peer reviewed ; The development of specific tools for the audit of air-conditioning systems is described in a companion paper. These developments are carried out in the frame of the HarmonAC project launched in 2007 in order to assist the implementation of the EPBD, article 9. The development of these tools was continously supported by the application to case studies. This paper reports on the application of the tools within an audit procedure applied to a typical office building in Belgium, which is in operation for more than 20 years. The first step of the work consisted in analyzing the current performance of the building which proved to point out a number of problems in the operation of the building. As it is not rarely the case for running office buildings, these problems were due to a lack of commissioning and a progressive decrease of the performances due to a poor management. Application of the first stage “Benchmark” tool to this building led to a very high discrepancy between the measured consumptions and the calculated ones. This appeared mainly for the electricity consumption but also for the gas consumption. Calibration of the simulation tool, mainly by representing more exactly the management of the building and the real internal gains improved the accuracy of the calculations to a large extent. After calibration of the building and HVAC model, the application of a second tool (SimAudit) allowed to evaluate the savings obtained by the implementation of a number of Energy Conservation Opportunities (ECOs). In this case, the dominant ECOs were in relation with the operation of the building: change of schedules, modification of setpoints; implementation of free-cooling strategies; provision of heat recovery and move to reversibility of the chillers. Reasonable estimates of the savings can be obtained following the calibration/simulation process. The paper will highlight some of the most relevant ECOs and provide an order of magnitude for the savings in this kind of building and in a mild climate. ...