A.Frank Vigil, senior building science specialist at Advanced Energy in Raleigh, N.C., responds: Reports are increasing of problems with black stain deposition on interior surfaces. Since particulate is attracted to cooler surfaces, it is not uncommon to see black staining following the framing members behind sheetrock. The source of the particulate varies. We have documented sources ranging from furnaces and water heaters to fireplaces, candles, outdoor cooking grills, and even automobile tires.

Although you say the fireplace has been tested for leakage, not all technicians test for all possible problems. Was the fireplace tested under pressure or simply examined? Is the fireplace a "sealed" unit? Some units appear to be sealed, but when tested under negative pressure can leak significantly. Is there a standing pilot light? If...

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