Published on December 5th, 2014 | by Henri


Soot gathers in house that is heated with gas

Q.  I have a gas furnace with radiators and hot-water heat but I seem to be getting black sooty film over my walls and windows. The gas company says that gas is clean and it must be something else. I closed up a wood stove in my rec room, relined my fireplace, re-vented my gas dryer and opened a vent closer to my furnace for better combustion but still have the problem.

It is especially heavy around my windows and TV screen. I wash my windows frequently because of this and repaint my rooms every two years. This has been an ongoing problem for several years, since I converted to gas from oil. No one seems to be able to find out the cause. Hoping for your assistance.

A.    If your boiler is an old style that is vented through the chimney, it draws its combustion air through any openings, cracks and crevices in the house (fireplace, wood stove and around older windows).

If the flues to the wood stove, the fireplace and the boiler are next to each other in the same chimney, and there are flat caps on them, when the boiler fires up, its gases may be drawn back in the house through the wood stove and /or the fireplace flues. And if the boiler’s flue was not thoroughly cleaned at the time you changed from oil to gas (as it must be), the oil deposits on the flue walls are desiccated by the heat and drawn into the combustion gases of the boiler. The clean combustion gases associated with natural gas are now contaminated with the soot from the oil and are recirculated throughout the house.

Another possibility is that, if you use the fireplace, it may draw its make-up air from the boiler flue, thus drawing the desiccated oil particles into the house.

Have the boiler flue thoroughly cleaned by a chimney sweep; remove or change the caps to tunnel-type caps parallel to each other (never facing each other) and see if that does not improve the situation.

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