Stains on a carpet edge and walls may be caused by soot from an attached garage, a malfunctioning furnace or even candles. Here are some tips for preventing soot and carbon monoxide problems in the home.

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Q: We have a modified two-story home. The carpet in the upstairs bedrooms has a dark 1-inch stain on the edge above our garage. Our carpet is 18 years old, and we will probably replace it in the next couple of years, but first I want to find a solution to this problem.

A: It sounds like a case of carbon soot stains. Sometimes called sooting or ghosting, it can be found along edges and under doors of carpeted rooms. But take a closer look. It also accumulates on flat surfaces, electronic devices such as TVs, computers, VCRs, etc. In some cases, ghostly stripes show up on walls as it collects over vertical wall studs.

Surprisingly, the source of the soot most often is simple candle-burning. Burning candles can be extremely dirty, especially scented ones. Although they appear to burn cleanly, they produce tiny particles of soot that become airborne. The microscopic particles tend to accumulate on cooler surfaces over time, creating a noticeable stain. If you burn candles in these rooms, stop.

Soot also can come, as you suspect, from the garage. Just starting or parking a car can create soot, especially if you back in a vehicle or it’s out of tune and burning dirty. Attached and tuck-under garages can look as if they are separated from the house, but they often aren’t, not to small particles like soot. Air typically is pulled from the garage into the house, bringing the soot with it.

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You may be concerned about the soot, but vehicles are big sources of carbon monoxide, or CO, as well, and that’s more dangerous. You can see soot, but not CO, which at low doses can make people feel unwell. And if it comes and goes, as it would from a garage source, it might never be connected to vague symptoms such as headaches or tiredness. It is critical that you install a digital electronic carbon monoxide alarm in your home, especially in bedrooms adjoining a garage.

Soot and CO also can come from a malfunctioning furnace, water heater or fireplace. In those cases, however, you’d tend to find soot stains throughout the house. Have a technician check appliances to make sure they are burning cleanly and venting properly.

To prevent soot stains in a house, occupants should:

• Avoid or restrict the use of candles in houses (the National Candle Association Web site, www.candles.org, gives specific information on how to choose and burn candles so as to avoid soot problems).

• Have combustion appliances in the home checked regularly for proper operation and venting effectiveness.

• Never idle vehicles in an attached garage; ensure that the garage is properly air-sealed from the house.

Builders should:

• Build tighter houses and specifically ensure that air leakage pathways at floor-header assemblies are well sealed.

• Use insulation techniques and wall details that minimize thermal bridging and cold spots.

• Ensure that garages are air-sealed from the home and any ductwork running through the garage is tightly sealed.

• Ensure all combustion appliances are properly installed and vented. Use direct-vent, sealed combustion appliances.

Includes information from the Canada Mortgage and Housing Corp.

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