"Colbyt" wrote in
:

The potential problem that I see from your description is that the
current metal sleeve is that it MAY only vent the furnace. If both
are vented into the sleeve go for it and save about 1K$.

OK. They Y into the same flue at the base of the chimney.

Depending on your location you might need to convert the existing
sleeve to double-wall pipe to be "code compliant".

I believe it already is since it was put in with our 80% furnace 4 years
ago.

Sounds like I'll be removing the chimney top while they rip off the roof.
Woohoo! I love tearing things down.