Most homes in this area were built with a chimney that vents a gas or oil furnace, boiler, and/or water heater. When these chimneys were built, they were lined with clay flue tiles. Over time the acids in the exhaust will break down the face of the tile lining. Once the facing is gone the tiles will break down much faster (they flake apart like a clay flower pot left outside). This deterioration process is accelerated with today's higher efficiency heating appliances. Once the tiles start to fall apart, they could block the flow of exhaust gasses up the chimney causing flue gasses (including carbon monoxide) to spill into the living area.
A common misconception is that this flue is inspected by the heating contractor when the furnace is being serviced. It is not.
This is one of the reasons why an annual inspection should be done on your chimney, whether or not you have or use a fireplace.
