This method is described in a small practical guidebook (40 pages). It includes three levels of inspections or diagnosis: - DIAGVENT 1: no measurements, only visual checking, for commissioning new installations. The main objective is to verify that the installed system is in accordance with the expected one. The system is started on ; - DIAGVENT 2: is the main part in the method ; it is a more detailed inspection, both for new and existing installations ; it includes not only visual checks but also performances measurements (total and local air flow rates, pressures, electrical power) ; it also includes analysis of the results and indications for feasible improvements or more detailed investigations, if necessary ; - DIAGVENT 3: corresponds to specific measurements, when a strong problem has been revealed after DIAGVENT 1 or 2, or after a complaint from the users. It may lead to a very detailed inspection and may include sophisticated measurement techniques (for acoustics, air leakages, air pollution transfers, etc.). Specific measurements are not detailed in the guidebook, but it is shown, depending on the nature of the problem, which points should be checked and what kind of measurements or analysis should be made.