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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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.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

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.

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

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.


Continue reading at DUCT & AIR HANDLER ODOR FAQs or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see SLAB DUCTWORK to track down and fix leaks, odors, mold, health hazards from in-slab ductwork



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ODORS in AIR HANDLERS & DUCT WORK at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


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