The interior of the enclosure is divided into three sections. The back section (the filter end) has an opening into which a 8.5 x 17 x 0.5 filter can be fitted. Also within this section there is a subhorsepower permanent-split capacitor motor with a centrifugal fan mounted on each end. A vane connected to a microswitch in the left fan output connects to two positions of a four-contact two-row terminal strip mounted at the bottom of the back section. The other two positions of the terminal strip provide power for the motor; one of the power leads is routed to a fuse holder mounted on the back of the enclosure over the filter, and then back to the motor. Attached to the other row of the terminal strip there is a cable consisting of four individual wires dressed with nylon spiral wrap and terminating in a seven-pin round connector that includes a tie-down screw. The fans exhaust into the center section, which comprises a plenum that changes the direction of the air flow from horizontal to vertical. There was some kind of material to seal the opening of the plenum to the unit it serves. A Magnehelic manometer with a maximum scale of 2 inches of water is mounted in the front section. The manometer measures the air pressure in the center section. A threaded mounting device fitted with two large knurled knobs extends from each side of the top through the enclosure to mount it to the unit it serves. Four locating pins extend from the bottom of the enclosure to help position the object properly. A handle is mounted on each side toward the bottom of the enclosure.