System effect is the negative impact on fan performance that results when the fan is connected to a duct system. Fan curves and fan tables are developed by laboratory measurements with the fan installed under ideal conditions. However, when the duct system is connected to the fan, the fan operating conditions are changed from those of the laboratory and the fan will have a different performance than as rated.
The designer first calculates the system resistance. This is the amount of resistance to airflow (static pressure) that the system will impose, measured in inches of water gauge (inches wg). The system resistance depends upon these factors: