In order to assess the real performances of different demand controlled ventilation (DCV)systems, two of them were installed in meeting rooms of an office building.The first system is controlled by movement detection on terminal units and has been installedin a small meeting room which is regularly used.The second system is controlled by CO2 detection and frequency variation on fan. It has beeninstalled in a large meeting room (30 persons seated, up to 50 persons standing).The systems have proved to be energy saving with correct CO2 levels. Meeting rooms havelow occupation rate due to the fact there are generally less people in meetings than themaximum allowed. Simulations have also been run considering full office buildings toestimate the variation of savings for different occupation rates and the influence of DCV inmeeting room and offices on the total building ventilation losses. Practical rules for ownerson how to realise a correct DCV systems were drawn in order to realise a guide.