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

  • DUCT & AIR HANDLER ODORS - CONTENTS: How to Find, Test, & Remove or Cure Odors Smells, & Gases that Appear in Ductwork. How do we find and cure smelly heating or cooling ducts or air handlers? What are the causes or sources of odors in air ducts? What are the sources of odors in or around heating or air conditioning equipment. How are pet odors transported in air ducts or ventilation systems? Examples of how to track down and cure odors in HVAC ducts.
  • POST a QUESTION or READ FAQs about the causes of & cures for HVAC duct system odors & smells

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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, dead animals in the ducts or air handler, or even a bad blower motor that is overheating.

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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

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.

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.

Checklist 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.

  • ANIMNAL 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.

    You won't smell Carbon Monoxide: 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. CO itself is odorless.

    You may smell heating combustion flue gases: 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 & MOLDY or MUSTY or "MILDEW" 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.

    • MOLD or MILDEW or 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 at our Reviewers List at the end of this article.
    • MOLD ODOR 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 & 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 or URINE ODOR SOURCE DETECTION
  • 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 HAZARD TESTS & MITIGATION provides information about radon gas that can accumulate to unsafe levels in buildings. However keep in mind that radon gas itself is odorless.
  • SEPTIC or SEWER GAS SMELLS: 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,

    collapsed, 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 SEWAGE ODORS in WET WEATHER: 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 at 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 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


    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 FIND ODOR SOURCE - 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

HVAC Duct Odor Diagnosis & Cure: What to Do Next

  1. If the nature of the odor didn't suggest to you to follow one of the common odor problem pathways listed above, then see our odor diagnosis home page: ODORS GASES SMELLS, DIAGNOSIS & CURE .
  2. If the duct odor seems to smell like heating fuel, see OIL HEAT ODORS & NOISES, as well as OIL BURNER NOISE SMOKE ODORS.
  3. If the duct odor seems to smell like sewer or rotten egg smells transported through ductwork see SEWER SEPTIC ODORS in HVAC DUCTS.
  4. 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 FIND ODOR SOURCE by testing different building surfaces & materials.
  5. For examples of steps followed by readers in trying to track down smells ascribed to the building air duct system see the EXAMPLES of TRACKING DOWN DUCT ODOR just below

Examples of Tracking Down Odors Ascribed to HVAC Ducts

Question: HVAC system odors when changing from cooling to heating mode - "Evergreen Smell"

I have another puzzle for you. Back in August 2013, we moved into this house. It is a 2-story house built in 1983. It has no basement and no crawl space. The day before we moved in, we had the air ducts cleaned and sanitized. We put the AC on and all was good for 2 months. The cooler temps came in October and we no longer needed the AC on. A few days went by with no AC or heat on because the house temp was good.

However, an evergreen smell started to enter into the ductwork. Yikes. I had handymen out, HVAC guys out, had my husband clean the evaporator coil, etc. We spent way to much time & money and were getting nowhere. Low & behold, the heat kicked in then and the evergreen odor was gone for several months. Unfortunately, the evergreen odor is back now that AC season is here. My husband cleaned the evaporator coil again. The odor is still there.

I checked with the previous owners, and they never used pan tablets which may explain the evergreen odor being in the ductwork and coil housing. I spoke to several HVAC companies, too. Honestly, it will be a total waste of money to have HVAC guys out because we have been there and done that. I need a different spin on this and how to fix it. The newest twist in this happened a few hours ago. I went to open the frig and smelled the evergreen odor in there. I thought I was having a nightmare.

Any helpful advice you can provide Daniel would be greatly appreciated.


I think I know what is causing the musty evergreen odor in our air ducts & coil. I think it is a slow freon leak. Call me crazy, but it is the only thing that makes sense right now to me. Go figure that the outside temp dropped from 75 degrees to 30 degrees. I need a warm outside temp in order to have an HVAC service come out to do the dye test for a possible freon leak. I just wish I could do something now to patch up the leak, so that the fumes will stop entering our house. The fumes are not serving our immune systems well around here. - S.N. 4/14/2014

Reply: refrigerants are generally odorless: look for odor source & odor transport

It may be obvious but worth saying, one would tackle the question by asking what's different between the two HVAC modes, then look closely at those details. The air is moving from and to the same places via the same fan and in the same ductwork, right? It's only heat vs cooling, temperatures, moistures that may be different - or something else different that's less obvious.

'Evergreen odors" sounds (smells) to me like an air freshener product or possibly a cleaning product someone has used somewhere. Refigerant gases are themselves odorless.

Reader follow-up:

Thank you for your feedback. From all the research and feedback I am getting, it looks like freon travels outside during the winter. When it warms up, it travels inside the house. That would explain why the evergreen odor suddenly reappeared last week when the temp rose to 70 degrees. I just have to tackle finding the leak now and go from there. Not giving up.

Reply: refrigerant gases are odorless

I may be missing something but refigerant, existing in either a gas or liquid state, remains inside an enclosed HVACR system of pipes, valves, controls, and a receiver and a compressor. It doesn't come indoors and outdoors, it doesn't leak out of a system in normal operation, and it is odorless and colorless.

Reader follow-up:

I agree Daniel. My head is about to explode from all the information that is hitting me left, right & center.


Yesterday, we had a video camera inspection done to see all the ductwork in the house. It was recommended we do this because concrete slab homes like ours tend to get water in their in-ground ductwork. As a result, odors are caused. The inspection proved there is no water. Thank goodness. However, we still need to find the source of the evergreen odor. When the AC was put on, the technician was very concerned because the coil was staying warm. Most likely the refrigerant (freon) is leaking out somewhere. The HVAC service that installed the furnace & AC back in 2009 is coming in a few hours to check the charge on the AC. If the charge test is not good, the next step will be the sniffer and/or dye test to locate the leak.

Reply: Notorious problems with ducts in slabs

Freon (or other HVAC refrigerants) does not smell. It is odorless.

Re-charging a refrigerant gas is only part of a proper repair. The leak needs to be found and fixed.

Ducts in slabs are a notorious source of contaminants: moisture, leaks, rodents, mold; Perhaps someone sprayed a sanitizer or odorant in the ductwork.

See SLAB DUCTWORK for an explanation of the problems commonly found with HVAC air ducts placed in or below concrete floor slabs. For example, anything sprayed into or even leaking into the in-slab ducts could be a source of odors later detected in the building.

Reader follow-up:

The HVAC company that installed the furnace & AC in this house back in 2009, came out. We had to wait for at least 60 degree weather for them to check the charge on the AC and do the sniffer test. As I expected, the refrigerant was bone dry. Also, the leak is in the coil. That being the case, the entire coil is being replaced. We are still under warranty thank goodness for parts & labor on the furnace & AC.

Reply: look for cleaners, deodorants, and compressor oil leaks

The relationship between odor and no refrigerant is a tenuous one at best. Perhaps that the system was not cooling and not dehumidifying is a factor in odor development (e.g. mold in a damp area) or odor transmission. In short, you still need to track down the "evergreen" odor source.

Have you determined if a deodorant or sanitizer or cleaner was sprayed or used in the HVAC system or in a building area where such odorants might be picked up by the HVAC air handling system?

Sheila it occurs to me that your installers, in finding the refrigerant leak, should also look for a compressor oil spill or leak anywhere in the system. While refrigerant gases are inert, odorless, colourless, lubricants within the system might have a smell - though not one I'd describe as "evergreen". It remains that an evergreen smell seems more likely to trace to a cleaner or deodorant.

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


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



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: oil fired furnace pulls smoke & fumes into house during burner shut-down

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


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:

  1. 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)
  2. 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.
  3. 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.

  4. 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.


    also OIL HEAT ODORS & NOISES 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.
  5. 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.
  6. Look for these other possible heating system problems that could be delivering odors to the building:
    1. a heating air supply or return duct installation error such as the return duct near furnace oil burner I mentioned above
    2. 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.
    3. a combustion air supply defect such as inadequate combustion air when a furnace room door is shut. See BACKDRAFTING HEATING EQUIPMENT
    4. 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
    5. See our other Heating System Odors diagnostic suggestions above in this article.


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