What Does Black Soot Mean on a Furnace Filter?

Carbon Monoxide

• Black soot on your furnace filter may be an indication of carbon monoxide, an odorless, tasteless and potentially deadly gas. Typical causes for carbon monoxide include improperly vented cooking appliances, car exhaust and fuel-burning appliances. Be aware of the dangers of carbon monoxide, and use precautions. Make periodic checks to ensure your appliances are working properly. Do not leave your car or lawn mower running in an attached garage, and routinely maintain your furnace. If you or a family member experiences any flu-like symptoms, leave your house immediately and call for emergency assistance. Always seek medical care for suspected carbon monoxide poisoning even if you consider it to be minor.

Candles

• As unlikely as it may sound, candles could be the culprit of the black soot on your furnace filter. According to the U.S. Environmental Protection Agency, burning candles are a common cause of carbon soot in households. This is particularly evident in candles made of a lower-quality paraffin wax and scented candles. Test your candle by burning it on top of a turned-off television set. Allow the candle to burn for several hours before extinguishing. Wipe your television screen with a clean white cloth. If there is a black sooty residue on the cloth, consider purchasing a different type of candle.

Puff Back

• A puff back is a misfire of a furnace's ignition system. This happens when an oil burner does not ignite at the correct time and vapors are allowed to accumulate before ignition. This causes an explosion similar to the backfiring of a car. The explosion discharges soot throughout the furnace system -- or, if you are unlucky, your entire home. Puff back can be a chronic problem where small amounts of soot are distributed at each ignition or a sudden explosion that sends soot throughout your entire system.

Precautions

• Proper maintenance can help to prevent damage and potential safety concerns caused by soot in your home. Make sure each venting system is adequate for the appliance it serves. Have your furnace and chimneys checked and cleaned annually by a professional. Most of all, do not ignore excessive soot on your furnace filter. When in doubt, turn your furnace off and call a professional.