start with installing a carbon monoxide detector in the basement with a
digital parts per million display. then continue reading....

your answer is not in just the manual you need to read for your
specific model, but also in what the installer did, and add to that
what the previous homeowner might have done.
if it is presently ok by the installation manual to be sucking on your
stale basement damp radon air for its combustion, you might be happy to
let it continue doing so.
if on the other hand your basement doesn't have sufficient make-up air
and the furnace is sucking on the hot water chimney flue and causing
exhaust gas corrosion of the flue pipe atop the gas hot water heater,
you have a needed repair.
you will need to know the requirements for that furnace model.

....if the CO alarm has not rung, we're safe for now...