My house is a 1950's brick walkup. There is a chimney that runs from
the basement and up through the kitchen and out the roof. There is no
fireplace or woodstove hooked up to this chimney; just the gas
furnace. The chimney is brick. It appears to have a liner of some kind
of clay or ceramic stuff. There is a metal roof over the top of the
chimney opening.

Where the chimney passes through the kitchen, it juts out from the
wall and is plastered over. Just this winter, I have started noticing
water damage in the plaster. It appears moisture is destroying the
plaster from the inside, discoloring it brown and making it bubble up.
The damage does seem to be located more toward the top of the chimney,
near the ceiling of the kitchen.

I have only owned the house for two years, so I don't know for sure if
this is a recurrent problem. I'm guessing it is. It could have been
patched over before we bought the house, and only now starting to show
damage.

I went up on the roof to see where water might be getting in. The
concrete crown on top of the chimney looks to be in good shape. All
the brickwork and mortar looks sound up there. There are no obvious
problems with the flashing or the shingles around the base of the
chimney. The flashing material looks to be some kind of brown-painted
sheet metal. Someone has caulked around the edges of the sheet metal
with clear silicone caulk.

Since there are no obvious problems up above, I am wondering if there
may be moisture coming from inside the chimney somehow? It is
suspicous to me that we only started having this problem when the
weather got cold and we were running the furnace. Of course that could
be a coincidence, if the previous owner did a patch job that held for
2 years, and the underlying problem is only now starting to reveal
itself.

Does anyone have any suggestions to help me diagnose and repair this
problem?