After poking around the vents today, doing more work to insulate and clean up before winter I noticed that the vent stack on my furnace is for the most part coated in a white fine powder material, as well as rust flakes. I understand from reading about CO, that the white powder indicates CO problems.
But what does that mean, does it mean that the vent is leaking CO, or does it indicate the furnace is venting excessive CO (maybe burning inefficiently due to age)? The powder is only forming on the joints between pipes, which to my mind says exhaust gas leakage.
Just for reference my furnace is 23 years old, and none of the venting joints are sealed.