I have a mid 90's Carrier furnace that in recent weeks the smell coming from the outside vent while its running has become strong and smells like combustion and a bit of propane like smell. I can feel the intake beside it pulling in air and both vents are clear. Its always had a smell when its first started up outside - but this smell seems to now continue outside whenever its running.
All 3 visible burners seem to fire quickly and well when it starts up, and as far as I know the water seems to be draining out ok into the floor drain.
No smell or CO is detected in the house or around the furnace, but I'm concerned about this smell as I'm wondering if its not burning properly.
Any idea what may be causing this issue? Could it be a heat exchanger?