I need advice on my Carrier split system. The fan coil located in the attic is rusting on the lower wing of the coil. The installer says that this is expected and not to be concerned with it. Given that the system is less than four years old I am concerned. Although the system works I do not think rust is acceptable. Is this an installation issue? Contractor was out several times a few years ago to empty the secondary pan. He finally changed the white drain pipe. Should I have the coil replaced? Should I have the unit replaced? How can rusting be minimized in the future? Thank you for your help.