System Effect:

A pressure loss which recognizes the effect of fan inlet restrictions, fan outlet restrictions or other conditions influencing fan performance when installed in the system.

Duct elbows, transitions or other disruptions to uniform airflow may contribute to system effect, by the proximity to walls, beams and other obstruction to air flow in case of unducted fans.

For a quantitative discussion of system effects refer to AMCA publication 201.

The diagram below shows some more common causes of system effect: