Photograph of  thermal tracking or soot tracking - cool surface, moisture condenses, soot is deposited - Daniel Friedman 02-12-16 Moldy Ceilings? How to Recognize & Diagnose Stains at Heating or Air Conditioning Supply Registers

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Dark heating or air conditioning air register stains on ceilings & walls: this article illustrates what mold looks like on interior ceilings and how to distinguish that mold presence from stains found around HVAC air supply registers. In fact sometimes stains at HVAC registers are mold, and sometimes the stains are soot - possibly indicating very dangerous conditions. Most often the stains are ordinary house dust and debris, as we explain here.

We provide details about how to recognize & diagnose stains at heating or air conditioning supply registers and explain how to distinguish these common debris marks from possibly more serious mold, thermal tracking, thermal bridging stains, building air leaks, and building insulation defects.

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Dark stains on building interior walls may appear in other patterns and could be from other causes - we provide photographs, description, diagnosis, and advice for many of these indoor stains in this article.

How to Diagnose Stains and Debris at Ceiling and Wall Heating or Cooling Air Supply Registers

Mold on an air supply register © Daniel FriedmanWhen investigating a building for a mold problem, you can save mold test costs by learning how to recognize Stuff that is Not Mold or is only Harmless Mold but may be mistaken for more serious contamination - save your money.

These stains and marks are often substances that you can easily learn to recognize in buildings.

[Click to enlarge any image]

In our photo at left the stains were indeed mold, formed due to moisture collecting at the corners of the ceiling register. But usually these marks are simple dust transported by the air handler and ductwork.

Save your mold test money, and increase the accuracy of your mold contamination inspection or test for toxic or allergenic mold in buildings: review these items to learn recognize non-fungal materials or even possibly harmless cosmetic "black mold" often mistaken for "toxic fungal growth."

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Photograph of dirt on a ceiling at an HVAC supply register Dirty HVAC duct work, lack of air filter maintenance, or furnace malfunctions can all be indicated by black stains that appear around warm or cool air delivery registers in building ceilings or walls.

This is usually a deposition of house dust (skin cells, fabric fibers). Mold would not be expected to grow in just this pattern around a supply register (though we agree that air movement is related to moisture deposition or condensation which is related to mold growth.)

An investigation of what was at first called KILLER HOUSE DUST from an HVAC system showed that the dust of concern turned out to be cotton and other carpet fibers having nothing to do with the Heating or Cooling equipment was discussed at our FEAR OF MOLD WebLog or "Blog" where we periodically post results of interesting forensic investigations.

We welcome more thermal tracking, soot tracking, air bypass leaks, and similar photos of indoor stains as well as text suggestions to expand this detail and would be glad to credit contributors.

Thermal tracking on a wall above a hot water heating baseboard is shown

See OTHER STAINS on WALLS & CEILINGS for further discussion of this photograph and for additional photos and examples of stains on building surfaces that are caused by problems with building heating or air conditioning systems or from other sources.

Dark Stains & Debris Around an Air Supply Register May not be Mold but Might Mean Trouble

Watch out: dark stains deposited around HVAC supply registers might spell trouble. Normal house dust, comprised mostly of skin cells, fabric fibers, and perhaps road dust, looks gray or black on surfaces where it is accumulated, and may be relatively harmless. But here are some other, more serious clues that could result in dust and debris and "soot stains" around air registers:

  • An unsafe gas fired warm air furnace that is depositing soot is very dangerous, should be shut down at once, and inspected by an expert. Sooting in gas fired heating equipment can be a clue of dangerous, potentially fatal carbon monoxide gas production. See COMBUSTION AIR DEFECTS
    And s
  • A potentially unsafe oil fired warm air furnace that is not working properly, one that is producing soot, may be sending that material through the heating ducts, especially if there is an air return close to the furnace. An air return inlet close to any heating appliance is dangerous because it can cause improper combustion, cause production of dangerous carbon monoxide, and can interfere with proper safe operation of the heater. A result might be a dangerous "puffback".
  • Air filter problems: no air filter, or a leaky air filter, or a damaged one can send dust, dirt and debris through the HVAC duct system.

Mold Can Also Be Found on and Around HVAC Air Supply Registers

By simple visual inspection (by the naked eye) it can be difficult to know if the black or gray debris on air supply registers is moldy crud or common house dust.

The ceiling air supply register and aerobiology lab mold photos below show debris deposited on HVAC ceiling air supply registers in a Kentucky building.

In response to a history of a building leak event and odor complaints as well as the appearance of mold on some building walls, we analyzed samples of the dust from the ceiling air supply register shown at left. At right you can see that the dominant particle in the sample was Cladosporium sp. C. sphaerospermum spores were also present in this sample.

Mold on an air supply register © Daniel Friedman Cladosporiium mold on an HVAC ceiling air supply register © Daniel Friedman

Don't panic even if you do find small amounts of mold in an air supply register. Even in a healthy building we might expect condensate forming on ceiling air supply registers in some conditions, and thus an accompanying growth of small areas of mold. If that's the extent of mold contamination, ordinary household cleaning procedures are sufficient.

As we suggest at BLOWER LEAKS, RUST & MOLD, if the building leak history or complaint history suggest that a larger mold reservoir could be present, including other genera/species of mold, further investigation is warranted. Leaks into an air handler unit (blower assembly) or into the building ductwork can increase the mold contamination in those components, and leaks into building ceilings, walls, or crawl spaces can produce large mold reservoirs of a variety of mold genera/species.

While Cladosporium sp. (photo above right) is the most common mold found on earth and while it's just about everywhere, for some people this is an allergenic mold. We don't want it being blown around by our air conditioning system nor its growth improved by mishandling of HVAC condensate.


and MOLD RELATED ILLNESS GUIDE for a description of the health effects and air quality complaints associated with various kinds of mold.

Actual Ceiling Mold Contamination - Examples

Mold contamination on building ceiling surfaces can appear in a very wide range extending from trivial to extensive and likely to be unhealthy.

Comparing Two Extents of Mold Growth on a Ceiling

Our ceiling mold photo at below left illustrates a mold growth pattern that appears to follow the properties of a prior rolled-on paint application - we speculated that factor because the edge-delineated mold growth patterns need an explanation, yet they occur at right angles to one another, suggesting that the ceiling framing (joist direction) alone would not explain this pattern.

This growth pattern could have occurred either due to high indoor moisture or due to leaks into the ceiling above. We suspected the former, but further investigation was needed.

Mold contaminated apartment ceiling (C) D Friedman and SM Mold contaminated apartment ceiling (C) D Friedman and SM

Our second moldy ceiling photo (above right - click to enlarge) shows isolated spots of mold over a larger ceiling area. It might be tempting to just surface clean and paint this second ceiling but that option should not be chosen before a more thorough building inspection and mold cause diagnosis.

Depending on the moisture or leak source the mold you see on a building ceiling may be often divided into two location-dependent cases:

  • Surface mold contamination driven by in-room moisture or high humidity will usually appear more extensively on the exposed or visible ceiling surface than in materials and on surfaces hidden in the ceiling cavity.
  • Hidden mold contamination within the ceiling cavity will usually be driven by leaks into the ceiling cavity from above, such as from roof or plumbing leaks. Mold within a building cavity can also occur, however, if there is sufficient moist air movement into the cavity from the building interior space.



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