I just got back from a no heat call. Furnace is a 1993 Comfortmaker 80%, 150000 btu's. The basement is stone and very moist (mushrooms growing on the dirt floor).
Furnace flue was very rusty, took the covers off and found a lot of moisture in the vestibule. Took the cover off the burners and found the tubes of the heat-exchanger filled with an 1 1/2" water. I figure it was caused by warm air circulating through the furnace during the summer and condensing in the heat exchanger. Also there is no A/C. I condemned the furnace and recommended replacement. This is the first time I have seen this. Does my theory of how it happened sound right? Also they never ran the blower during the summer.