I have a 4 ton heat pump(Goodman #GPH1348H12A) set up in a 24x56 (1344 SQ FT) mobile home. I have two 16" return ducts that tie into one main 16" return line to the unit (Flex hose). The Filter size at each is 14x20x1. I have two 14" (Flex) lines that tie into each side of the mobile home's rigid ducts (One single long line). I have had a problem needing to charge the A/C each summer and a tech has told me it is due to the main return line needing to be 20" with both 16"s joining to it. Also the vents need to be 20x20x1. Are the 16" returns okay or do I need to plan a replacement project?