Air handling units do not always function as planned: airflow rates are often larger than required, the recirculation rate is not at its set-point value and parasitic shortcuts sometimes decrease dramatically the ventilation efficiency. A dedicated diagnosis, based on the tracer gas dilution technique can easily detect such dysfunction, and help to cure the defects. Within the framework of the Joule-Thermie AIRLESS project, a measurement protocol, which includes the planning of measurement and the interpretation algorithms, was developed on the basis of years of practice in such measurements and implements and implemented in a user-friendly computer programme. The protocol was applied to several air handling units in various buildings. This contribution presents the method applied, the principles used in the test protocol, and some measurement results.