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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

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.

InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

- Daniel Friedman, Publisher/Editor/Author - See WHO ARE WE?

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.

Flooded hvac ducts (C) InspectApedia.com

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

Given in thereader Q&A below.

...


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Reader Comments, Questions & Answers About The Article Above

Below you will find questions and answers previously posted on this page at its page bottom reader comment box.

Reader Q&A - also see RECOMMENDED ARTICLES & FAQs

Reader question: Does mold or mildew smell like extreme dust?

Well, I've read every article here related to duct smells. I've looked at every article, it feels like, on the internet about duct smells too. All of the obvious suggestions (cleaned ducts, look for leaks, clean furnace (I have no air conditioning), etc.) have been considered/done.

This 1921 bungalow has always smelled like "old wood", but ever since we cleaned the ducts, now the smell is unbearable. We naturally thought that once you clean ducts, the air quality would improve...not get worse. The smell is not like those listed in articles. It's just an extreme dust smell. Enough to bother my throat.

I wonder if it's mold. But we cannot see any signs of it inside or in the area. Does mold or mildew smell like extreme dust? We shut the whole thing down a few weeks ago now while cleaning, so no air flow. It's been hot and humid. Is it possible that mold grew in that short time period? Or can ducts smell worse after cleaning? If so, why? It's maddening. Thanks. 2024-09-20 by Ann

(This lengthy Q&A discussion was originally posted on the HVAC SYSTEM ODOR FAQs page)

Reply by Moderator: Mold has a pretty distinctive odor

@Ann,

Well certainly we're not going to give up. I take it you've already read the diagnostic suggestions at

DUCT & AIR HANDLER ODORS (where we will move this entire long discussion with your followup questions and our responses)
and
HVAC SYSTEM ODORS - topic home

as you're commenting on a FAQs page on those topics rather than the topic home pages. (That's fine I just want to avoid repeating what you've already read).

You're right that I haven't discussed "old wood" smell sources and I'm not sure what to make of it. You'll probably agree that not only do individual's odor sensitivity vary but so does their description.

Mold (people say "mildew" but there's no mildew in buildings - mildew grows only on living plants) has a pretty distinctive odor that most people agree smells musty or moldy. If you smell "mold" there is mold somewhere. Yes there could be mold contamination that we don't smell since depending on the mold genera/species, location, surface on which it's growing, and building moisture and temperature, its offgassing of mvocs (which are what we smell) varies alot.

Here are a few suggestions, and depending on what you find we'll have more to say:

1. You want to be sure that your inspection of the ductwork and air handling system is complete: the full length of the duct interiors from air handler to air supply registers and from air return(s) back to the air handler as well as inside the air handler itself, its return and supply plenums, filter, and the cleanliness of the blower fan blades and assembly too.

2. Look around each of the air return inlets in your home, since the return air system can pick up a smell from one place and transport it to elsewhere in the same building. At each air return (if there's more than one) is there a dust source, damaged wood, insect damage, rot or water or leak damage? Any spills of chemicals or other smelly substances?

3. Pin down what you've already suggested: when was this smell first noticed? Before the ducts were cleaned? Stronger after cleaning? What was disturbed during cleaning.

4. Identify exactly what "cleaner" products were used inside your ductwork: product brand and ID, then we can together look at its description and also perhaps determine if it was properly applied.

In nearly 5 decades of investigating building smell sources too often I've found that someone sprayed a magic "cleaner" or "neutralizer" or "sanitizer" over a problem that instead should have been cleaned and its source corrected or removed. (One applicator sprayed the supply registers and ducts in a home and missed a dead animal in the air handler's return plenum.)

5. What kind of ducts are installed: metal, flex duct? fiberglass lined ductwork? That makes a significant difference in just how and even whether ductwork can be cleaned. How old is the ductwork? Same as the building? Have there ever been leaks into the ductwork, such as a plumbing leak that dripped onto the ducts or a building water entry problem that wet the air handler?

Let me know what you think and we'll keep at this until the problem is solved.

Daniel

Reader follow-up:

@InspectApedia Publisher, First, thank you so much for responding! No one else has on other forums. Your comments:

1. Inspection of the ductwork and air handling system - we haven't gotten at a couple of locations but will this weekend. We've looked at the air handler, its return and supply plenums, filter, and blower fan blades/assembly too.

2. Air return inlets - We don't see anything at each air return. No dust source, damaged wood, insect damage, rot or water or leak damage, spills of chemicals or other smelly substances. Nothing of the sort.

3. Smell - unfortunately....the house has smelled "old" for 15 years. Smells like an antique store. Now, I will admit that we aren't the cleanest people as far as dusting. So, we assumed it was dust because it always smells better after we dust. I thought after we cleaned the ducts, it would naturally smell better. Well, that antique store smell got 10 times worse. So yes, stronger after cleaning. And these ducts...oh boy, they had so much junk in them.

4. Cleaners - my husband did them and used nothing. He just vacuumed them out.

5. Kind of ducts - the house is from the 20s, so it's a hodge podge, but mostly wood and metal. We found some asbestos tape here and there (duct boot/supply registers) and it will be removed Tuesday professionally. Leaks - we've been in the house since 2009 and haven't known of leaks.

At this point, we are removing one 12-foot duct w/ 1x8 pine sides and sheet metal tops/bottoms that we think has mold. Also removing bottom sheet metal on two panning returns with 2x6 fir floor joists, tops of subfloor planks, and sheet metal bottoms.

How might you approach mitigating possible mold on the 2x8 floor joists/subflooring? Once mitigating, we're having the cold air returns replaced with metal.

Next concern - vertical supply ducts leading to the 1st/2nd floor. We're hoping the supply ducts are metal ducts. Our fear is that they're empty stud wall cavities leading to a register boot. In your experience, would the vertical ducts in a 1920 bungalow be metal or uninsulated wall cavities?

If wall cavities, what is the best prevention method for dealing with mold in those cavities? Is there a non-toxic fogging or the like? If the vertical ducts are metal and by chance contain asbestos tape on the inside, what could our next move be? Can those ducts be lined or new ducts inserted inside old ones, or would that cause more mold?

I'm terrified that we are breathing wall cavity air...asbestos and lord knows what. Again, thank you again so much for responding. We're freaking out a bit.

Also, wondering what you think about mold bombs in the attic? Do people use them as preventative maintenance and are they toxic? The insulation is 7 years old now.

Moderator reply: a mold bomb won't help and could make it worse

@Ann,

I can say right from the start: don't even think about a mold bomb in your attic. It wouldn't help and could make problems worse. If there is mold contamination we need to find and remove the mold and fix its source. Simply trying to kill it can leave harmful or toxic particles in the building.

Other comments:
It was more common to use space between joists or studs covered with sheet metal as a return duct but occasionally we see that as supply ducts as well. That means of course if anything that's leaked into the wall including from the exterior or pests in the wall or in some houses even a dead animal in the wall can cause a smell.

It's also the case that the surrounding materials such as drywall or carpet or other porous materials can absorb odors and feed them back to you.

The most diagnostic detail we have so far is that the smell got much worse after the ducts were cleaned.

If I understand you're correctly you didn't hire a duct cleaning company but your husband tried vacuuming the ducts.

That would certainly stir up whatever was in them and furthermore unless he was using a
HEPA rated vacuum cleaner he also blew into the air fine dust particles during that cleaning process, which would also increase smells.

If any of that duct work was fiberglass lined, just vacuuming it can also damage the fiberglass.

So we need to know what your ducts are made of and what's inside those spaces.

It's also the case that you've only been in the house for a tiny portion of its total life, so the fact that you haven't observed leaks doesn't mean that there weren't any. Frankly would be unusual for a house built in the 1920s to have never had leaks anywhere over the ensuing 100 years.

So if you haven't done so, hiring an experienced home inspector or building inspector who could point out to you evidence of where there have been leaks or other damage would be useful.

It would also help you set priorities of repair and tell you when and how to spend your money on repairing or improving the home.

Very important in all of this is to not let yourself panic as in my opinion the anxiety itself maybe a worse health hazard than the things that you're worried about.

A combination of damp wiping and HEPA vacuuming of building surfaces will reduce the indoor dust levels and that may help at least reduce your anxiety if not the smells.

At SMELL PATCH TEST,

you will see a simple method you can use to test various building surfaces to see if those are odor sources. It's very inexpensive and easy.

Sometimes, in an older home when we think the smell has been absorbed into surfaces, we clean those and then paint them with a sealant paint of the type often used after building fire or following mold remediation.

You can use the same procedure if you actually find mold growth on building surfaces.

Clean the surfaces, seal them with any fungicidal or post fire sealant paint, and make sure that there's no continuing cause such as a moisture trap or building leak.

Reader follow-up: what is the best mold treatment for the subfloor/joists after pulling cold air returns out?

@InspectApedia Publisher, Again, thank you for your thoughtful reply. We are so grateful. Your comments,

1. "It was more common to use space between joists or studs covered with sheet metal as a return duct but occasionally we see that as supply ducts as well." - does that mean that it's most likely that our supply ducts are unlined with sheet metal or something else? And if so, how on earth do we clean them? And is it sucking in everything bad into the house?? Seems we should try to line them or insert micro-ducts? Surely, we're not talking about ripping into the walls? Trying not to panic as you say, but yikes.

2. "...surrounding materials such as drywall or carpet or other porous materials can absorb odors and feed them back to you." - no drywall or carpet or the like in the house.

3. " Your husband tried vacuuming the ducts." - yes, and we've cleaned it off for 2 weeks now as some came back into the house even with precautions thinking that was the smell. Then I realized it was coming directly from the ducts.

4. "If any of that duct work was fiberglass lined, just vacuuming it can also damage the fiberglass." - no fiberglass on the parts he cleaned in the cold air returns. We are replacing the duct work in the basement. There are 3 nine-foot-long supply lines leading to registers upstairs that we have yet to inspect. We are hoping those are lined with metal. If not, do you think those could be lined? All 3 run through 2x4 stud wall. We hope to drop a camera through vertical supply lines after asbestos abatement at the boots.

5. "You've only been in the house for a tiny portion of its total life, so the fact that you haven't observed leaks doesn't mean that there weren't any." - this is what I've been thinking about. Once we remove the 12-foot duct w/ 1x8 pine sides and sheet metal tops/bottoms, hopefully we can investigate old leaks. Hiring an experienced home/building inspector might be a good idea. I'll ask around.

6. "A combination of damp wiping/HEPA vacuuming of building surfaces..." - we've been doing the entire house and that's how we realized where the smell was directly coming from - the cold air returns.

Comment on trying to figure out moisture - our hot water HPVC is covered in insulation. They are beneath the existing cold air returns. No water entering that way.

Gonna check for condensation/moisture on the duct in the basement or an old plumbing vent that we think ties in with the roof vent. Should there be a cap on that on the roof? None of the houses have that. We'll know more about if it intersects with the returns when we get in there. We don't think it does. We'll know more after this weekend after removal of return ducts/panning.

Question - After we remove the old-style panning, what is the best treatment for the subfloor/joists after pulling cold air returns out? White vinegar? That seems to work so well for all mold we've treated in other capacities. Thank you! It's like we have an older uncle helping us.

Moderator reply: any ordinary household cleaner is fine for cleaning mold from a hard material surface such as wood

@Ann,

Using any ordinary household cleaner is fine for cleaning mold from a hard material surface such as wood.

If you're actually seeing mold on the studs or similar surfaces see ACTION GUIDE - WHAT TO DO ABOUT INDOOR MOLD

at MOLD KILLING GUIDE we explain that even dead spores may be toxic so proper treatment means cleaning - removing mold from surfaces - not just spraying with a disinfectant.

MOLD ODORS, MUSTY SMELLS may also be helpful as there we discuss tracking down moldy or musty odors and addressing their cause

About your ductwork, materials, and smell sources I want to be helpful but at this point making more suggestions when I haven't seen your home nor even photos (you can post one per comment, use more comments for more photos) would be so speculative as to risk being misleading or useless.

Daniel

 

Question: possible A/C drip pan mold is clue to duct mold odor source?

Black marks on plastic A/C condensate drip tray don't look like mold (C) Inspectapedia.com capelliIn an effort to determine the source of an unpleasant odor that appears to originate somewhere in a central location of our HVAC system, the HVAC company we used recently replaced the coils (which were only a year old). When they removed the prior coils my husband took photos, one of which is attached.

It's the edge of the drip pan that faced the back so we were unable to see it prior to removal of the coils. The pattern appears mold-like, but perhaps it's from water staining and dirt (thought that would become a haven for mold growth, I suppose).

The thing that's wierd is that even after they replaced the coils, the odor returned according to prior patterns which is that it only emerges if we've used the AC, then stopped using it for a day or so (as happens during seasonal transitions) and then started using it again. It's as if water stagnates somewhere and grows something (mold? bacteria? algae? etc)

The odor disappears if we clean the coils and most recently, cleaning the condensate line also seemed to help.

You've suggested having the ducts inspected, but so far we've been unable to find a company that does this. The best we found so far is a company that has a camera on a 6' length, so they'd need to cut holes in our ducts every 6 feet to be able to inspect all the ducts and at this point, since the odor comes out of all the ducts, we're leaning away from this approach to diagnose the problem.

The HVAC company is throwing up their hands claiming they just fix stuff that's not working.

In any case, if you could look at the photo and let me know if you think that's looks like mold. Our IAQ consultant said she can't diagnose it from a photo, while noting the pattern is mold-like. - Anonymous by private email 2021/06/18

Moderator reply:

That black pattern looks a bit odd; not quite how I expect to see mold growing, but sorry, I can't rule it out completely - take a look at

MOLD APPEARANCE - WHAT MOLD LOOKS LIKE

and also at our library of what mold looks like on various building surfaces including plastic at

MOLD GROWTH on SURFACES, PHOTOS catalog of images of mold growth on various materials & surfaces - https://inspectapedia.com/mold/Mold-on-Building-Surfaces.php

where you will find in that alphabetical index these examples

MOLD on/in AIR DUCTS in HVAC systems

MOLD in AIR HANDLERS of air conditioners and heating furnaces

and further in the alphabetical index you'll find htis link to

MOLD on PLASTIC SURFACES

About your condensate drip pan black marks or mold

When the pan was removed, could you see into the pan and could you see other sides of the pan? If this pattern was uniform all over the pan it could be an artifact of its manufacture.

If this is mold you'd be able to wipe it off - or most of it - with a paper towel dampened with glass cleaner or any household liquid cleaner.

In any event even if that was mold it would not explain a moldy odor complaint in a building; (though if it were mold it might be a symptom of mold contamination in the HVAC system, including air hander, duct work, filters, etc.)

Why? It's simply not a sufficient reservoir of mold.

It always makes sense to start diagnosing an HVAC system odor problem with a combination of visual inspection of all accessible areas and a review of the history of the system and of the building where it is installed, looking for visible mold or for water - follow the water path. As you'll read in our article series on duct odor and mold odor diagnosis and cure, duct systems can also pick up odors in the return ducts and transport them throughout the building.

So don't rely on a too-narrow scope of inspection.

Set priorities for areas of invasive inspection of ductwork for mold odor sources

When invasive inspection of ductwork is difficult and costly, it makes sense to start by inspecting the most-suspect areas first.

Those would be any spot where there is greater risk of water inside the duct system:

- sections of duct at which an external inspection shows stains indicating leakage either outside that leaked onto the duct system (and maybe into it) or leak stains at duct sides and bottoms indicating that water was in the tion f
ducts and leaked out

- sections of duct closest to common water sources such as just downstream from the air handler and cooling coil - and in the air handler itself, including its blower assembly and filters

As it will be helpful to other readers, and as its publication may prompt other readers to offer a helpful suggestion, I will include a redacted (to respect your privacy) version of this discussion [above on this page].

Do take a look, and do let me know how you progress on this irritating problem.

 

On 2020-08-10 by Lynne

Help, help, help (please). In May, I had to replace the compressor and air handler for my 20 year old high velocity AC system. They installed a variable stage Bosch and Unico air handler. The ductwork remained the same.

It's been a nightmare. I am getting a sour, acrid, vinegar like smell coming from the system. It's a heavy odor, it settles on everything and permeates bedding, clothing, paper products, towels. It seems to be worse when the system first comes on, and when the variable compressor is running at a lower speed, and not quite as noticeable as when it's running full power.

But as it cycles off and on I am getting full blasts of this horrible smell. The contractor came out and cleaned the coils and checked the drainage, and everything looked fine, but the smell is still there.

There is also an IWave air cleaner in place. We tested removing it, it's not helping or hurting. Would a duct cleaning help? Can you clean the small high velocity ducts? I would be so appreciate of any info. Thank you.

On 2020-08-10 by (mod) - sour, acrid, vinegar like smell coming from the A/C system

Lynne:

An "air cleaner" can never remove an odor in a building if the odor source itself is not found and removed or cleaned-up.

Check the duct odor sources listed above on this page, including attention to spills that may have got into an air handler or into the duct system and to odors that might be picked up at the air return registers.

See also

HIGH VELOCITY HVAC DUCTS


On 2020-08-02 by Ronald K Williams

Question: Help! I had fiberglass ductboard recently installed in my crawl space (entire supply ducting, not return). Since being installed I have been smelling a strange chemical odor. It doesnt smell like mold. I called in a second HVAC company. They looked at the system but could not really inspect the return (they declined to cut a hole to look inside). They also declined to pull a flex duct to inspect that.

The second company looked in the air handler but the return plenum had been replaced so they said that was clean. I have been smelling the odor for about a month. The second company said use a Nu Calgon, Clen Aire product. Is this normal?

On 2020-08-02 by (mod)

Ronald,

With no clue as to the nature of the odor it's hard to guess at a possible source. Check both the examples of odor sources listed above on this page and especially chck for odors picked up at the return air inlets.

You may need a camera inspection of the duct system.


On 2020-02-23 by Ben W.

I woke up to a strong acrid odor from the vent in our bedroom -- the odor you get when you turn on the heat the first time in the winter, only a bit stronger, and we had been running the heat for several months. The odor seemed to come only from the one register. I checked the others in the house and detected nothing. The odor dissipated but didn't totally disappear in a few minutes. Could it simply have been dust in that one vent? Thanks!

On 2020-02-23 - by (mod) -

Ben

I'd take a close look at the "heating vent" in your bedroom - I infer from your phrasing that this is a forced hot air system. Actually inspect the duct interior and if you see nothing obvious like a spill into the duct, accumulated dirt and debris, or a dead animal, then you might ask for a duct inspection by a company who can send a camera through the ducts to inspect sections whose exterior is not readily accessible.

On 2020-02-24 by Ben W.

I appreciate it. Somehow a couple of tissues were at least over this particular vent, so I wonder if some got pulled into it. I cleared it away and it's been fine since -- no odor at all -- but I may take up your advice about having the interior inspected. Thank you for the response.


On 2019-12-24 by Scarlett

Just moved into a 2-level house with basement. 3 vents stink when the furnace is off. But when the hot air blow, the smell disappears. Metal ducts, no crawl space. The vent that Stinks the most is on 2nd floor, another stinky vent is basement, same location at a different level. The last one is also on 2nd floor, next door to the first one. Had a furnace guy here today, he didn’t think it smelled like a dead animal in ductwork, otherwise it would be stronger.

And he didn’t think it’s a dead animal on crawl space, cause there’s no crawl space. I suspected it smelled like sewage, but the guy disagreed. Called several duct cleaning companies but they don’t sounds confident to solve the problem. What should I do? The smell is like radish turning bad and a little sweet.

On 2019-12-24 - by (mod) -

If you don't think that the smell is coming from a contaminant in the ducts themselves then you need to look for an older Source, such as a tan are returned or near one. If you think the odor source is inside the ductwork that it would make sense to have the ducts cleaned and sanitized.


On 2019-12-11 by Paul Delger

The smell is a fine dust that just goes through the air when the furnace is on. The duct guy said everything is clean. He put a filter outside one supply duct and it looks clean too. I have spent much money chasing this problem. The duct guy wants to seal the system--furnace but don't think my wife will want me to spend more money. Extremely frustrating. I put a air purifier inside the furnace by the blower fan and that's not helping either. Do you have any other suggestions. Could carbon monoxide becoming in play here. How can I actually find the irriating particulars. Thank you so much.

On 2019-12-11 - by (mod) -

Paul

CO (carbon monoxide) itself is odorless and colorless. However CO leaked from a heating system will usually be mixed with other flue gases and might have an odor - depending on the fuel type being burned - of gases or heating oil.

The difficulty in "finding the irritating particles" is that in my opinion we do not want to hire an expensive consultant who stops by to collect some sort of generic sample, send that to a lab, and have the report sent to you. Such shooting in the dark is costly and rarely hits a useful target.

However you could try collecting particles from the surface of one of your air filters, using clear adhesive tape, and having that analyzed by a forensic lab to see if there are abnormal levels of any particular particle type.

See details at

DUST SAMPLING PROCEDURE https://inspectapedia.com/sickhouse/Dust_Sampling_Guide.php

and

TAPE & BULK SAMPLING & TESTS for MOLD or DUST at https://inspectapedia.com/mold/Mold_Test_Adhesive_Tape.php


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

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: 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

Reply: tips for tracking down odors blamed on heating or air conditioning systems

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.

    Also see COMBUSTION GASES & PARTICLE HAZARDS.
  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.

    See HEATING SYSTEM ODORS and

    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.

      See CHIMNEY INSPECTION DIAGNOSIS REPAIR
    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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