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Transition must be planned for each job. Reduction should not be more than 4 inches per linear foot, approximately 20 angular degrees (Figure 3). AIR FLOW C B A Figure 4 LOCATING THE AIR FILTER AN AIR FILTER MUST BE MOUNTED IN THE RETURN AIR DUCT OF A CENTRAL FORCED-AIR SYSTEM, ON THE AIR ENTERING SIDE OF FURNACE. Figure 3 Model NO. ACM1000M-XXX ACM1400M-XXX ACM1600M-XXX ACM2000M-XXX A LENGTH 19-13/16 24-13/16 19-13/16 24-13/16 B HEIGHT C WIDTH CFM RATING 18-5/16 5-7/16 600 TO 1200 18-5/16 5-7/1