I have a chimney with the basement's woodburning insert's pipe running up parallel to an abandoned vent for an indoor grill. The grill's vent on the chimney is capped with about a 1" thick concrete block caulked in place.
One wet winter when I got the woodburning insert hot enough, lots of water vapor (and some accompanying smell) was driven into the house through the old indoor grill location.
I don't know if water seeped through the concrete cap or if it seeped in through the red brick. The overall poured chimney cap seems OK. Should I try to apply a sealer of some sort to the small cap block? Other ideas?