You may have noticed I find chemical reactions (especially in the systems of a house) fascinating. And so, here's another... Have you gone into the basement and noticed the bottom of the furnace chimney has a circle around it of red dust? Ever wondered what that was? (I did-so you don't have to...) Most of the furnaces in Portland use the original brick chimneys to vent toxic gases outside. When a lot of these chimneys were built, they didn't have chimney caps. (Picture a little umbrella over your chimney. Or look at this image.)
One of the by products of burning fuel (especially coal, oil or wood) is creosote. Creosote contains sulfur. Creosote's the fancy name for the black ash that remains when you make a fire in your fireplace.
Without a chimney cap, rain water drops into the bottom of the chimney, and gets the creosote wet. Water and creosote interact chemically (in the bottom of your furnace's chimney) and produce sulfuric acid. Yes, sulfuric acid. That's the stuff that will eat a hole in just about anything-including brick. So, the sulfuric acid degrades the brick and creates that halo of dust around your chimney.
So the chimney sweep contractor's not just trying to sell you a chimney cap to make money-it'll lengthen the life of your furnace's chimney.