"INTRODUCTION Plenum fans have become popular with some segments of the HVAC industry because their use can lead to shorter cabinets (read less expensive) and quieter applications. As with all engineering choices@ there are trade-offs and pitfalls to be considered. Plenum fans cannot compete with properly utilized housed fans for efficiency. Properly utilizing a housed fan means a ""draw-through"" unit. This configuration puts the fan at the discharge end of the unit@ drawing the air through the coil. The fan must be connected directly to the ductwork with two to three equivalent diameters of straight duct before any turns. In many applications@ however@ housed fans are not used in ""draw-through"" configurations or are not properly ducted. In these configurations@ the efficiency gap narrows considerably. As with any component@ you must exercise care in design@ construction@ and handling of plenum fans. Several examples will be presented@ documenting problems that were traced back to problems with frame design@ component alignment@ and component handling. These issues are important for all fans but some at least can have a uniquely plenum fan twist. For example@ any fan can have a problem with frame resonances@ making them impossible to balance properly. Plenum fans@ however@ with their large@ flat inlet plates can transmit these resonance frequencies into the airstream like a loud speaker."