Black Smoke Is Coming out the Chimney of a Gas Furnace

Carbon Monoxide

• Carbon monoxide is a colorless, odorless, tasteless gas that forms when there’s not enough oxygen to completely burn the natural gas in your furnace. Soot, which is mostly carbon, also forms when there’s incomplete combustion. If you see soot, you know that carbon monoxide is also present. Other signs of carbon monoxide include a large, yellow unsteady burner flame and a pungent smell when the furnace is running. You and other family members may also suffer from dizziness, nausea, headaches, shortness of breath and confusion. Immediately leave the house if you experience any of these symptoms or if your carbon monoxide detector goes off.

Venting/Draft

• Incomplete combustion can occur if your furnace isn’t properly vented. This can happen if the chimney is blocked by leaves or a bird’s nest, or if the flue is too small or too short. Since many gas furnaces draw air from the room, it can also happen if newspapers or other objects are piled up around the furnace, obstructing airflow. Another problem occurs with highly efficient modern furnaces that send less heat up the chimney. This can cause the flue temperature to drop so low that the chimney can’t provide a proper draft. If this is the problem, you may need to install a new liner or rework the connector pipe located between the furnace and the chimney.

Combustion Test

• Carbon monoxide can also spill out of the draft hood and into your house. Perform a simple test to see if your furnace is receiving enough air for combustion, but make sure you have a carbon monoxide alarm before you do. Close all windows and doors, along with the dampers on any wood stoves or fireplaces in your house. Open the interior door to the furnace room and any interior doors located between the furnace and the exhaust fans for vented appliances such as the clothes dryer or exhaust fans in the kitchen or bathrooms. Turn on the furnace. Wait a few minutes, then hold an incense stick or a smoking match 2 inches from the draft hood opening. Your furnace is venting correctly if the smoke is drawn into the draft hood; if it blows away from the hood, you have combustion gases, such as carbon monoxide, entering your house. Call your heating contractor right away.

Prevention

• Check the air filter every month and change it if it’s dirty. Change filters at least every three months or more often during the heating season. Vacuum the blower every few months, along with the base of the furnace, the gas burners and any other areas you can reach. This will reduce dust that blocks the flow of air. Have your gas furnace inspected for leaks every year and tuned, if needed, so it will burn more efficiently. In addition, have the heat exchanger and the combustion chamber checked. A cracked heat exchanger may allow carbon monoxide to escape.