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How to Find & Remove Odors, Gases & Smells Heating and Air Conditioning Duct Work
- Air Duct and Air Handler Odor Guide: How to Find, Test, & Remove Odors, Odor Detection, Smells, & Gases that Appear in Ductwork
- What are the possible causes or sources 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
- HEATING SYSTEM ODORS - separate article
- ODORS GASES SMELLS, DIAGNOSIS & CURE - separate article
- OIL HEAT ODORS - separate article
- Questions & answers about the causes of & cures for HVAC duct system odors & smells
- References
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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. 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.
Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman.
Building Air Duct and Air Handler Odor Guide: How to Find, Test, & Remove Odors, Odor Detection, Smells, & Gases that Appear in Ductwork
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, PROCEDURE 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 HVAC Air 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 INSTRUMENTS 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 Odor Sources: 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:
- 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 (SLAB DUCTWORK) 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 (SLAB DUCTWORK) 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.
- 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 sewer 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.
- SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs
- 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 sulphur & sewer gas odors in buildings
Frequently Asked Questions (FAQs) About Odors in or From Heating & Air Conditioning Air Duct Systems
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Question:
foul odor coming from in-slab HVAC Ducts - how do we fill in the ducts
We have had our furnace duct work moved to the attic to eliminate the foul odor coming from the in slab duct work. It is still coming in to the house and we need to fill all the duct work with something that will seal off the system. Can we have all of the ducts filled solid with spray insulation foam to correct this. The odor is making my wife and I Ill. She is allergic to mold & mildew also! Please advise! Thank You Tom - 3/12/2012
Reply:
Tom, SLAB DUCTWORK - catalogs the functional and environmental problems found when HVAC air ducts are routed in or below floor slabs IN my experience, if we seal the air supply and return registers for the in-slab duct system we don't expect to find odors coming from the remaining ductwork. But if you are sure that you need a more thorough fill-in, I would consider pouring concrete in the entire duct system - that material will fill the in-floor ducts completely, eliminating any concern for stagnant water, rodents, etc.
(By the way, there is no mildew in buildings - mildew only grows on living plants. If you smell "mildew" inside a building, it's some other genera/species of mold.)
Question: puff of smoke with A/C on is drawn into ductwork
A/C on, doors closed, in the desert. Light, puff, smoke. For 4 years. It Draws into ductwork, & exchanger, paint,& clean all you want. When it gets warm, & the A/c goes on, the house will stink of smoke. - DD 8/8/2012
Reply:
DD:
Watch out: an air conditioner has absolutely no business emanating a puff of smoke during any part of its operating cycle. This sounds dangerous. You need a service call by an expert.
Question:
I have a forced air oil furnace. It began putting strong fumes into the house just before and during shut down of a cycle.
There was no c02 detected in the home. I had multiple HVAC companies inspect the furnace, the problem continued. To the point that windows had to be left open, the family was suffering respiratory distress.
I had the furnace replaced. I had the chimneys cleaned. I had the vents cleaned. The problem continues. The HVAC company is stumped, they say there would be co2 present, yet they confirm they smell odor. To me, it's the same as car exhaust.
The chimney cleaning company said if I continue to have problems, they can install an insert into the chimney. ? The HVAC company wants to install an electronic whole house air filter and if that does not resolve the issue, they will put in an electric furnace and heat pump.
This is becoming a very expensive and I'm afraid health risk issue. Two furnaces with the same issue?
Thanks for any advice. - D.M., Chardon OH
Reply:
A competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with a heating system - it sounds from your description as if perhaps your onsite people lacked that expertise or perhaps did not accurately understand your heating system odor complaint.
That said, here are some things to consider:
- Confirm that the odor is from or related to your heating system. For example, confirm that the odor only occurs when the system is or has just been in operation (it sounds as if you have already done this)
- Is it the furnace? Considering that as the odor problem has continued after a complete furnace replacement, one would speculate that the problem is not due to improper furnace operation, but in fact improper operation could still be the problem if the new installation were faulty, or if one of the chimney, venting, duct, or combustion air issues we outline here is discovered.
- About CO2 and heating systems vs CO: I am guessing that you are mistyping, and that the heating company tested the building for CO (carbon monoxide - dangerous, potentially fatal) not CO2 (carbon dioxide) which is always present in air, outdoors and inside. If I am correct and the test was for CO, then the heating company is mistaken in that CO is NOT necessarily detected in building air if odors are coming from a heating system. Especially with oil fired equipment and with properly adjusted oil fired equipment, CO levels may be below limits of detection; yet flue gases will still smell of oil fumes and combustion products.
Watch out: for safety, be sure that your home has working and properly located smoke detectors as well as carbon monoxide detectors. See CARBON MONOXIDE - CO. Incidentally, CO itself is odorless. Also see COMBUSTION GASES & PARTICLE HAZARDS.
- The best approach to tracking down this odor is to pinpoint the time (as you may have done), equipment operating conditions, and by making a careful inspection of the entire heating system (not just the furnace itself) track down the exact sources of odors; for example if odor of oil burner combustion is delivered out of air supply registers, I'd expect either a hole in a heat exchanger, faulty equipment installation, or quite possibly, a common and significant supply or return air duct design error, such as placement of a cold air return intake close to the oil fired equipment itself.
See HEATING SYSTEM ODORS and also OIL HEAT ODORS for more suggestion about tracking down heating system-related odors
Watch out: an air intake or return inlet that can draw oil burner gases and fumes into the duct system is unsafe and can also cause improper heating system operation even if CO is not immediately detected.
- Do NOT try to solve this problem by installing an electronic whole house air filter - that is treating the symptom, not the cause, and risks leaving a dangerous condition int he home.
- Look for these other possible heating system problems that could be delivering odors to the building:
- a heating air supply or return duct installation error such as the return duct near furnace oil burner I mentioned above
- a chimney and venting or draft error or blockage or improperly operating draft inducer or vent damper. Before installing a chimney liner, have the chimney inspected by an expert (certified chimney sweep for example) to diagnose any leaks, construction or draft problems. I don't understand why the chimney company you consulted would install a chimney liner unless they could also explain to you what problems exist in the present chimney. See CHIMNEY INSPECTION DIAGNOSIS REPAIR
- a combustion air supply defect such as inadequate combustion air when a furnace room door is shut. See BACKDRAFTING HEATING EQUIPMENT
- an oil burner operation defect such as improper oil burner shut-down that could be leading to a puffback and that might be belching oil burner fumes into the furnace area at system startup or shutdown, where they are picked up in an improperly located return air inlet
- See our other Heating System Odors diagnostic suggestions above in this article.
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Technical Reviewers & References
Related Topics, found near the top of this page suggest articles closely related to this one.
Click to Show or Hide Citations & References
- 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 (SLAB DUCTWORK) , and the use of a duct scanning camera to inspect the condition of ductwork located in the building floor slab - January 2010.
- "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]
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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