Q: I am fixing to buy a house that is 5 years old and while inspecting it I noticed on the AC unit for the upstairs that the top of the plenum had large spots were it was almost rusted thru. The owners realtor is having a AC company come out tomorrow. What suggestions do you have that might be causing this?

The rust spot is most likely caused by a torn or missing spot of insulation inside the plenum. We suggest having this looked at to make sure that there is no moisture build up.