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Moldy Ceilings? How to Recognize & Diagnose Stains at Heating or Air Conditioning Supply Registers
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InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.
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. 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
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. 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." Readers should see HEAT LOSS in BUILDINGS. Also see AIR SEALING STRATEGIES and AIR LEAK DETECTION TOOLS as well as AIR LEAK MINIMIZATION. For photos of snow melt on rooftops that also show points of building heat transfer and loss, see STAIN DIAGNOSIS on BUILDING EXTERIORS.
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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 at Wall Thermal Tracking Stains. See Other Stains on Indoor 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 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.
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 Atlas & Particles List 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 (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.
Frequently Asked Questions (FAQs)
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
- Thanks to Alan Carson, Carson Dunlop, Associates, Toronto, for technical critique and content suggestions regarding thermal tracking.
 Carson, Dunlop & Associates Ltd., 120 Carlton Street Suite 407, Toronto ON M5A 4K2. (416) 964-9415 1-800-268-7070 info@carsondunlop.com. The firm provides professional home inspection services & home inspection education & publications. Alan Carson is a past president of ASHI, the American Society of Home Inspectors. Thanks to Alan Carson and Bob Dunlop, for permission for InspectAPedia to use text excerpts from The Home Reference Book & illustrations from The Illustrated Home. Carson Dunlop Associates' provides:
- Commercial Building Inspection Courses - protocol ASTM Standard E 2018-08 for Property Condition Assessments
- Home Inspection Education Courses including home study & live classes at eleven colleges & universities.
- Home Inspection Education Home Study Courses - ASHI@Home Training 10-course program.
Special Offer: Carson Dunlop Associates offers InspectAPedia readers in the U.S.A. a 5% discount on these courses: Enter INSPECTAHITP in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference Book, a reference & inspection report product for building owners & inspectors.
Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
- The Home Reference eBook, an electronic version for PCs, the iPad, iPhone, & Android smart phones.
Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter inspectaehrb in the order payment page "Promo/Redemption" space.
- The Illustrated Home illustrates construction details and building components, a reference for owners & inspectors.
Special Offer: For a 5% discount on any number of copies of the Illustrated Home purchased as a single order Enter INSPECTAILL in the order payment page "Promo/Redemption" space.
- The Horizon Software System manages business operations,scheduling, & inspection report writing using Carson Dunlop's knowledge base & color images. The Horizon system runs on always-available cloud-based software for office computers, laptops, tablets, iPad, Android, & other smartphones.
- Mark Cramer Inspection Services Mark Cramer, Tampa Florida, Mr. Cramer is a past president of ASHI, the American Society of Home Inspectors and is a Florida home inspector and home inspection educator. Contact Mark Cramer at: 727-595-4211 mark@BestTampaInspector.com 11/06
- ASHRAE resource on dew point and wall condensation - see the ASHRAE Fundamentals Handbook, available in many libraries. The following three ASHRAE Handbooks are also available at the InspectAPedia bookstore in the third page of our Insulate-Ventilate section:
- 2005 ASHRAE Handbook : Fundamentals : Inch-Pound Edition (2005 ASHRAE HANDBOOK : Fundamentals : I-P Edition) (Hardcover), Thomas H. Kuehn (Contributor), R. J. Couvillion (Contributor), John W. Coleman (Contributor), Narasipur Suryanarayana (Contributor), Zahid Ayub (Contributor), Robert Parsons (Author), ISBN-10: 1931862702 or ISBN-13: 978-1931862707
- 2004 ASHRAE Handbook : Heating, Ventilating, and Air-Conditioning: Systems and Equipment : Inch-Pound Edition (2004 ASHRAE Handbook : HVAC Systems and Equipment : I-P Edition) (Hardcover)
by American Society of Heating, ISBN-10: 1931862478 or ISBN-13: 978-1931862479
"2004 ASHRAE Handbook - HVAC Systems and Equipment The 2004 ASHRAE HandbookHVAC Systems and Equipment discusses various common systems and the equipment (components or assemblies) that comprise them, and describes features and differences. This information helps system designers and operators in selecting and using equipment. Major sections include Air-Conditioning and Heating Systems (chapters on system analysis and selection, air distribution, in-room terminal systems, centralized and decentralized systems, heat pumps, panel heating and cooling, cogeneration and engine-driven systems, heat recovery, steam and hydronic systems, district systems, small forced-air systems, infrared radiant heating, and water heating); Air-Handling Equipment (chapters on duct construction, air distribution, fans, coils, evaporative air-coolers, humidifiers, mechanical and desiccant dehumidification, air cleaners, industrial gas cleaning and air pollution control); Heating Equipment (chapters on automatic fuel-burning equipment, boilers, furnaces, in-space heaters, chimneys and flue vent systems, unit heaters, makeup air units, radiators, and solar equipment); General Components (chapters on compressors, condensers, cooling towers, liquid coolers, liquid-chilling systems, centrifugal pumps, motors and drives, pipes and fittings, valves, heat exchangers, and energy recovery equipment); and Unitary Equipment (chapters on air conditioners and heat pumps, room air conditioners and packaged terminal equipment, and a new chapter on mechanical dehumidifiers and heat pipes)."
- 1996 Ashrae Handbook Heating, Ventilating, and Air-Conditioning Systems and Equipment: Inch-Pound Edition (Hardcover), ISBN-10: 1883413346 or ISBN-13: 978-1883413347 ,
"The 1996 HVAC Systems and Equipment Handbook is the result of ASHRAE's continuing effort to update, expand and reorganize the Handbook Series. Over a third of the book has been revised and augmented with new chapters on hydronic heating and cooling systems design; fans; unit ventilator; unit heaters; and makeup air units. Extensive changes have been added to chapters on panel heating and cooling; cogeneration systems and engine and turbine drives; applied heat pump and heat recovery systems; humidifiers; desiccant dehumidification and pressure drying equipment, air-heating coils; chimney, gas vent, fireplace systems; cooling towers; centrifugal pumps; and air-to-air energy recovery. Separate I-P and SI editions."
- Principles of Heating, Ventilating, And Air Conditioning: A textbook with Design Data Based on 2005 AShrae Handbook - Fundamentals (Hardcover), Harry J., Jr. Sauer (Author), Ronald H. Howell, ISBN-10: 1931862923 or ISBN-13: 978-1931862929
- 1993 ASHRAE Handbook Fundamentals (Hardcover), ISBN-10: 0910110964 or ISBN-13: 978-091011096
- Energy Savers: Whole House Systems Approach to Energy Efficient Home Design [copy on file as /interiors/Whole_House_Energy_Efficiency_DOE.pdf ] - U.S. Department of Energy
- "Energy Savers: Whole-House Supply Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Supply_Vent.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11880?print
- "Energy Savers: Whole-House Exhaust Ventilation Systems [copy on file as /interiors/Energy_Savers_Whole-House_Exhaust.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11870
- "Energy Savers: Ventilation [copy on file as /interiors/Energy_Savers_Ventilation.pdf ] - ", U.S. Department of Energy
- "Energy Savers: Natural Ventilation [copy on file as /interiors/Energy_Savers_Natural_Ventilation.pdf ] - ", U.S. Department of Energy
- "Energy Savers: Energy Recovery Ventilation Systems [copy on file as /interiors/Energy_Savers_Energy_Recovery_Venting.pdf ] - ", U.S. Department of Energy energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11900
- "Energy Savers: Detecting Air Leaks [copy on file as /interiors/Energy_Savers_Detect_Air_Leaks.pdf ] - ", U.S. Department of Energy
- "Energy Savers: Air Sealing [copy on file as /interiors/Energy_Savers_Air_Sealing_1.pdf ] - ", U.S. Department of Energy
Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair
 The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 25th Ed., 2012, is a bound volume of more than 450 illustrated pages that assist home inspectors and home owners in the inspection and detection of problems on buildings. The text is intended as a reference guide to help building owners operate and maintain their home effectively. Field inspection worksheets are included at the back of the volume. Special Offer: For a 10% discount on any number of copies of the Home Reference Book purchased as a single order. Enter INSPECTAHRB in the order payment page "Promo/Redemption" space. InspectAPedia.com editor Daniel Friedman is a contributing author.
Or choose the The Home Reference eBook for PCs, Macs, Kindle, iPad, iPhone, or Android Smart Phones. Special Offer: For a 5% discount on any number of copies of the Home Reference eBook purchased as a single order. Enter INSPECTAEHRB in the order payment page "Promo/Redemption" space.
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