Demand-Controlled Ventilation

Across the U.S., many stores keep long hours every day, and although they may be full of browsing shoppers during some hours of the week, relatively few customers will be milling about the floor space at other times. Occupancy fluctuations like these offer retail stores and other commercial facilities an opportunity for annual energy savings that can amount to as much as $1.00 per square foot (ft2). Instead of continuously ventilating the space at a constant rate designed to accommodate the maximum number of customers, building operators can implement demand-controlled ventilation (DCV) so that the amount of outside air drawn in for ventilation depends on the building’s actual occupancy at any given time. This strategy results in energy savings because it reduces the amount of air that needs to be conditioned as well as the fan energy used to move that air. DCV primarily refers to when actual occupancies are approximated by measuring carbon dioxide (CO2) levels within a building with sensors.

Measure occupancy patterns. If actual, estimated, or proxy data (for example, receipts) on occupancy aren't available, one low-cost way to determine the applicability of DCV is to use a portable CO2 sensor to measure the effective ventilation rate for a given facility. These devices won’t measure occupancy directly, but they will determine the effective ventilation rate per person, based on the difference between measured interior and ambient outdoor CO2 concentrations.

To assess the viability of DCV in a particular facility, locate the portable sensor away from doors, windows, loading docks, and other potential sources of bias, and let it record for a period of at least one week. If CO2concentrations are below 800 parts per million (ppm) much of the time, the facility is probably a good candidate for DCV. Concentrations consistently above 1,000 ppm suggest that DCV is unlikely to provide much in the way of energy savings. However, if CO2 concentrations rise above 1,500 ppm on a regular basis, DCV may be desirable for an entirely different purpose—air-quality improvement. If interior CO2concentrations are getting this high, it's probably an indication that body odors and pollutants—such as off-gassing from building materials, furniture, or other products—are accumulating, and occupant comfort could be improved by increasing ventilation.

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