A new study from the American Society of Heating, Refrigerating and Air-Conditioning Engineers, co-funded by the Air-Conditioning Engineers and Air-Conditioning and Refrigeration Technology Institute, finds that despite common perception, parallel fan-powered air-terminal units, which are used to distribute cooled or heated air in commercial building cooling systems, may not be more efficient than series fans.

When designing a cooling system, building designers can choose between either a series or parallel configuration for a building’s fan-powered, air-terminal unit. Since parallel fans run intermittently while a building is occupied, this configuration has been thought to be more efficient than series fans, which run continuously during the work day. According to ARI’s press release, in the study, tests conducted by investigators at Texas A&M University’s Energy Systems Laboratory found considerable air leakage from parallel fans’ back-draft dampers and terminal box seams, greatly reducing their efficiency. When no leakage occurs, the parallel fan terminals are more efficient, consuming 17 percent less energy than series fan terminals; however, tests showed that when leaks are present, series fans appear to outperform parallel fans.
 
“For manufacturers and building design engineers, this research provides new insights into the magnitude of air leakage in parallel fan powered terminals and its impact on system operation and overall energy consumption,” said Karim Amrane, ARI’s Vice President of Policy and Technology. “It also serves as a useful reference in prompting changes in design practices to provide more energy efficient building.”