I have a Nordyne M3R series furnace in my Modular home, it is 5 years old. Last year it kept tripping the limit switch located on the exhaust motor area. I replace the plastic orifice located in the unit and it worked fine. This year, it started tripping it again, I replaced the orifice again, and the limit switch. The switch keeps tripping and i had to remove it from the furnace area so the heat wouldn`t trip it. i know this is a bad idea, but I needed heat at that moment. The motor on the exhaust fan is exposed in a way i can touch it. it gets hotter than hell, almost too hot to touch, the PVC exhaust pipe gets pretty war too. Any ideas why it might be tripping the switch? I`m going to climb up on the roof tomorrow and see if the exhaust pipe is clogged.