I had the above model installed about 8 years ago when our Westinghouse unit failed. The installer created a rectangular plenum that measures 11.5" wide x 18" inches deep at the air handler output (per the manual), but necked it down to 11.5" wide x 14" deep x 16" tall to fit it onto an existing 14" x 24" inch supply elbow interface that is clocked 90 degrees from the air handler installation. The supply elbow terminate into a 10" x 24" main supply duct. The unit seems to routinely underperform, and I have always wondered if my cfm was negatively impacted by necking down this plenum's topside instead of creating, say, a 14.5" x 14.5" trapezoid on the plenum top end that would reflect the same amount of square inches as the base. I have attached a rough drawing of my plenum, as well as pertinent pages from my manual. I don't have a working knowledge of static pressures and such, and was wondering if some experts out there could weigh in on this before I hire someone to rework the plenum? Thanx!