Fault Detection & Diagnosis (FDD) is at the heart of the PERFORMER European FP7 project that aims at reducing the gap between expected and actual energy performance. The purpose of the article is to describe the FDD methodology used within the Performer project and illustrate the benefits of its application to a positive energy building. The rules were defined according to the Building Manager's requirements and were applied to the data collected from the BMS. The proposed approach is very easy to apply and to use in any type of building, including nearly zero energy buildings. Expert rules address very technical and recurring issues in buildings and the work undertaken within PERFORMER is to help Building Managers in their decision-making process by giving them an accurate picture of the level of performance of their buildings. The whole study and final results will be available by the end of the year.