I think you mean the chimney is way too large.

That said, I've helped install flue liners, for the
water heater, in the cases where people put in
a 90 percenter furnace.

Christopher A. Young
Learn more about Jesus
www.lds.org
..

wrote in message
...


Not a strange world. The issue is that an orphaned
water heater typically winds up on a chimney that is
now way too small, because it was sized for both the
water heater and a gas furnace. Drafting is NOT the
issue. Condensation is. In the winter in a chimney that
is not entirely contained within the heated part of the
house, condensation occurs because the gases now
cool off too much. With a furnace also running, the
chimney was kept warm enough so that this would
not happen. And that condensation, over time, will
destroy the mortar in a chimney.