Demand controlled ventilation (DCV) is the process of automatically modulating the rate of outdoor air supply (i.e., rate of ventilation) as the "demand" or need for ventilation varies. The objective is to keep ventilation rates at or above design specifications and code requirements and also to save energy by avoiding excessive ventilation rates, as energy is normally required to heat, cool, and dehumidify the ventilation air supplied to buildings. The need for ventilation is increased when the rate of air pollutant generation from indoor sources is high. People and their activities are among the important indoor pollutant sources and in many indoor spaces occupant density is highly variable. Thus, DCV is most often implemented in spaces with sometimes high and temporally variable occupant density, for example meeting rooms and theaters. In the usual application of DCV, ventilation rates are automatically modulated based on measured indoor concentrations of carbon dioxide (CO2), as CO2 is emitted by people as a metabolic by product and more easily measured than other air pollutants resulting from occupancy. When the indoor occupant density is increased, the indoor concentration of CO2 increases, unless the ventilation rate also increases. Carbon dioxide is not generally considered a directly harmful air pollutant at the concentrations found indoors — rather the concentration of CO2 is considered a proxy for the concentration of a variety of other odorous or potentially harmful pollutants emitted by people or their activities. A typical DCV system is designed to modulate ventilation rates over time so that indoor carbon dioxide concentrations do not exceed a set point, or target, value. The set point CO2 concentration is typically between 800 and 1000 parts per million (ppm). The control system modulates the rate of outdoor air supply, when the economizer is not activated, to maintain the indoor carbon dioxide concentration near the set poin. Prior research results provide a scientific underpinning for the practice of modulating ventilation rates based on measured indoor CO2 concentrations. A variety of studies have shown that occupants of buildings with higher indoor concentrations of CO2 more often have building-related adverse health symptoms and are less satisfied with indoor air quality. Some research has found that occupants of buildings with higher CO2 concentrations have more absence from work or school and perform work more slowly. More information on this subject is available at the Indoor Air Quality Scientific Findings Resource Bank web site. DCV can significantly reduce building energy consumption and also prevent potentially harmful low ventilation rates. For many spaces with a high and variable occupant density, California’s Title 24 Building Energy Efficiency Standards, as of 2016, require demand controlled ventilation (DCV) systems. (For details on requirements go here.) The main purpose of DCV is to save energy by allowing reduced ventilation when occupancy is less than design occupancy. Title 24 also requires provision of a minimum rate of outdoor air supply per unit floor area during occupancy, regardless of the indoor CO2 concentration. DCV is sometimes used in other space types, even when not required by codes. The energy savings potential of DCV varies with climate, building or space type, occupant density, variation in occupancy over time, and DCV control scheme. Also, energy savings depend on the minimum ventilation rates that would have been provided if DCV was not employed. |
Recommendations >
