When Is Demand Control Ventilation Required?
Demand controlled ventilation, or DCV, is one of the most effective methods to improve the energy efficiency of HVAC systems. DCV is based on a simple principle: when indoor spaces are at partial occupancy, the full design airflow is not needed. The equation is simple: moving less air requires less fan power, reducing electricity expenses. Since the air volume that must be conditioned decreases, there are heating and cooling savings as well. All of the savings happen without compromising the temperature setpoint, and hence perceived comfort.
ASHRAE Standard 62.1 is one of the main design references for airing systems, and its requirements have been included in building codes worldwide. The operating conditions for a DCV system are covered by the standard, but DCV itself is not specified as a mandatory requirement. However, local building codes often make DCV mandatory for certain types of edifices. Two examples are the California Energy Code and the New York City Energy Conservation Code, which require DCV in many commercial occupancies.
In a few words, DCV can be mandatory or optional depending on local building codes and the type of construction. However, given its efficiency benefits, DCV is highly recommended even when optional. Note that ASHRAE 62.1 provides two procedures for ventilation system design:
- The Ventilation Rate Procedure (VRP), based on prescriptive outdoor airflow values. These are determined by the building type, floor area, and number of occupants.
- The Indoor Air Quality Procedure (IAQP), based on air contaminants of concern. The ventilation system relies on the readings from a dedicated sensor for each of the pollutants in order to keep their concentrations below a threshold. Airflow values below those in the VRP are acceptable. However, the thresholds must be based on an authoritative reference such as the World Health Organization.
Building code requirements should be reviewed carefully before proceeding with a DCV design. Many codes only recognize the VRP as an accepted method, since it uses airflow tables that have been developed with extensive research by ASHRAE. For example, classrooms require 10 cfm per square foot and 0.12 cfm per occupant, according to the standard. The IAQP has the potential to achieve a lower airflow and so larger energy savings, while conserving air quality. However, there is no global standard for air pollution limits that can be referenced in this case.
How DCV Influences Energy Efficiency and Air Quality
HVAC savings are a direct benefit of DCV, but the outlook is different for indoor air quality. Based on how DCV is implemented, air quality can improve, worsen or stay the same. Ideally, the airing system design should improve both efficiency and air quality.
Note that ASHRAE standards and local building codes consider both efficiency and air quality. Therefore, a DCV design that worsens air quality is most likely against the codes. There are two main ways in which a poor implementation of DCV can have a negative impact on indoor environments:
- Reducing outdoor airflow excessively: A building interior does not need the full airing capacity during partial occupancy. However, if the OA flow rate is reduced too drastically, pollutant concentrations start increasing because the air is not renewed fast enough.
- Missing key pollutants when applying the Indoor Air Quality Procedure (IAQP): The IAQP does not specify a minimum outdoor airflow, and is instead focused on controlling the concentration of pollutants directly. This is effective as long as the design process accounts for all relevant substances. However, if one air pollutant is missed, the airing system will not have a dedicated sensor for that pollutatnt and there will be no direct control over its concentration.
To summarize, a DCV system must not reduce airflow to a point where air pollutants can accumulate and reach harmful levels. The design must also consider long-term health effects: some air pollutants are not immediately dangerous at low concentrations, but may lead to health issues in the long run. Since DCV responds to CO2 concentration, which is related with occupancy, key pollutants may be missed.
A well-designed DCV system reduces airflow as much as possible while keeping pollutant concentrations below the established thresholds. There is no global standard for air pollution levels, but the reference values from the World Health Organization offer the closest thing.
Depending on local building codes and the occupancy classification of a property, DCV can be mandatory or optional. DCV is also included among the prerequisites for LEED certification – buildings without it are not eligible, regardless of how high they score in the LEED rating system.
Building codes that make DCV mandatory often demand a VRP-based system. This is a very important requirement, since an airing system designed with the indoor air quality procedure may not be eligible. However, the IAQP still provides a useful approach to control air pollutants independently from occupancy sensors. A combined design approach that uses both the VRP and IAQP can meet building codes that demand the VRP, while bringing the air quality benefits of the IAQP.