Chimney Dust

The Chemistry Behind Furnace Chimney Deterioration: How Sulfuric Acid Forms and Impacts Your Home

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 is the fancy name for the black ash that remains when you make a fire in your fireplace.

Without a chimney cap, rainwater 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.

Have more questions or want professional advice on buying or selling a home?