Despite common perception, parallel fan-powered air-terminal units may not be more efficient than series fans, according to a recent study.

The study was sponsored by the American Society of Heating, Refrigerating and Air-Conditioning Engineers, and co-funded by the Air-Conditioning Engineers and Air-Conditioning and Refrigeration Technology Institute.

The associations say 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.

In the study, tests conducted by investigators at Texas A&M University’s Energy Systems Laboratory found considerable air leakage from parallel fans’ backdraft dampers and terminal box seams, greatly reducing their efficiency. Leak rates for tested parallel fan terminals were found to be, on average, between 10 percent to 20 percent, and in some cases higher than 30 percent. These leaks reduced the airflow from the central air handler and caused the air to bypass the room to be conditioned, resulting in more energy needed to move more air to maintain comfort in the conditioned space.

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. When a 20 percent leakage rate was introduced, the series terminal unit outperformed the parallel unit and used 5.5 percent less energy.

“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 buildings.”

To read the final report,