Ventilation is required so that the human occupants in buildings are provided with fresh air. The purpose is to provide oxygen and dilute other gases such as CO2 and human odors.

ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) is a technical organization in the United States that recommends the systems and procedures for ventilation equipment. It has focused on two basic methods. The "dilution method" is used most frequently where pollutants are diluted by fresh air. Demand-controlled ventilation (DCV) is considered a dilution method. The second method, called the "Indoor Air Quality (IAQ) Procedure," is used to identify and remove specific contaminants. The IAQ method is not addressed in this guide.

The concept of automatic ventilation control based on occupant demand has been known for over 20 years. The barrier to widespread implementation was not having a cost-effective, simple, and reliable sensor. Early sensors did not provide the reliability that was needed in many applications.

In addition, the cost for the sensors was high. They were used in specialized applications primarily for indoor air quality purposes. Other methods were also explored. For example, as computers became used more in theater operations, the operators began to use the computers to track ticket sales to control ventilation rates.

In recent years, advances in sensor technology have shown that demand-controlled ventilation is now both feasible and cost-effective. Interpretations of the ASHRAE guidelines indicate that demand-controlled ventilation is acceptable when properly designed and installed.

 

Demand-controlled ventilation adjusts outside ventilation air based on the number of occupants and the ventilation demands that those occupants create. DCV is part of a building's ventilation system control strategy. It may include hardware, software, and control strategy and is an integral part of a building's ventilation design.

 

Large assembly spaces such as gymnasiums, auditoriums, lecture halls, conference rooms, churches, and theaters are good candidates for DCV. These spaces are designed for large numbers of people with high outside air requirements. However, the spaces are frequently only partially occupied. It is expected that, in the future, most spaces with ventilation air capacities of at least 1,500 cubic feet per minute (CFM) and serving areas having an average occupant load factor of 20 or less will be designed with DCV features.

 

DCV is a ventilation control strategy that provides just the right amount of outside air that is needed by the occupants. Active control of the ventilation system can provide the opportunity to control indoor air quality. It can save energy.

Figure 1 shows the ventilation savings potential (area in gold) for a typical application where DCV replaces fixed scheduled ventilation.

 

Figure 1 - Comparison of ventilation rates for different system designs

 

Many of the components that are used in controlling outside air may already be in place. These existing components could include an economizer or air makeup unit with modulating dampers. The additional components would be control sensors to communicate either directly with the economizer or with a central computer. The extra components might include carbon dioxide (CO2) sensors, occupancy sensors, or turnstile counters. Programming, while not a physical component, is a critical element of system operation.

 

Good communication among those implementing DCV systems is critical to a successful project. The building owner, architect and engineer, installer, commissioning agent and building inspector have different perspectives, but should work together to produce a good ventilation system.

This guide is organized into multiple sections that focus on the needs of a particular group. The sections are: