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.

around here in pittsburgh the furnace typically used one flue a
seperate flue for the water heater.

after all for a large part of the year the furnace would rarely if
ever run..

in pittsburgh late april till mid october///

and its not a biggie to cut a hole in a concrete wall or block wall
despite what some people think, direct vent is way better.......

overreaction to safety and efficency rules is just that
overreaction...

i suggest to those people bothered by this that most rules are
reasonable.

Take the NEC, we all benefit from safer and more efficent electrical
systems, safer vehicles with better gas mileage etc etc