutant sensors.
A major purpose of ventilation systems is to control relative humidity. Relative humidity (Rh) compares air’s water vapor concentration to the air’s water-vapor saturation point. Thus, Rh indicates water-vapor saturation level of the indoor air. Furthermore, Rh indicates the probability of water-vapor condensation within the building. Condensation threatens human health and building durability.
Building materials emit volatile organic compounds (VOCs) that are detrimental to human health. VOC emission aren’t related to occupancy but instead to the emission rate of building materials. VOC sensors are available, but their performance isn’t as reliable or precise as Rh sensors and CO2 sensors. Because of these shortcomings, few DCV ventilation systems use VOC sensors.
Where to Use DCV
Ventilation research indicates that DCV is cost-effective in these situations.
- The building has a high occupancy.
- One or two pollutants dominate. Ventilation, sufficient to control the target pollutants provides sufficient control of other pollutants.
- The occupancy schedule, occupancy level, or the occupants’ activities that generate pollutants, are variable and unpredictable. (If pollutant concentrations are predictable, a time schedule is sufficient for control.)
- Space heating and cooling is expensive due to a severe climate, expensive energy, or both. Therefore, building owners can save a lot of money by minimizing ventilation.
Where DCV Doesn’t Work Well
DCV doesn’t generally work well in residential buildings for the following reasons.
- Occupant densities are lower and the available CO2 signal is more difficult to discern from background concentrations.
- Occupants may not be the major source of pollutants especially in new dwellings or newly remodeled dwellings.
- Considerable delays may occur between occupants entering the building and CO2 levels reaching the control limit for operation of ventilation system. Therefore, the occupants experience a high exposure when they enter.