The "white powder" is residual salts left from the water vapor that
condenses within the flu [chimney] pipe. This water vapor is the result of
burning natural gas. An abundance collects inside the flu piping, and
condenses to water. When the water finally dries, it will leave a residual
salt. Which by the way has the relative same pH as a glass of red wine
[generally not exactly.] It does not mean you are leaking carbon monoxide,
although, there is some carbon dioxide leakage occurring and is likely
minimal.

What you don't want, is the flu piping to rust through, and allow excessive
amounts of "products of combustion" leaking under your home or in your
attic.

Seek the advice of your friendly neighborhood licensed furnace repair guy
and have your heating unit inspected annually.

--
Zyp
"Eigenvector" wrote in message
...
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.