I had a chimney sweep do a checkup of my fireplace and furnace chimeys and
he found that the furnace chimney lining is cracked. The liner is apparently
made of clay, and the crack looks like if you took a your fist and punched
it somewhat through sheetrock, from the inside and left a the outer piece
protruding. Additionally, there was a fist-sized chunk just missing in
another area. He found all this right by where the flue from the furnace and
water heater attach to the chimney - I can basically touch the piece that is
cracked and the missing piece too, once we removed the flue pipe extension.

He said patching is not an option (even though it's easily in reach), and
that I must get a $1,500.00 stainless steel flue liner, that he will gladly
install. He said patching will never hold. He said that if the piece breaks
off and blocks the exhaust, then I'll be in trouble. I agree - if that
happens. I just don't see why this can't be patched with high-heat resistant
patch, if it's within reach.

I have CO detectors connected to my alarm system that will alert me of any
issues. However, would you be overly concerned of a situation like this? Is
this a "must-fix" emergency situation, or do older homes commonly have these
types of cracks? Do you know of a way to safely patch-fix this type of
break? My home was build in 1964.