What if my chimney vents a furnace?
The furnace flue is used to vent furnaces for gas and oil. High efficiency furnaces do not waste energy by sending heat up the chimney. As a result, temperatures in the flue drop low enough to produce condensation. The condensation combines with acids in the flue to create a damaging acidic moisture. Understanding and maintaining your High Efficiency Furnace
Clay liners are particularly vulnerable to this acidic moisture assault.
Signs to look for of a deteriorating flue:
Joints between the tiles are not sealed.