I have a question, please .An agent came to check our 2005 Trane XB80 furnice because it stopped blowing hot air. He reset the little plastic part with the 2 ceramic wires and the furnace started to blow hot air again. He checked if there are any cracks, inside where the heat exchangers are, there or none, but he says that is time to replace the entire unit, because the heat exchangers plates start to have a discoloration, very little though, because the furnace overheats....He showed us some pictures with heat exchangers very burned, dark, but ours look like new, clean, with a very little, darker grey light brown spots, just in the top of the plate....Is this something that urges us to change our entire system, or we don't have to worry about this yet, maybe next few years....?