Photograph of this ugly duct routing risking water entry, mold, rodents, high operating cost.

How to Find & Remove Odors, Gases & Smells Heating and Air Conditioning Duct Work
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What are the possible causes of odors in ductwork?
How do we find and cure smelly heating or cooling ducts or air handlers?
How to find and remove heating oil tank odors, oil tank leak odors
How to identify and remove animal or pet odors in buildings
How to use gas detectors, gas sniffers, various gas testing devices
How to identify odors or gases by type, source, and toxicity. Noxious odors or smells in buildings can be diagnosed and cured
Questions & answers about the causes of & cures for HVAC duct system odors & smells

HVAC Duct odor diagnosis & cure: This article explains the diagnosis and cure of odors in HVAC ducts, air handlers, blowers, for both warm air heating and air conditioning systems.

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Building Air Duct and Air Handler Odor Guide: How to Find, Test, & Remove Odors, Odor Detection, Smells, & Gases that Appear in Ductwork

Duct and air handler odors in buildings can be traced to a variety of sources such as leaks and mold in the duct system, a leaky (and unsafe) heat exchanger sending flue gases or even carbon monoxide into building air (see CARBON MONOXIDE - CO and HEAT EXCHANGER LEAKS), dead animals in the ducts or air handler, or even a bad blower motor that is overheating.

This website provides articles on to diagnose, test, identify, and cure or remove a wide range of obnoxious or even toxic odors in buildings and in building water supply. We discuss odors from a variety of sources including animals including pets, dogs, cats, or unwanted animals or dead animals, formaldehyde odors in buildings from building products or furnishings, plumbing drains, plastic or vinyl odors from building products, flue gases, oil tanks or oil spills, pesticides, septic odors, sewer gases, and even abandoned chemicals at properties.

Readers should see our more extensive odor diagnosis home page: ODORS, Smells, Gases in buildings-Diagnosis & Cure. Also see OIL HEAT ODORS, as well as OIL BURNER NOISE SMOKE ODORS. Readers should also see our ODOR DIAGNOSIS CHECKLIST for a quick check that can help identify the source of smells in buildings and see SMELL PATCH TEST to Track Down Odors.

Tracking down building odors associated with the heating or cooling ductwork can be tricky not only because there is a larger variety of possible sources of duct smells and stinky ducts than you might guess, but also because once an odor source has invaded the HVAC system, smells can be delivered to other more remote building areas.

One IAQ investigator associate traced the mold-related-illness of a building occupant to the delivery of mold-contaminated air (MVOC's and mold spores) right to the occupant's head when she was asleep - a supply air register was close to the bed's headboard.

List of Sources of Duct System Odors / Smells

The checklist below addresses things to check if odors appear to be present in or coming from building heating or cooling ductwork, air handlers, or blower compartments, or at the heat exchanger.

Animal Smells may be due to current or prior pets in a building, pet urine or fecal waste, cat boxes, animal hair, dog dander, cat dander (are allergens and are indicators of the level of prior pet activity), dust tracked in by dogs. But animal odors in buildings can also occur when an animal such as a mouse or rat has died in a building cavity.

A dead animal smell has been described by our clients with a wide variety of terms ranging from a vague noxious stink that seemed to vary with humidity to a sweet sickly smell. Dead animals or even insect nests in building plumbing, especially building vents, can also produce unexpected sewer odors - see Septic and Sewer gas odor links discussed below.

A dead animal is occasionally found in building duct work, or in the air handler itself; we have found birds, mice, rats, and twice a raccoon and a dead cat in ductwork. Animals may find their way into the duct system where they cannot escape.
Carbon Monoxide Gas Toxicity hazard levels, poisoning symptoms, & testing. Some of our readers report testing for carbon monoxide to see if their heat exchanger was leaking combustion gases into the ductwork. While testing for CO is an important safety check, it is not reliable as a duct odor source test: you won't smell CO (carbon monoxide) in a building.

But if combustion gases from a heating or hot water system or possibly a wood stove or coal stove are not being safely vented to outdoors you may smell other telltale products of combustion including those gases entering the duct system either through a leak in the heat exchanger or by return air duct system defects that draw heating flue gases into the duct system through that route. Be certain that you have working CO detectors as well as smoke detectors in your building.
CHINESE DRYWALL HAZARDS that may involve sulphur or "sewer gas" odors in buildings may be due to the use of corrosive sulphur and other outgassing from Chinese drywall used in some buildings. These gases are also corrosive and can damage HVAC equipment as well as other building components.
Drain Odors: Plumbing Drain Noises - Diagnosis & Repair guide. This article discusses the cause, diagnosis, and cure of plumbing drain noises. A drain noise can also be a clue to plumbing drain odor sources.

That "blub blub" or "glug glug" noise you hear from a building drain might mean that there is a problem with the drain system itself, such as a partial drain blockage, a drain venting problem, a drain odor problem, or even a failing septic system. In the Drain Odors article we discuss the causes and cures for plumbing drain noises, and we refer to key companion articles that assist in that diagnosis.

On occasion we have found that sewer gases from the building drain waste vent system were being drawn into the building air duct system (a return air intake was located close to the bad building drain).
Gas Odors: A Toxic Gas Testing Sampling Plan for Residential Indoor Air Investigations. This document outlines gas toxicity levels and gas testing procedures we use in field IAQ and environmental health investigations for a range of indoor gases which may be produced by building product outgassing, mold and MVOCs, mechanical systems, fire damage, or contamination from nearby industrial, beauty parlor, dry cleaning, or other activities which often produce noxious or toxic odors and gases.
Gas Exposure Hazard Levels: for Toxic Gas Exposure to Ammonia, Arsine, Arsenic, Bromine, Carbon Dioxide, Carbon Monoxide, Hydride, Ozone - allowable exposure levels and hazard levels
GAS DETECTION & MEASUREMENT recommendations for selection and use of gas detection equipment and descriptions of how gas testing equipment is used
Gas Testing methods of screening for various odors, gases, and chemicals in the indoor environment
Hazardous Material Waste Site Maps from ATSDR - U.S. CDC Gather - "Geographic Analysis Tool for Health and Environmental Research" online public access to data pertinent to public health
Heating System Odors: odors of combustion gases or heating oil or natural or LP gas can all be indicators of serious safety hazards as well as malfunctioning building heating or water heating equipment. See these detailed articles:
- BOILER NOISE SMOKE ODORS
- BOILER OPERATING PROBLEMS
- CARBON MONOXIDE - CO
- CHIMNEY INSPECTION DIAGNOSIS REPAIR
- DRAFT HOODS - gas fired
- DRAFT REGULATORS, DAMPERS, BOOSTERS
- HEAT EXCHANGER LEAKS
- HEATING OIL EXPOSURE HAZARDS, LIMITS
- OIL BURNER NOISE SMOKE ODORS
- OIL ODORS, LEAKY OIL TANK PIPING
- Oil Tank Leaks and Smells
- Spill Switches used to detect flue gas spillage on gas fired equipment
- THERMAL TRACKING - indoor stains that can indicate an unsafe heating system
- Using the TIF 8800 Gas Detector to check for gas leaks and combustion gases
Mold Odors and Moldy or Musty Smells: Here are articles on diagnosing moldy or musty building odors
- BLOWER LEAKS, RUST & MOLD discusses mold formation in heating and air conditioner air handlers and duct work. Also see Mold Growth in Air Handlers.
- Mold Odors, Musty Smells in buildings: this article summarizes the common sources and causes of moldy and musty odors in buildings.
- Mold Odors - MVOCs: If we smell mold, is mold present and is that a problem? Most people have a pretty good idea of moldy or musty smell as associated with mold. If you smell mold or find it at important levels in screening samples of air, dust, or vacuumed surfaces, (by quantity or by particle type in samples) it is probably there. Testing and ASTM Test Standards for MVOCs are also cited below.
- Mold Odors FAQs: Why do mold odors occur in our home following rain? Odors at exterior outlets sure sound as if there has been leakage into the wall and a probable mold colony. We need an expert visual inspection and possibly invasive sampling, combined with building history, to find and follow leak paths and high humidity cavities in order to inspect the most-likely mold reservoir targets in a building. The odors may be MVOC's which may be produced by some mold genera/species at varying levels as humidity, temperature, air pressure, and other variables change.
- Mold Information Website: This website provides information and procedures for finding, testing, cleaning and preventing indoor mold, toxic black mold, green mold, testing building indoor air quality, and other sick house / sick building investigations. Here are research articles, inspection and testing procedures, and contact information for expert services.
Electric motor odors - from electric motors used in the air handler: You'll want to decide what sort of odors a motor could possibly produce - burning materials like windings, insulation, lubricants, and confirm that you think that's what's going on.
- We recommend that you have a heating technician test the motor amperage draw - if a motor is overheating it's probably failing and drawing excessive current.
- Don't rule out other possible odor sources like a dead animal on the heat exchanger (mouse, bird), mold in the duct system, leaks of something into the duct system.
- Not finding CO does not mean no flue gases are present - CO should never be produced - it occurs when there is not enough combustion air. So combustion gases could be leaking into the duct above the heat exchanger, just no CO. So whoever tested only for CO was not fully informed on this topic. After checking out the motor, you might find someone with more experience to check out the heat exchanger for leaks.
OIL ODORS, LEAKY OIL TANK PIPING at the oil storage tank filler or vent piping can be diagnosed and repaired
Oil Tank Leaks and Smells are discussed at our website on handling above ground or buried heating oil storage tanks. These online articles answer most questions about above ground or buried oil storage tanks. Extensive free un-biased oil storage tank inspection and testing advice for property buyers and owners.

Depending on the combination of return air register location and the source of an oil piping or oil tank leak, oil smells may be drawn into a duct system and delivered elsewhere in the building.
Paint Failures & Odors: How to Diagnose, Correct, & Prevent Paint Failure on buildings. Paint odors: solvents and other chemicals in building paints or coatings are often a source of odor or paint smell complaints, even where low-VOC paints are in use.

Painted ducts that become warm may give off odors, and a return air register located close to a paint odor may also deliver paint odors elsewhere in the building.
Pesticide Odors in ductwork: Review the history of use of pesticides at the building, and make a thorough visual inspection of the duct work for holes or capped holes that may indicate that some idiot sprayed pesticide on or in the duct system or into the air handler. While pesticides that have been properly applied according to EPA and other standards, serious health risks could be present if pesticides were improperly applied.
Pet Odors may enter ductwork from a return air register located in an area occupied by pets: odors from dogs, cats, or other pets, source identification, testing, removal, are discussed in detail at ANIMAL ODORS IN buildings, with special focus on cat odors and allergens beginning at Cat Dander: how to inspect and test a building for past or current presence of cats, cat hair, cat dander, and cat allergens" . Also see Pet Allergens.
Plastic odors: Plastic Odors, including Siding Odors. This discussion also pertains to other vinyl or plastic materials used in buildings such as diagnosing odors from plastic trim, plastic or vinyl windows, window screens, doors, or similar materials. This article includes a plastic odor diagnosis checklist and it lists common sources of plastic-like smells in buildings.
Plumbing System Odors: problems with open sewer lines, plumbing vent systems, plumbing fixtures, plumbing drain traps, and septic systems can produce troublesome indoor or outdoor sewage smells that are sometimes dangerous or unhealthy. Here is our guide to tracking down and curing building odors due to plumbing drain, waste, vent, fixture, or septic systems: ODORS, SEPTIC or SEWER
Radon Gas U.S. EPA Radon level maps
Septic or Sewer Odors: Diagnosing and Curing Sewer Gas Smells and Septic Tank Odors. This series of detailed diagnostic procedure articles describes how to diagnose, find, and cure odors in buildings including septic or sewage or sewer gas smells or "gas odors" in buildings with a focus on homes with a private onsite septic tank but including tips for owners whose home is connected to a sewer system as well. A case of cast iron drain leaking sewer gas into a transite asbestos heating air duct is illustrated at CAST IRON DRAIN PIPING

Sewer gases may also appear in building duct work, either from a return air register drawing sewer gases from a nearby improperly-working drain (such as a drain that is not properly vented), or sewer gases may enter duct work from a leaky sewer line where both the sewer line and the duct work are located in the building floor slab. In general in-slab ducts are a recurrent problem, with mold in the duct work or air handler unit, water, dirt and contaminants from ductwork flooding, collapse, asbestos (transite ducts) and other problems.

A good solution when you face costly in-slab heating or air conditioning duct problems is to abandon the in-slab ducts entirely, routing new duct work in a ceiling or wall chase. In the less common case of both failing in-slab heating and cooling ducts and a furnace that needs to be replaced, some building owners take that opportunity to convert to forced hot water heating - it's less costly and less disruptive to route hot water heating piping through the building than to re-route the more space-consuming air ducts. Thanks to reader Conrad for discussion of finding sewer gas leaks into HVAC ducts located in the floor slab.

Sewer gases are more than an obnoxious odor. Because sewer gas contains methane gas (CH₄) there is a risk of an explosion hazard or even fatal asphyxiation. Sewer gases also probably contain hydrogen sulfide gas (H₂S) In addition some writers opine that there are possible health hazards from sewer gas exposure, such as a bacterial infection of the sinuses (which can occur due to any sinus irritation).
Septic or Sewer Odors: Wet Weather or Cold Weather Sewage or Septic Odors: Diagnosis and Repair Guide. This article discusses the diagnosis and correction of sewer gas or septic odors with focus on diagnosing odor sources and causes in cold weather. Some of the diagnostic steps pertain to all seasons.

Photo of tranite cement-asbestos material used for air ducts in a slab over a sewer pipe (C) Daniel Friedman and Conrad

Sewer Gas Odors from drains: Diagnosing Clogged Drains & Septic System Backups: Is it a blocked drain or the septic system? - A First Step for Homeowners.

This swer line repair article explains how to investigate slow or blocked drains and septic system backups to distinguish between a probable septic system failure versus a probable blocked building drain. When a building drain is clogged or slow, or when there is a septic system backup, it's important to determine where the problem lies, since the repair steps can be quite different and costs can vary widely.
Sewer Gas Odors in Heating or Cooling HVAC Ductwork: sewage gas may appear in HVAC ductwork, being picked up from a plumbing drain or waste line gas leak near a building air return, or sewer gas may be entering in-floor-slab ductwork from a nearby leaky sewer or septic drain line. More illustrations of this leaky sewer line that sent sewer gases into the building's heating duct system can be seen at TRANSITE PIPE AIR DUCTS and at CAST IRON DRAIN PIPING.

The photo (above-left) of a sewer line routed immediately below a transite asbestos in-slab floor heating duct was provided courtesy of reader Conrad. More about sewer gas odors, the common causes and remedies, can be found at SEWER GAS ODORS.
SMELL PATCH TEST to Track Down Odors - a simple procedure using paper towels and aluminum foil can help track down a building odor to a specific surface indoors.
SULPHUR & SEWER GAS SMELL SOURCES list of sources of sulphyr & sewer gas odors in buildings

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ODORS GASES SMELLS, DIAGNOSIS & CURE - see detailed links at page left; also see DUCT SYSTEM & DUCT DEFECTS

ASTM E2600 - 08 Standard Practice for Assessment of Vapor Intrusion into Structures on Property Involved in Real Estate Transactions is available from the ASTM at astm.org/Standards/E2600.htm .

"This practice is intended for use on a voluntary basis by parties who wish to conduct a VIA on a parcel of real estate, or more specifically conduct a screening evaluation to determine whether or not there is potential for a VIC, and if so, identify alternatives for further investigation."

The standard goes on to emphasize the uncertainty in testing any site for gases and vapor intrusion.
Thanks to reader Conrad for discussion of tracing duct odors to sewer gases leaking into nearby in-slab duct work, and the use of a duct scanning camera to inspect the condition of ductwork located in the building floor slab - January 2010.

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Complete List of Air Conditioning & Heat Pump Design, Inspection, Repair Books at the InspectAPedia Bookstore.

The Home Reference Book - the Encyclopedia of Homes, Carson Dunlop & Associates, Toronto, Ontario, 2010, $69.00 U.S., is available from Carson Dunlop. The Home Reference Book 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. InspectAPedia.com ^® author/editor Daniel Friedman is a contributing author. Field inspection worksheets are included at the back of the volume.

"Air Conditioning & Refrigeration I & II", BOCES Education, Warren Hilliard (instructor), Poughkeepsie, New York, May - July 1982, [classroom notes from air conditioning and refrigeration maintenance and repair course attended by the website author]
Fiberglass: Indoor Air Quality Investigations: Fiberglass in Indoor Air, HVAC ducts, and Building Insulation
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Building Air Duct and Air Handler Odor Guide: How to Find, Test, & Remove Odors, Odor Detection, Smells, & Gases that Appear in Ductwork

List of Sources of Duct System Odors / Smells

Questions & Answers regarding this article

Ask a Question or Search InspectAPedia

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Books & Articles on Building & Environmental Inspection, Testing, Diagnosis, & Repair