Demand controlled ventilation in practice: Case study

Demand controlled ventilation (DCV) can reduce the energy use significantly compared to a constant air volume (CAV) system. However, there is still a large uncertainty about the real energy savings and the ventilation efficiency. Furthermore, control and operation of the system are more complex. To formulate answers to these questions, measurements on a DCV system in a university building in Ghent, Belgium provide insight in the system operation and performance and the air distribution in the classrooms. Monitoring is carried out in March and May 2015.
During the measurements the VAV operation related to the set points was analysed. The active power demand for the fan and the thermal power demand for the heating coil was monitored. Furthermore, the CO2 concentration was measured at five positions in the classroom to determine the ventilation efficiency at maximum and reduced air flow rate.
The results for the operation of the air handling unit (AHU) show that DCV is able to control the indoor set points for both CO2 concentration and room temperature. When there is no occupancy in the classroom the DCV system operates at 32% of the time at a minimum air flow rate during operating hours, which decrease the active power demand for the fan.
The CO2 concentration was measured at five positions in the classroom to characterise the ventilation efficiency. The measurements show that at a high air flow rate the lowest concentrations were found for the position in the middle of the classroom. Comparing a high and low air flow it was found that during a low air flow the difference in CO2 concentrations for the five positions is at maximum 150 ppm. For a high air flow the position in the middle of the classroom has a maximum difference of 500 ppm lower compared to the front and back of the classroom. Furthermore, it was found that the room sensor did not deliver representative results especially in case of low air flow rates.