I rent my home and have a gas furnace exhaust chimney problem. Our furnace is in the living room in front of the exhaust chimney. When the temps get lower than ten degrees outside and then warms up we have a dark, sticky, liquid tha runs down the walls surrounding the chimney. When looking at the chimney in the attic it is sweating the ooze that we see on the walls. Any idea what our problem nay be? or what we can do to fix it? have pics if you need to see.