On the Level: Candles could be source of soot-like substance

We replaced our old gas furnace last fall with a new high efficiency gas furnace model. Since then we have noticed a black soot-like substance on white tiles, the top of the refrigerator and the TV screen. Is this caused by the gas heat? We called the company who installed the furnace and they say it's new and won't do that, but we aren't sure. Is this safe to breathe? We never noticed this when we had the old furnace.

It's not likely that the soot is caused by such a new gas furnace, but anything is possible. What is really not safe to breathe that could potentially come from a gas furnace is carbon monoxide, and that is something you cannot see or smell.

Anyone with a fuel burning device in the home such as a gas or oil furnace, gas stove, gas water heater, gas fireplace log or attached garage should own and maintain a carbon monoxide detector and alarm. I like the kind with the digital read-out because it tells you what the background carbon monoxide level is and the very fact that you can read the numbers tells you the thing has electrical power.

Do a few things to rule the furnace in or out as a source. Check the furnace's filter to see if any of this soot-like material is ending up there. You haven't mentioned if the substance shows up around the air registers that put heated air into the rooms. If it's coming from the gas furnace, it's likely you'd spot it there as a shadow darkening the wall near the grilles, and if you wiped the grille with a damp rag or paper towel it would show up there. A technician with a combustible gas detector can check the air at the register to determine if anything is getting into the air stream that shouldn't be, signaling a defective heat exchanger inside the furnace. Rare for such a new furnace, but not out of the question.

Some years ago a similar problem began to show up in new homes with heat pumps, and homeowners, insurance companies and heating and cooling contractors were driven to distraction trying to pin down the cause. It was ultimately determined that due to a combination of factors of such tight, modern construction with a "closed loop" heating and air-conditioning system that soot production from candles - especially scented candles - was the primary cause. Other factors also contributed such as poorly vented fireplaces, inside gas cooking devices, etc., but the main culprit was candle use.

Since you've switched from an old style gas furnace to high efficiency gas model, it may be that the fan speed of the new unit is different.

Increased fan speed can dislodge dust and dirt inside of ductwork that previously just stayed put. That should have showed up almost immediately after startup of the new gas furnace. Take a flashlight and peer deep inside the ducts to what's there, and if it looks nasty, consider duct cleaning.

Double-check and be honest about your candle usage inside the house. Remember, any flame you can see, especially the yellow flame of a candle, is a soot producer. The scents added to paraffin to produce those nice aromas also throw out a greasy soot that has particles so fine they can pass through most garden variety air-system filters only to roost on things both smooth and those which have minute amounts of static electricity on their surfaces, such as plastic picture frames or TV screens. I was called to a house where no one could seem to locate the cause of the soot issue - no gas furnace, no fireplace - none of the usual suspects. Then I spotted one of those plug-in air-fresheners.

Actually, by the time I was done, I counted 17 of them. They put out fine oil-borne particles and that was it.

In the worst cases I've seen the soot created "ghosting" on wall and ceiling surfaces. The resulting look is of large dark splotches reflecting framing and insulation behind the drywall. The only remedy for such an extreme staining is - after eliminating the cause - to prime and repaint. Those who try to shortcut and not prime first soon learn that only the wealthy can afford to do that because they have to do it twice - the soot will bleed through latex paint.

Keep the mail coming. If you've got a question, tip, or comment let me know. Write "On The Level," c/o The Capital, P.O. Box 3407, Annapolis, MD 21403 or e-mail me at jimrooney@jimrooneyonthelevel.com or inspektor1@mac.com.