What do you automatically assume when you see a horizontal unit in an attic with water beads all over it,and you know the drain is not backed up.
The significant problems we face cannot be solved at the same level of thinking we were at when we created them,Albert Einstein.Stop,Think,and Act!Find the PROBLEM before ever considering the SOLUTION,Carey Collins. What would Chesty Puller do?
If your talking on the coilbox I would think it might be there being the temperature differace between the metal of the coil box and the hot air in the attic. Now if it was an insullated coil box, you would think that would keep it from having moisture on it. thats just a guess from what little information you gave .