"Randy" wrote:
Hi Joe and DNB, I guess you were right. The fresh air damp duct is probably
to provide fresh air to the home, not for combustion. So both the furnace
and water heater would draw air for combustion from the garage. There isn't
a closet for them. They are surrounded by walls on three sides but the one
large side is open to the garage. The garage size is about 650 SF (2 car).
Is it OK to keep the garage doors and window closed? I do have the window
near the furnace but keep it crack 2"-3" would let the rain in.

Garage doors are far from airtight. Even with everything closed up, you'll
have enough plenty of openings for the combustion air.