KC-84-04-3 -- Retrofit of a Constant Volume Air System for Variable Speed Fan Control PDF

Retrofit of air distribution systems for energy conservation frequently involves conversion of a fixed air volume system to variable air volume (VAV) operation. Two single-duct systems utilizing a single supply and single return fan for cooling the interior zone of an office building were modified. The retrofit to VAV operation involved the application of variable frequency drives to the fans and the testing of several alternative techniques for airflow control. An on-site data acquisition system was used to record the effect of several control strategies on minimum outside air control, room temperature, and fan energy consumption.

Energy consumption of two systems was monitored during a one-year period of regular operation. Energy consumption data recorded before and after the retrofit are presented. For one system, the retrofit yielded an annual energy savings of 46.5 percent relative to constant volume operation; the other system, requiring a slightly lower average cooling load, provided an average reduction of 53.9% for the one-year monitoring period.

The paper describes the particular control strategies tested with the variable speed fan control system and makes several recommendations regarding their suitability and effect on other control loops used in VAV systems.