New findings contradict perception of parallel fan efficiency

Various factors contribute to differing levels of configuration performance.

By Staff September 21, 2007

A new study from the American Society of Heating, Refrigerating and Air-Conditioning Engineers 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 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 and 20%, and in some cases higher than 30%. The 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% less energy than series fan terminals. However, tests showed that when leaks are present, series fans appear to outperform parallel fans. When a 20% leakage rate was introduced, the series terminal unit outperformed the parallel unit and used 5.5% 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, vice president of policy and rechnology for the Air-Conditioning and Refrigeration Institute . “It also serves as a useful reference in prompting changes in design practices to provide more energy efficient building.”
View the final report, co-funded by the Air-Conditioning Engineers and Air-Conditioning and Refrigeration Technology Institute, here .