Now look inside, see the problems?

A look at the inside of this same chimney shows how acid-laden residue from the furnace has attacked this flue. The original clay liner has eroded away; pieces of it are even missing. (Old chimneys might not even have a liner.) Mortar and bricks are loose and falling, and moisture has leeched through to the home's interior walls..

With continued use, more erosion will take place, possibly leading to leaks in the flue, complete flue blockage, or possibly even carbon monoxide poisoning of the home's occupants.