How to Fix a Furnace That Is Blowing Black Soot

Switch to a higher quality fuel. Low grade fuels create more soot when burned, especially if you have an oil furnace, and you may be able to reduce your furnace's soot production simply by using a higher grade of fuel.

Clean or replace your furnace filters. A clogged filter makes your furnace run less efficiently than it should, which in some cases may lead to increased soot production.

Clean your furnace. A small amount of soot production is normal, but over time it can build up and give the impression that your furnace is producing more soot than it really is. Perform a thorough cleaning of all accessible parts of your furnace, including the flue, chimney and duct work, with a chimney-cleaning brush or contact a professional.

Run your furnace, after cleaning it, to see if any soot reappears. A small amount of soot immediately after a cleaning is normal; but if soot continues to build up, you may have a more serious problem.

Contact a licensed HVAC technician to have your furnace inspected and serviced if the appliance still produces large amounts of soot. There may be an issue with the unit's burner, heat exchanger, fan limit switch, barometric damper or another component.