It has long been known by air balancing technicians that most HVAC systems do not perform as tested in the factory. Several years ago the term “system effect” was coined. ASHRAE calls system effect a “phenomenon”. While perhaps a phenomenon people in the air balance industry have known about it for years. In the same paragragh it alludes to the recognition of acuracy of the testing and balancing reports depending upon the recognition of this phenomenon.
Basically as defined in ASHRAE 111-2008 TAB, System Effect Factors are allowances that “should be used to reduce the performance rating of a fan when it is installed under conditions that are different from those present when the fan was performance tested” (ASHRAE 111-2008 6.3.1).
The air balance tecnician needs to recoginize where and how “system effect” takes place. Basically you need to carefully look at, fan inlet conditions, fan discharge conditions, ductwork system losses (including elbows, branches, extractors, area changes) ductwork construction, duct leakage, and of course air balancing devices.