Question: I have a gas fired furnace (hot water radiator system) and have noticed droplets of water dripping from a seam in the flue that runs from the furnace to the brick chimney. There also appears to be a slight white corrosive build up around the seam. What is it? What should I do? Answer: This is condensation from the gas which can be quite corrosive in certain cases. All gas appliances which are vented into masonry chimneys should be installed into an aluminum liner which runs up the entire inside length of the chimney. This code change was put into place years ago after a lot of condensation problems. Have a heating contractor check both your chimney (for the liner) and your connector pipe (for corrosion) and replace/install if needed.