Demand controlled ventilation in school and office buildings: lessons learnt from case studies

Demand controlled ventilation (DCV) refers to a ventilation system with air flow rates that are controlled based on a measurement of an indoor air quality (IAQ) and/or thermal comfort parameter. DCV operates at reduced air flow rates during a large amount of the operation time. Due to this decrease, less energy is needed for fan operation and heating/cooling the supply air. However, uncertainty still exists about the IAQ performance and ventilation efficiency in the room, especially at lower air flow rates. Aim of this paper is to evaluate the IAQ performance, ventilation efficiency, operation and energy efficiency of DCV in school and office buildings based on measurement results. This research was done in cooperation with Flemish manufacturers of ventilation components, engineering offices designing HVAC and industrial federations.
Four case studies with large and varying occupancy and with different use and ventilation systems are selected. First case study is a school building with natural air supply and mechanical extraction controlled by measured indoor CO2 concentration. Second study consist of an educational building with lecture rooms. In this building balanced mechanical ventilation is controlled by CO2 and temperature measured at the extract air grill. Finally, in two different office buildings, three landscaped offices are evaluated. Here the balanced mechanical ventilation is controlled by CO2 concentration and temperature.
To evaluate IAQ, ventilation efficiency and the energy savings of the system, the following parameters are monitored: CO2 concentrations and air? temperatures at different positions in the room and at the extract air grill, position of the variable air volume (VAV) boxes, supply and extract air flow rates and the occupancy of the room. Measurements lasted for at least two typical weeks in autumn and winter 2015-2016.
The results show that a DCV system is able to guarantee a good IAQ in all the studied cases even at reduced air flow rates. The VAV boxes react well to predefined set points for CO2 concentration The effect of the reduced air flow on the ventilation efficiency is negligible. Moreover, during the measurement period, the reduction of the fan energy ranges from 25-55% compared to a constant air volume system (CAV). For the heat losses, the reduction was 25-32% compared to a CAV during the measurement period. Energy reductions of both the fan and heat losses are calculated according to a design airflow rate of 29 m3/h.pers, i.e., IDA3 in EN 13779. However, commissioning of the DCV is necessary to maintain these high energy reductions and good IAQ and ventilation efficiency. To conclude, DCV is an interesting ventilation system in rooms with a large and varying occupancy such as lecture rooms and landscaped offices.