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Finding the Source of Black Soot in a House

Category Miscellaneous
Soot could come from a furnace or from a fireplace and chimney. This is a page about finding the source of black soot in a house.


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November 4, 2015

We suddenly found black soot like powder on the floor, walls, baseboards, and furniture about 3 weeks ago when dusting. We attributed it to the furnace though we have no vents since we have in floor radiant heat, or the fireplace, which had just been cleaned. We also then had the furnace cleaned. About a week later, we noticed a slight musty smell which has since intensified. The smell is in the same area of the house but strangely also upstairs on the opposite side of the house in a bedroom. Airing out the house helped, but the smell immediately intensifies when the windows are closed.

Initially, we did not associate the two, thinking the smell may be from a dead mouse in the wall. We are still searching. During the search, we have found the soot again even though we have not used the fireplace. The black powder is on the baseboard and the windowsill of the half round windows above the lower casement windows. The lower sills do not have the powder. What can this be? Are the two issues related or do we have two separate problems? We would appreciate any insight or information about similar experiences. Also, I would gladly call a professional, but I don't know who to call.


November 5, 20150 found this helpful

This is a puzzle. Soot must come from somewhere where there is combustion. If you have in floor heating, (water pipes in the floor, right?) you must have a boiler. Does your boiler run on natural gas? Perhaps there is some problem with that, although how soot would get all over the house is beyond me. I have a natural gas boiler for hot water heat (radiators) and I had to have the burner replaced, because it was producing a sooty mess, but only inside the furnace.


However, I would call someone about the furnace first. I would suspect that if your boiler burns some other fuel, it would be even more likely to produce the soot. If that is not the problem, could the soot have gotten all over when the fireplace was cleaned?

I would not worry about the smell until after I had the furnace dealt with. They may be the same problem. If not, I think that you might be on the right track with the mouse search.

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November 6, 20150 found this helpful

Could what you think is soot really be mold? That would give your house a musty smell.

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May 20, 20190 found this helpful

The appliances are doing what they are supposed to do, but there's still soot and dirt. In fact, the soot is oozing out of attic vents, through attic hatchway doors, and into and out of vents. Is this the mystery?


No mystery, but a phenomenon with a rather sinister-sounding name: depressurization combustion re-entry.

A mouthful, maybe, but easily explained.

What happens: In a house that is well insulated and nearly airtight, the air pressure is lower indoors than out because air movers (dryers and exhaust fans, for example) are pushing air out of the house, and the house is too tight for new air to come in to balance off the pressure. Or, wind on one side of the house is pulling out the air, a venturi effect.

This happens often in new, tight houses. By itself, it is not a bad thing. But what happens is that the house wants to bring in air to even out the air pressure; trying to inhale, as it were.

If the outside air is clean, there is no problem. But if the outside air is contaminated, that contamination will get into the house. Yes, but how does that outside air get contaminated?


Sometimes a power vent (a burner or stove vent that does not need a chimney) that goes through the wall will contaminate the air, which is then drawn into the attic through the soffit vents.

Sometimes a low chimney, one that is not higher than the roof ridge, will not exhaust smoke and particles high enough, and instead waft smoke and particles into the attic by way of the ridge vent.

Or, particles will follow the vent stack into the attic, and land on the attic insulation. Dirty insulation is a sure sign of this.

Preposterous? Not at all, although you may have to be a physicist or engineer to understand it entirely.

Take an engineer's word for it. It does happen.

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Home and Garden Cleaning MiscellaneousNovember 4, 2015
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