What do the dirt marks on my carpets have to do with my furnace and ductwork and what is this term "Carpet Filtration"?

We're glad you asked!

Many consumers have been blaming markings and soot deposits on carpeting, walls and furnishings on their new furnace and heating system. During the last year much research has been done to scientifically prove the phenomenons, cause and effect.


Carpet Filtration

Though the issue of carpet filtration has been present for quite some time, it has been more of a problem lately with the trend toward the use of lighter-colored carpet for both replacements and in new homes.

It is quite common for dirt and dust to accumulate under interior door undercuts when the door is left closed. Positive pressure in the closed room, caused by supply air registers blowing air into the room, forces air to flow past the largest available hole to a lower pressure area (return air) within a home. The undercut door is generally the largest available hole. As the air passes between the door undercut and the carpeting, the carpet acts like a filter taking out dirt and dust particles from the air. Over a period of time, the carpet will darken as the dirt accumulates. This can easily be mistaken for soot staining.

In-Home Air Pressure
Positive and negative pressure within a building can also create stains. Air that is entering the home through holes and cracks will leave dirt and dust on outside walls and on the insulation covering the cracks. If the home is exposed to pressures from high stack effect from warm air rising or from mechanical pressures from duct leakage or even exhaust fans, carpet filtration can sometimes occur at the carpet edge where the interior or exterior wall joins. Again, the carpet will serve as a filter as the air exits under pressure. If the wall is open to the attic, dirt and dust stains will be present all along the baseboards or ceiling.

While the movement of air under pressure causes carpet filtration and the use of lighter-colored carpets make it more apparent, the furnace/air handler and system may be blamed. If pressure differences are corrected, carpet filtration should be lessened. This does not mean however, that the heating/cooling equipment can be ignored. Maintenance and cleaning of the heating/cooling equipment should be done annually and the ductwork should be cleaned every 3 years to ensure a properly tuned and efficient system. In addition, qualified trained technicians should perform all work




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