What Causes Black Powder to Build Up In a Furnace?

Poor Maintenance

• Soot develops in furnaces that aren’t properly maintained. Gas and oil furnaces need routine maintenance, in addition to an annual inspection, to keep them running efficiently. Check the filter every month and change it when it becomes dirty. Vacuum any areas you can reach every few months to reduce dust and dirt that can block the flow of air, and keep the furnace room clean. Inspect your furnace for leaks every year. Make sure your chimney is inspected and clean.

Nozzle

• A dirty oil spray nozzle or a clogged gas orifice can cause soot to form in your furnace. Nozzles usually come with a filter, but they’re easily plugged up because the hole on the nozzle is extremely small. Nozzles can also wear out. This furnace part is very inexpensive and costs less than $5 as of the date of publication. Most service technicians recommend replacing the nozzle every year.

Burner Problems

• Soot can also develop when the burner hasn’t been properly adjusted or if there’s dirt on the seat of the valve that’s responsible for cutting off the flow of fuel during the “shut-down” phase of the burner operation. Flames in an oil furnace are yellow, but gas flames should be blue. If the flames in your gas furnace are yellow or orange, it means the burner isn’t getting hot enough and you have a combustion problem.

Heat Exchanger

• The heat exchanger is the core of your gas or oil-fired furnace. If it becomes damaged or cracked, soot can build up on it, reducing the transfer of heat, overheating the heat exchanger and sending soot back into the house. In addition, carbon monoxide can leak into the house. Soot should never be seen on a gas heat exchanger. A small amount might be visible on an oil furnace’s heat exchanger, but a thick buildup indicates a problem. When a heat exchanger becomes cracked or develops a large hole, the only solution is to replace the furnace.

Chimney

• Soot is vented out of the house through the chimney and venting system of your gas or oil furnace, but soot can build up on the chimney liner and flue pipes. If the flue is too small or too short, your furnace can’t vent soot and gases out of the house as it should. High-efficiency gas and oil furnaces produce a lot of water vapor that contains chlorides from your house. These vapors are very corrosive and can damage your chimney liner and interfere with combustion. Inspect your chimney every year for soot buildup and to ensure that your chimney is safe to operate.