The FANS (Fan Assessment Numeration System) Unit is a device, originally developed and constructed by the URSDA-ARS Southern Poultry Research Laboratory and refined at University of Kentucky, to measure in-situ air flow of exhaust ventilation fans. The FANS Unit has been adopted as a standard method of measuring fan airflow at different static pressures in livestock barns for numerous field research projects. However, procedures for using FANS to conduct in-situ fan tests are not completely standardized. One procedure for changing the static pressure is turning on and off different fans inside the barn. Still, it is not known if the FANS Unit may affect the results of a fan tested in-situ when different fans are operating simultaneously. Therefore, this project aimed to determine if the operation of different fan combinations during an in-situ fan performance test would affect results obtained from the FANS unit. Tests were conducted in eight tunnel ventilated broiler houses, in which one (1.22 m) exhaust fan per barn was chosen for repeated testing while different combinations of fans were operating. The combinations were: FANS unit in the upstream position; FANS unit between adjacent operating fans in the upstream and downstream positions (Middle position); FANS unit at the tested fan operating alone, with no other fans operating. Results showed that the air flow was up to 20% higher, when testing a fan with no other fans operating, than the air flow from the same fan tested with FANS in an upstream position. Air flow was also up to 16% higher with the FANS unit in the Middle position than when the same fan was tested with the FANS unit in the upstream position. A smaller difference range (-5.4% to 3.9%) was also found between the test results from FANS with no other fans operating and FANS in the Middle position.