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Black Soot Gas Furnace
10 January 2004


Q. Two years ago we replaced our heat pump with a gas furnace. Since then we have noticed a black soot like substance on white tiles, top of refrigerator and TV screen. Is this caused by the gas heat? Is this safe to breathe? We never noticed this when we had electric heat.

A. It’s not likely that the soot is caused by such a new gas furnace it’s but not out of the realm of the impossible. What is not safe to breathe that could potentially come from a gas furnace is carbon monoxide and that 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 CO levels are and the very fact that you can read the numbers tells you the thing has power to it.

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 -like darkening of 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 bad heat exchanger inside the furnace. Rare for such a new system 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 a heat pump to gas it will be fairly safe to conclude that the fan speed of the new unit is different and may be a bit faster than that of the old heat pump. Increased fan speed can dislodge dirt inside of ductwork that previously just stayed put. That should have showed up almost immediately after start up of the new gas furnace. Take a flashlight and peer deep inside of the ducts to what’s there and if it looks nasty consider duct cleaning.

Double-check and be honest about candle usage in the house and remember that 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 that they can pass through most garden variety air-system filters only to roost on things both smooth and that have minute amounts of static electricity on their surfaces, such as plastic picture frames or TV screens. 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 paint. 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.

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