9 ConclusionsThe objective of this document is to present the main causes and quantification of the system effect (SE) induced by a fitting, appurtenance or obstacle close to the inlet or outlet of a fan. The physical mechanism differs according to the location of the flow disturber (fitting) with respect to the fan: when it is at the inlet the fan curve is modified whereas when it is at the outlet the fan curve is usually not affected but the system pressure drop may increase due to the presence of the appurtenance. That is the reason why two different definitions of the SEF have been proposed, one for the inlet side and the other for the outlet side. In both cases the pressure drop of the obstacle has to be taken into account if relevant in the evaluation of the SEF.