flue cap, rain water is getting down and corroding the vent pipe and then the draft motor and heat exchanger. Or a bald eagle has made a nest in the flue cap, if so leave the nest alone and run a new vent line. Ha you thought I didn,t know the answer
"Nothing else can poison our culture, corrupt our society or ruin the character of our people like unearned money or unearned opportunity." -- James R. Cook
"Fooling around with alternating current is just a waste of time. Nobody will use it, ever." Thomas Edison, 1889.