To prepare this recommendation, many national standards, as well as those adopted by individual fan manufacturers, in particular DIN 24154/2 and 24154/4, VSM 18695 and 18705, NEN and VIS 714, were examined. No one existing standard was considered suitable as Eurovent recommendation. This recommendation adopts the metric R 20 Series, as given in ISO Document R 497, as the nominal dimension for the inside diameter. It takes into account the maximum casing thickness at the circular flange, as used for general purpose fans. This document is available in English, French, German and Italian.