Dark heating or air conditioning air register stains on ceilings & walls:

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

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.

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

When 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."

Article contents

• STAINS at HVAC REGISTERS
  • DARK STAINS & DEBRIS at SUPPLY REGISTERS
  • MOLD ON & AREOUND HVAC SUPPLY REGISTERS
  • CEILING MOLD CONTAMINATION

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 "Blog" .

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

at WALL THERMAL TRACKING STAINS.

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
  see CARBON MONOXIDE - CO
• 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".
  
  See OIL BURNER SOOT & PUFFBACKS
• 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.
  
  See AIR FILTERS for HVAC SYSTEMS.

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.

See BLOWER LEAKS, RUST & MOLD

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 above.

Below you can see that the dominant particle in the sample was Cladosporium sp. C. sphaerospermum spores were also present in this sample.

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.

See MOLD CONTAMINATION LEVELS

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.

Our second moldy ceiling photo (below - 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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Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

On 2018-04-11 - by (mod) -

Sorry DRP, when readers post a photo along with a comment they should see a message that the photo awaits moderator approval. That's to protect both readers and our website from malware or from inappropriate content.

Your photos show dust and dirt deposits at a air supply register - that can occur regardless of the soot content: house dust, for example will leave such marks; but also an improperly operating oil fired heating system, use of scented candles, smoking, or any other indoor soot source could be at fault.

On 2018-04-10 2 by DRP Landlord

These pictures were taken after a Tenant moved out after one year.

Any ideas about what caused this black “sooty” film?
It is all over everything, in every room of the house.

IMAGE LOST by older version of Clark Van Oyen’s Comments Box code - now fixed. Please re-post the image if you can. Sorry. Mod.

On 2018-02-27 - by (mod) - If you smell mold

Craig

If there were water leaks into a ceiling it is possible that insulation, wood framing, and drywall all have been wet - inviting mold growth, rot, and insect damage, depending on how much material got wet and how long those conditions existed.

If you smell mold, or if you believe that you suffer a (possible) building-related illness, those would increase the level of concern. "Suffering a (possible) building-related illness" means, for example that you have respiratory issues when in the home but not when out of it.

Water dripping off the outside of a roof when there is no rain could also mean that water is leaking from the swamp cooler - possibly that equipment is not working properly.

You can ask your health department to take a look at your home.

On 2018-02-27 by Craig - mold contamination in Palm Springs

My email,
Mauispirits@yahoo.com.

Any suggestions?

Hi.
I live in palm springs. When I moved in, the outside roof would pout down water on the pavement in intervals like a waterfall. Little did I know about old swamp cooler and slum landlord.

After about a month I noticed rippling in the ceiling, then brown drops which eventually turned to buckets of brown nasty thick water.

The landlord finally replaced the swamp cooler but did nothing on my ceiling which is still cracked and rippled with brown stains and the paint is bubbly around the area as well.also in the bathroom.

Do I need to worry about this?

I now notice water dripping off the outside roof as before but only in the night and early mornings. If I dare mention this to my landlord, oh I'll have hell to pay and possibly will be kicked out . I'm disabledand worried this 1957 apartment may be filled with mold above the ceiling and walls where paint is puffing out.

Please help me figure out what. Who, where and what to do to make sure it's not a toxic environment for me and my guests and pets.

Any ideas or leads would be greatly appreciated.

On 2017-12-04 - by (mod) -

CK

To me that looks like a dirty air conditioning or heat pump evaporator coil. It may be some corrosion as well.

If you're asking what are the constituents of the deposits on the cooling coil, I can't really say from your photo. Usual contents of house dust are dominated by fabric fibers and skin cells. Occasionally we see some mold growth on that organic material.

On 2017-12-03 by C King

Condensor Coil

IMAGE LOST by older version of Clark Van Oyen’s Comments Box code - now fixed. Please re-post the image if you can. Sorry. Mod.

Just “dirt” or Mold? My kid and I have been reoccurently ill since moving into this apartment and the minute I mentioned the WORD “mold” the maintenance and property managers have been hateful and rude and I was given a lease renewal 90 days before my lease is even up and the rent is over $100 higher with no explanation. I feel almost like I’m being bullied, it’s parhetic!

Paying rent and utilities on an apartment that I’ve basically abandoned for the sake of my child’s health, specifically, because when she’s out of that apartment for 24-72 hours her symptoms subside!

We have been prescribed antibiotics more than 5 times each since August! I have a sinus infection that won’t go away?! Someone please help!

IMAGE LOST by older version of Clark Van Oyen’s Comments Box code - now fixed. Please re-post the image if you can. Sorry. Mod.

On 2017-12-03 by C King

What’s this?
IMAGE LOST by older version of Clark Van Oyen’s Comments Box code - now fixed. Please re-post the image if you can. Sorry. Mod.

On 2016-10-11 by Carol Williams

I have more light brown water stains above my hot water heater. what may be the problem, and who do I call?

...

Continue reading at STAINS HVAC SOOT or DUST or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• AIRBORNE PARTICLE & MOLD LEVELS in DUCTWORK where we describe how to test HVAC systems and ductwork for mold.
• BLOWER LEAKS, RUST & MOLD
• IAQ DIAGNOSIS via THERMAL TRACKING STAINS
• MOLD on/in AIR DUCTS in HVAC SYSTEMS - photos of in-situ mold growth in or on actual ductwork
• MOLD on DUCT SUPPLY or RETURN REGISTERS - photos of in-situ mold growth on air supply and return openings and grilles.
• OIL BURNER SOOT & PUFFBACKS
• STAIN DIAGNOSIS on BUILDING INTERIORS - home
• STAINS CANDLES FIREPLACE WOODSTOVE
• STAINS DIAGNOSE IAQ PROBLEMS
• STAINS on INDOOR SURFACES, PHOTO GUIDE
• STAINS CANDLES FIREPLACE WOODSTOVE
• STAINS at HVAC REGISTERS
• STAINS on INDOOR SURFACES, PHOTO GUIDE
• THERMAL TRACKING BRIDGING GHOSTING - home

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STAINS at HVAC REGISTERS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

Or see this

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING STAINS

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