Variable-air-volume HVAC systems using occupancy sensor control in commercial buildings can result in significant energy savings compared to typical HVAC systems in both new and retrofitted buildings.
These are the results of a study published by the US Department of Energy titled, "Energy Savings for Occupancy-Based Control (OBC) of Variable-Air-Volume (VAV) Systems." The study showed that on average, commercial buildings using variable-air-volume (VAV) systems achieved a 17% to 23% energy savings over the life of the buildings.
The energy savings were possible because typical HVAC control systems set air flow rates based on maximum occupancy for a given area to conform with minimum fresh air requirements. Building occupancy, however, varies dynamically. Conference rooms, cafeterias, auditoriums and other assembly spaces are often unoccupied for significant periods of time. Office occupancy varies throughout the day. This results in over-ventilation, which wastes both heating and cooling energy.
But how does the study work in the real world? One example is the Empire State Building, which had a energy-savings retrofit in 2011 including VAV systems controlled by CO2 transmitters. In 2014, the building management reported that they had surpassed the energy savings originally guaranteed for the third consecutive year. This year, the property beat its energy-efficiency guarantee by 15.9 percent, saving $2.8 million. Over the past three years, the program has generated a total of approximately $7.5 million in savings.
While there are many ways to determine room occupancy, carbon dioxide sensors mounted in wall boxes are the most common. They are low-cost, and are designed to easily integrate into HVAC systems using industry-standard protocols like 4-20mA or 0-10 VAC outputs.
For example, CO2Meter.com offers the aSense Ventostat CO2 + RH/T wall-mounted CO2 transmitter that is specifically designed to monitor CO2 levels in offices, classrooms and commercial spaces.