18 inch flex duct 4ton system - 1440 CFM
Currently getting my 4ton XL20I/XC80 Trane system installed, they replaced the old return that was a 16" that was bent and squished. so they did some framing and opened it up to fit a 18 inch r-6 flex return line, it is about 10-12 feet in length connected to a 20x30x1 return filter/grate. is this large enough to support the 4ton system with out causing damages? they did a CFM test with a cool hand held fan and it was reading 1440 CFM, they are coming back to try to straighten it up to get more CFM out of it but will it decrease the efficiency or hurt the system if it is getting 1440 CFM and not the 1600 every one recommends?