When discussing a new furnace with a contractor, if you have a chimney in your house that the furnace uses, you have issues to consider.
If you are choosing an 80% efficient furnace, chances are a metal liner needs to be installed in the chimney to protect against condensation damage. The newer furnace has much cooler and moister exhaust than the old, inefficient unit you are replacing—and that can cause problems for old chimneys.
If you are putting in a 90% efficient unit that uses its own separate PVC vent, a metal liner must still be installed in the chimney for your gas water heater, to bring your system up to code.
Unfortunately, too many dealers skip the liner, and many homeowners are finding out what a costly mistake that can be. Don’t be tempted to save a buck here. It doesn’t make sense in the long run.