Faults in building services systems contributes to a significant amount of the energy use in todays building stock. During the ninetiees SP worked in the field of fault detection and diagnosis (FDD). One of the results was an FDD prototype that showed promising results. This prototype was an off-line version that was applied on old collected data. In cooperation with Honeywell INU Control and Västfastigheter we now have made an on-line version. The goal was to implement and evaluate the the FDD method in the building energy management system (BEMS) at Borås Hospital. The FDD-method has with only minor problems been implemented as an integrated software in the controller i-30. However, the calibration of the mathematical model of the test system showed to be more difficult than expected. The main reason is that it was a rather old system with no direct measurement of the air flow rates. Hysteresis in control valves and a malfunctioning actuator also cause problems. Due to delays in the start up of the project the calibration for the winter/heating case could not be made until the following winter. Initially a fault in the implementation also caused a fault in the controller itself. Even though the threshold limits for fault detection has had to be set higher than initially expected and faults therefore more difficult to detect, a functioning FDD application is now running on one of the air-handling units. The lessons learnt is that the actual FDD method should preferably be applied on newer and more stable systems. The method will now be applied to other air-handling-units and further evaluated by the maintenance staff.