Ventilation can be one of the top energy uses in a high-occupancy building. Spaces designed for large numbers of people are required by code to have HVAC systems that can provide large amounts of outside air. However, these spaces are frequently only partially occupied or are unoccupied. A table developed by the American Society of Heating, Refrigerating & Air-Conditioning Engineers (ASHRAE) shows typical diversity of occupancy throughout the day (Washington State Building Code Council, 2012 Pg AE-34). Demand-controlled ventilation (DCV) represents an untapped potential source of energy savings for existing buildings and an excellent investment in new buildings.

CO₂ sensors are used to optimize the amount of ventilation air provided via the HVAC system. With a CO₂ sensor in the return air plenum, ventilation air will typically not need to be supplied until sometime after the room has been occupied. For HVAC systems serving multiple separate spaces, it is better to locate the CO₂ sensors in each space, three feet above the floor, and provide ventilation air whenever excessive CO₂ is detected in any space. If the control system includes a motion sensor, the supply can be shut off the moment the room becomes unoccupied, regardless of CO₂ levels, but this may not be a cost-effective strategy.

DCV strategies typically include:

• Integrated fan cycling to meet ASHRAE 90.1 requirements.
• Variable frequency drive or two-speed fan motors for additional fan energy savings.
• Fleet ventilation systems for large retail applications. Fleet ventilation is a strategy for areas served by more than one system, which allows selected system(s) to provide all of the space ventilation needs and the remaining system(s) to cycle on and off to maintain temperatures.
• Carbon monoxide monitoring systems for parking garages.
• Motorized damper on outside air to modulate based on CO₂ levels.

This strategy is used routinely, and may not be considered emerging for new construction of large, variable-occupant-density spaces such as auditoriums, gymnasiums, conference rooms, and performing arts centers. This strategy should be considered for retrofitting these types of spaces, and for other spaces where occupancy fluctuates, such as large open office areas and classrooms.

The standard practice for HVAC ventilation is to provide the code-required minimum outside air, which typically specifies minimum ventilation for the maximum design occupancy at all times. However, it has been documented that many buildings do not meet this code baseline, and other buildings provide many times the required ventilation, especially when not fully occupied, and regularly ventilate unoccupied spaces.

Some corporate chains are already adopting DCV as standard practice, in advance of code requirements.

The option for CO₂ control has been included in ASHRAE 62.1 since 1999, and some codes require DCV for larger spaces with high occupant densities. Most codes refer to ASHRAE standards 62.1 and 90.1, which define both the minimum outside air requirements and how DCV can be applied. ASHRAE requirements have changed with every version.

The State of Oregon building codes allow DCV. A proposed code change would require DCV for larger spaces with high occupant densities.

The State of Washington building codes allow DCV via the use of "alternate methods" utilizing ASHRAE 62.1.

The State of Idaho adopted the International Building Code, which permits reduced ventilation during reduced occupancy.

The State of Montana adopted ASHRAE 90.1 for commercial spaces, which allows ventilation to be adjusted for variable occupancy.

CO2 sensors are commercially available, but accuracy and long-term calibration concerns, as well as application and integration issues have prevented widespread adoption. Programming of energy management systems has not been standardized and is a critical element of system operation.

Potential obstacles include difficulties integrating with an existing energy management system or economizer controller, and determining the appropriate location for the CO₂ sensor(s). An effective DCV system requires more careful design than a standard system. For instance, each space being served should have some form of occupant sensing. In some cases, multiple sensors may be needed. For example, a large variable air volume (VAV) system serving multiple spaces would require multiple sensors.

If operators are concerned with the accuracy and tolerance of CO₂ sensors, they may adopt conservative set points and failure modes, reducing the effectiveness of the strategy.

Other factors may govern the ventilation system design and operation, such as pressurization between spaces (e.g., between kitchens and dining rooms), release of contaminants that are a health hazard to occupants, and changes to the return and exhaust air controls. Some applications, such as movie theaters or performing arts centers, cannot tolerate fan cycling.

Public schools may be reluctant to implement this technology because it may raise concerns about indoor air quality (IAQ) among parents who are convinced that poor IAQ may be contributing to their child’s poor performance or rashes. This obstacle may be overcome by letting parents know that the school will take extra steps to ensure that the children have a constant supply of adequate ventilation air by monitoring each space 24 hours per day and adjusting the ventilation air supply as needed.

If variable frequency drives, energy-efficient motors, duty-cycling of fans, dedicated outdoor air systems (DOAS) and performance-tested HVAC are already in place, the energy savings of DCV systems will be less. However, these technologies can be combined with DCV systems to maximize savings.