DCV Modelling


Demand control ventilation (DCV) is a ventilation system capability that provides automatic reduction of outdoor air intake below design rates when the actual occupancy of spaces served by the system is less than design occupancy. In most commercial buildings ventilation is provided to deal with two types of indoor pollution: odours from people, and off-gassing from building components and furniture. When a space is vacant, it has no people pollution so the people-related ventilation rate is not needed. Many types of high-occupancy spaces, such as classrooms, multi-purpose rooms, theatres, conference rooms, or lobbies have ventilation designed for a high peak occupancy that rarely occurs.


Demand-controlled ventilation has the potential to provide significant HVAC energy savings by conditioning only the amount of ventilation air necessary to maintain good indoor air quality. It can be particularly effective in buildings or zones that have widely varying occupancy.


DCV is most successful when reducing ventilation rates – and so reducing fan energy use – when meeting the demands of indoor air quality (IAQ). It is not necessarily so successful, in energy terms, for high design cooling loads, since airflows will increase above the required IAQ minimum to meet space cooling loads. Practically, this is likely to mean that IAQ-driven DCV is most effective in ASHRAE climate zones 4 and above.


Schematic of a typical DCV system from Demand-Controlled Ventilation and Sustainability with permission from author Tom Lawrence and ASHRAE.

Summary of Steps to Model DCV in DesignBuilder


This section provides a summary of the steps required to model an air system with DCV outdoor air control in DesignBuilder.


  1. Make sure you are using Detailed HVAC.
  2. On the Outdoor air system tab of the AHU dialog under the Demand Controlled Ventilation header, switch on Demand controlled ventilation. Recirculation must be selected on the AHU dialog to enable this setting.
  3. Select the System outdoor air method (VRP, IAQP etc).
  4. Ensure that the Simulate CO2 concentration option at site level on the Location tab under the Site Details > Outdoor Air CO2 and Contaminants headers is switched on.
  5. If one of the IAQP methods are selected then the CO2 and contaminant control option must be set on the HVAC Zone dialog for at least one zone connected to the AHU. CO2 setpoints must also be set. This is done on the Activity tab under the CO2/Contaminant Setpoints header if the Simple HVAC activity data option is being used or on the HVAC zone dialog if using the Detailed HVAC activity data option.
  6. You can check the success of the DCV control in various ways, for example:
    1. Use the ac/h data for zones and for the building within DesignBuilder:

    2. Check CO2 levels within selected zones using the Results Viewer. You should find that the Zone Air CO2 Concentration report is automatically included in the .eso output file when DCV control has been specified.