I'm willing to bet condensation. The first year we had our new furnace, the chimney was 32' tall and the liner was oversized. Everyday I was removing a gallon and a half of water from the cleanout of the chimney. If I would forget, it would leak out of the cleanout and into the house. After putting in a 5.5" liner our problem dissapeared. It's suprising how much liquid can condense in such short amount of time.