I didn't discount fume fading, Tony. I just doubted it was from combustion, since the combustion chambers gasses don't exit the registers, they exit the building through a separate vent. Fresh air is brought from the room via the furnace blower, then passing through an air-air exchanger, where it picks up only the heat of the combustion.
If the furnace were producing combustion gasses that entered the room, it would come from the combustion air intake, unless as i said, you had a serious complete failure of the HX AND poor combustion to boot. Even then, gases could only exit when the furnace blower shuts off, due to positive pressure on the clean side of the HX when the blower is on. On top of that, you would have to have a failed control for the burner, as it would be on, when the furnace blower is off. Talk about a perfect storm of failure, I imagine the odds against are several million to one.
Chances are it's something else. But then again, maybe the Shaw technician had a degree in engineering with emphasis on furnace design, and he did a complete on-site investigation.
Perhaps you have something else emitting vapors, or some other pollutant capable of doing the fading.
