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Community and Q&A

Carbon Furnace Filters and Ghosting Marks

suect | Posted in General Questions on

I noticed shadows on the corners of the exterior wall corners of several rooms and where the ceiling meets the wall on the out facing walls.

I have Ben using carbon filters for the past 2 years, just recently noticing the carbon layer is on the exit side of the filter as it enters into the living space.

Could this cause the shadow marks?

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  1. charlie_sullivan | | #1

    I've been using a carbon filter in my ERV/HRV system for years and haven't seen anything like that, but it might vary with the specific carbon filter. Is this on air from a furnace? It also might be that the carbon filter is damaged by heat and starts shedding particles, if that's the case. I'd also expect the carbon filter to work better on room temperature air.

  2. user-2310254 | | #2


    Are the shadows occurring where you air leaks? If so, it could be dust dust exfiltrating (along with air) to the outside.

  3. onslow | | #3


    I will guess you are in Connecticut (not a fan of computed tomography), so I suspect the shadowing you are seeing is the result of cold spots harvesting dust from the air. I have seen the effect most notably in homes with old style open fireplaces or heavy smokers. The lack of insulation at the top plate of your wall is a cold path from outside to inside. The equivalent of morning dew will collect on the wall's cold surface, which in turn makes airborne dust or soot collect there. In one house I worked on, the rafter pattern was quite obvious on the ceiling as well as the continuous shading of the wall at the top plate position.

    If you aren't using lots of candles or own an open fireplace that backdrafts occasionally, then the charcoal filters may well be the source of the soot. Activated charcoal can be dusty, but it really should be anchored to the filter media in a firm way. The shadowing material is typically the result of very small particles that are floated around the room with the humidity, a bit like snow nucleation.

    If you live near by industrial polluters, then the "soot" could be that and more. You might check to see if you have similar dark staining around your windows. Double hung window will often leak near the track sides. If the same effect is there, you may be stuck with an air quality issue that is beyond filtering.

    If the filters are shedding, one way to test them would be to take a white coffee filter opened flat then taped across the floor tool for your vacuum. Rubber bands might work depending on how strong the vacuum is. With the vacuum on, put the floor tool and coffee filter up against the air filter media you are using and leave in place for several minutes. If the charcoal is shedding you should see a ghost image of the tool opening appear on the coffee filter material. I would then suggest trying another brand of air filter or perhaps turning the the charcoal side in.

  4. suect | | #4

    Thank you. Over 10 years ago we had burned candles, now we melt them. We have 2 gas furnaces, no wood burning. I recently had the fire places cleaned. I’m now wondering if the carbon filters are restrictive, leading to potential problems.

  5. Expert Member
    BILL WICHERS | | #5

    Activated carbon can create a very fine carbon dust, but that usually clears pretty quickly. The dust is mostly created during shipping when the granules are bouncing around in transit. This same problem occurs in water filters that use granulated activated carbon.

    Any filter can get clogged up over time, and activated carbon filters are generally more susceptible. In the water filter world, you always put a particulate filter ahead of a carbon filter to protect it. In the air filter world, that's not commonly done.

    An easy test would be to replace your activated carbon filter with a high-MERV (MERV-13 or higher) filter. When doing this, check if there is more black dust on the "out" side of the carbon filter, which would tend to indicate some carbon dust.

    BTW, using a deeply pleated filter will reduce backpressure, and is especially helpful with the more restrictive high MERV filters. Deeper filters can usually go longer before needing to be replaced too. I use, and like, the Aprilaire filter system that uses very deep (~4-5 inch or so) pleated filters. These are much better than the common 1" deep filters used in residential HVAC systems. You do need to install their filter holder though. If you want to stick with a "slot", you can get deeper (2" and 4" depths are pretty common) pleated filters in cartridge form that can be slipped into a slot wide enough to accept them. Grainger has a good selection of these filters.


    1. suect | | #6

      Thank you.

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