Indoor climate and energy performance of Finnish low energy office building were studied to determine optimal control and operation solutions of demand controlled room conditioning and ventilation system with airflow rate controlled active chilled beams. Onsite measured temperature, CO2, occupancy rate were used to calibrate a dynamic simulations model. The results showed an average occupancy rate of 0.55 during office hours (OH) offering a good energy saving potential for demand control ventilation (DCV) system. DCV system used 7-8% less total primary energy compared to constant air volume (CAV) system depending on control and operation strategy used. DCV system saving was 33-41% if only heating, cooling, fans and pumps primary energy were considered. Supply air temperature controlled according to extract air temperature instead of outdoor air temperature minimized overheating problem which occurred with design solution during both seasons and systems. DCV system with active chilled beam complied at least 94% and 90% of OH with Category II indoor climate with water priority cooling and air priority cooling control respectively. Ceiling cooling panel showed an effect of decoupling cooling from ventilation resulted only in 0.9% additional primary energy saving compared to DCV system with active chilled beams. (C) 2015 Elsevier B.V. All rights reserved.