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 on a thermal comfort parameter. DCV operates at reduced air flow rates during a large amount of the operation time and thus consumes less energy for fan operation and heating/cooling the supply air. The aim of the present research is to assess the IAQ ventilation efficiency, and the operation and energy efficiency of real operating DCV systems in moderate climates. Measurements are carried out for at least two weeks in autumn and winter 2015-2016. The following parameters were 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. Four case studies with large and varying occupancy rate and with different use and ventilation systems are selected. Two classrooms and three landscaped offices were evaluated. The results show that a DCV is interesting in rooms with a large and varying occupancy rate such as lecture rooms and landscaped offices. A good IAQ is measured in all cases studied even at reduced air flow rates. The effect of the reduced air flow on the ventilation efficiency is negligible. The VAV boxes react well to predefined set points for CO2 concentration. During the measurement period, the reduction for fan energy ranges from 25 to 55% and ventilation heat losses 25-32% compared to a constant air volume system (CAV) with a design airflow rate of 29 m(3)/(h.pers), i.e., IDA3 in EN 13779. However, commissioning of the DCV is necessary to obtain and maintain these performances. (C) 2018 Elsevier B.V. All rights reserved.