I had a customer complaint about smelling oil from furnace during a call for heat but only before fan kicked on. Once fan energizes, the smell went away. I smelt a faint oil smell once I arrive. I'm suspecting possible heat exchanger. This system is a 1994 lowboy front flue. By removing the blower assy. and/or burner assy. will I be able to gain enough access to heat exchanger with a mirror and flashlight to look for cracks? Short of depaneling the furnace and without a borescope, what is the best way to gain access. Also, I had another customer ask if a 90% direct vent gas furnace can be vented through a chimney with sch 40 pvc. I have never seen it done nor have I done it myself. I looked in the code book and I couldn't find where it says you can't. Anyone with an answer I would appreciate it and a possible reference.