I have a Goodman HVAC unit both indoor and outdoor. It's about 4 years old now. Last time a HVAC tech came out to redo the plumbing that the original installer messed up he commented about how rusty the coils were, at the time the unit was only 3 years old. This weekend I had to clean the drain line and opened up the case, I noticed that the coils are all completely rusted now. Is this normal? We live in Texas so humidity is high but how much longer are these coils going to last if the rust keeps up?