We bought a 2nd home recently (built in 1923) that my wife uses during the week, empty on weekends. New Oil boiler was installed shortly after we bought the home, New Yorker that the manufacturer states is 86% efficient.
The boiler ties into the homes original masonary flue, I do not think it is lined with clay but I'm not positive. I would guess the prior owners, an old couple, did little to no maintenance on the flue, and it certainly seems from the basement that there was a lot of soot at the bottom of the flue.
It was rather cold this past weekend and when my wife returned to the home on Monday evening there was clearly a leakage of "stuff" on the basement floor. My heating company originally was going to put in a gas boiler, including a metal liner, but we were forced to stay with oil and when we changed the unit out for an oil boiler they felt the flue did not need to be lined.
From my brief research it seems as if what has leaked onto the floor is a result of condensation accumlation inside the flue falling back down and getting dirty with soot along the way. Manufacturer literature states the minimum flue size is 8"x8" rectangular or 6" circular by 15'. The 15' is not a problem and I think it probably meets the 8"x8" inside dimension but I'm not positive.
The heating company is coming back out tomorrow to see, what would the Pro's suggest I discuss with them? Thanks.