Heating & air conditioning system odor cause & cure questions & answers.

This article series describes & lists sources of smells that are traced to HVAC systems such as air conditioners, heating boilers, furnaces, duct systems, heat pumps, and water heaters.

HVAC Equipment Odor FAQs

These questions and answers about heating or air conditioning system (HVAC system) odors, their cause, prevention, or cure were posted originally

at HVAC SYSTEM ODORS - be sure to see that list of diagnostic procedures.

Watch out: 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.

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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

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 our entire long discussion and Q&A has been moved)
and
HVAC SYSTEM ODORS - topic home

(please see the full discussion of this Q&A now found at DUCT & AIR HANDLER ODORS)

On 2019-10-22 by Anonymous

Posted this in the “Odor” section, but posting here, as well, should it be the better location….

I have an odor coming from my HVAC. Here’s the history. Not sure if some things are relevant or not.

End of September: Drain had backed up in my unfinished basement: puddle about 3’ in diameter. Likely occurred a couple of hrs prior to finding it. Vacuumed the black water with a shop vac, called a plumber, cleaned the area w/Clorox and subsequently w/food grade hydrogen peroxide. Was careful to get under the HVAC and water heater as water had reached underneath both.

After realizing I’d failed to clean the inside the drain lines, cleaned those w/vinegar using a small scrub brush inside the line a couple of days ago. (Will clean again w/hydrogen peroxide which should arrive in the mail any day now.)

Early October: Routine pest control inspection that included exclusion for mice since a few droppings were seen in my attic. Pest control saw no sign of infestation or mouse taking up residence, but one just checking things out.

I wanted a complete exclusion done even though they said it was overkill given the evidence. Part of the exclusion involved setting glue traps around the basement to monitor for a month.

After the pest control techs left I turned my heat on for the first time all season. Because it was extremely dry, the inline humidifier also kicked in. As soon as the air came through the vents, a sweaty locker room smell emerged. The smell then morphed into a more perfume-ish odor (though that doesn’t really describe it).

After sleuthing for the cause, I realized the glue traps were emitting a nasty smell and as soon as I took them up, the perfume-like odor abated. (Also called the pest control company to confirm they didn’t do anything in or around the HVAC, which they hadn’t.)

Was using heat for a couple of weeks w/no problems. During this same period of time started putting up insulation around the basement stairwell in an effort to implement noise reduction measures because the HVAC is too loud on the main level/living space.

Mid October: Noticed an odd smell at the bottom of the stairs to the basement. (Of note, there’s a door I keep closed btwn the main level and the basement.) Consulted w/an IAQ person I work with who thought it might be SVOC’s and suggested vacuuming the stairwell every day for a week to see if the smell improved

. (Am trying that now but skeptical since this problem is new and I have to ask myself what’s changed. In thinking about that, started wondering if enclosing the stairs with insulation might be affecting that odor at the bottom half of the stairwell. Not sure.)

A couple of days ago when the weather got warm again, turned on the AC and the locker room smell was there again. Did some research online, learned about Dirty Sock Syndrome and wondered if that what is going on.

HVAC tech who installed my new AC coils and a few other upgrades a few months ago come out today. He smelled two different smells, both of which were so unique he had difficulty describing them, and each in a different location: one at the furnace and one at the AC coils

. Unable to diagnose or fix the problem, he’ll have the company send out a service tech. He did say it wasn’t Dirty Sock Syndrome because the system was far too new for that and because inspection showed no sign of anything suspicious. All was extremely dry and clean.

Of note, the basement it self does not smell like the odor that comes through the vents when the AC (or just fan) is on. And the HVAC installer felt confident that whatever is going on is inside the system somewhere.

In the meantime, since I don’t know what I’m breathing in, I’m reluctant to turn on my system, which is making my house uncomfortable.

Do you have any ideas or direction to point me? This is very concerning and I need a solution sooner rather than later. If the odor was just an annoyance I’d deal with it until it’s fixed. Not knowing the cause, I’m a bit worried about what’s being circulated throughout the house if the system runs.

Thank you for reading this length history and for your much-valued assistance.

Update: The tech who was here yesterday said the smell he smelled in the furnace had a sight electrical odor. His explanation for why there’s a locker room smell when the AC is on was the smells can change so perhaps it’s one odor at one source that’s changing as air moves over the coils.

Also, ran the heat for a few minutes today. No locker room smell but a vague note of something mildly chemical in the background that’s very difficult to describe. Nothing overpowering, but not neutral either.

Lastly, forgot to note in original post that I have a gas furnace.

HVAC tech coming out this afternoon. Looking forward to hearing your thoughts….

On 2019-10-23 by (mod) -

It's an interesting point that an odor might be affected depending on the temperature of the system. We're still left with looking for the odor source. There are several sources of odors that people might describe as electrical, including ozone from an ozone generator, burning electrical contacts or contact doors, or overheating electrical wires.

There's a simpler heated dust odor is produced when dust on an electrical heating element is oxidized. If you think that it's an electrical odor you want to look carefully is there could be an electrical safety hazard involved.

On 2019-10-26 2 by Anonymous

Thanks, Dan. The electrical odor was very fleeting, didn't persist, and was never smelled again by anyone other than the first HVAC guy who only got a momentary whiff and then it was gone.

The smell that is persisting is not electrical at all, yet no one can figure out what it is or what the source is.

We did switch the brand of media filter we were using because the kind we needed was unavailable, but now we have it and it's in place. In any case, the sudden smell that began with sweaty locker room did not coincide w/the temporary filter change, so that doesn't seem like the culprit.

And the filter didn't have an odor that was anything like what we've been smelling in the vents when we run the a/c. It's very very odd. Any other thoughts??? Again, both HVAC guys did thorough inspections and found our system extremely clean.

The second guy even ran an air quality check and the folks who gave the results said they'd never seen results so clean. (We do work hard to maintain excellent indoor air quality, so that was nice to hear.) Meanwhile, the mystery smell remains unsolved.

On 2019-11-12 by Anonymous

Following up on prior posts regarding an odor from our registers that is quite pronounced when we run the AC: Multiple inspections from HVAC specialists cannot figure out the cause of the smell when we run our AC.

The system has been thoroughly inspected, we have been asked many questions, the technicians have been incredibly thorough and creative in their thinking, yet the smell persists

It's now being recommended that we get a UV light to aim at the ac coils w/the assumption that some microscopic bit of bacteria is growing that is causing the smell, despite the fact that the coils are brand new and the entire system is immaculate. Any thoughts?

On 2019-11-13 by (mod) - will ozone cure our HVAC duct odors?

Anon

Well it can't hurt to actually CLEAN the cooling coil and inspect that component; A UV light - IF there are contaminants on the coil - will be a longer recovery time, give ambiguous results, and may address only biological contaminants.

It may be useful to

1. try to characterize the odor so that the smell type might suggest the odor source - else everybody is flying blind

2. look for odor input sources near return registers or near return ducts first - something spilled into a duct or onto it or near an air inlet;

3. Send an inspection camera through both supply and return systems

Also see OZONE MOLD / ODOR TREATMENT WARNINGS

On 2019-11-24 by Anonymous

Thanks, Dan. We are now thinking of having our (brand new!) Ac coils cleaned, though concerned about having toxic disinfectant wafting through the house from the process.

We (me, my wife, and all the HVAC folks, have tried to describe/name the odor. Unfortunately, smell is a bit subjective and while everyone can smell something, there's no agreement on how to describe it.

We've gone round and round about your second suggestion and just can't come up with a single thing. The inspection camera is a good idea. Obviously it won't be able to identify anything microscopic, but in our no-stone-unturned process, this makes sense to do.

Meanwhile, back to cleaning the coils, do you have any suggestions as to the best kind of products to use and/or avoid? We work very hard on our indoor air quality and, among other things, try to reduce toxins that we introduce into the environment... Thank you again for you time and ideas!

On 2019-11-25 0 by (mod) -

An orderly approach to finding an odor source - for which there are of course more suggestions in this article series - basically involves

1. identify the odor source by guessing at its olfactory nature

2. inspection of all of the ductwork and air handler for signs of spills, leaks of something into the system, or animals or other unwanted substances therein

3. inspection of the air inlet registers and filters and things near those air inlets that could be an odor source

About cleaning, typically the coil and duct cleaning products used do not leave a lingering odor; about their toxicity, just take a look at the product MSDS. (I do not have a specific product recommendation).

I do not like to spray "deodorants" or other substances that pretend to treat an odor source by covering it up with something else. That whole concept leaves me nervous. I prefer to find and remove the odor source.

Do keep us posted; what you find, try, do, decide will help other readers.

On 2019-11-25 by Anonymous

Thanks, Dan. We had our ducts cleaned this summer. Everyone who's seen/inspected our system comments son how well maintained and clean it is. We would never use deodorant to fix anything and we often shake our heads at places that suggest doing such things. Like you, our goal is always, find the source of the problem and fix it, don't mask it.

Glad to hear AC coil disinfectants don't tend to leave lingering odors. And, finally, just a quick reminder that the smell only comes when the AC runs, not w/heat or just the fan. So far no one has been able to crack the code, including excellent HVAC specialists and a top-notch IAQ professional who we've worked w/for several years. So it's all on you to figure it out! Ha! (No pressure.)

On 2019-11-25 by (mod) -

Right. Oh how often we fall victim to the fantasy that somebody from more than 100 miles away (I write to you from Mexico - over the wall) is smarter than the onsite guys (never likely) - moreso of the somebody shows up carrying a briefcase or an impressive gas sampling tool.

Only when AC runs means only when cool, when producing condensate; that may be a clue. Oh and don't forget to follow out the condensate disposal piping, traps, etc.

On 2019-11-27 by Anonymous

Thanks, from the other side of the wall!

On 2019-11-27 by Anonymous

Ugh, Sorry. Not sure what happened in last post. Thought I'd typed that the overflow onto our unfinished cement basement floor was about 3' in diameter. Don't know how "The Small About got in there...

Forgot to mention, we had a back up of the drain a couple of wks before The Small.

About a 3' area. Removed the water, had a plumber assess, cleaned area where water has overflow w/clorox and a couple of days later w/food grade hydrogen peroxide. Forgot to clean inside the condensate drains, so looped back and took care of that.

HVAC guys (no suitcase or fancy gas sampling tools in hand) said it couldn't be those pipes because of how air flow works (doesn't pull air up from the pipes, but rather air moves in the opposite direction.

Hmm. What do you think of this perspective? I started wondering if some nasty bacteria was growing in the pipes, even though I cleaned them using a skinny brush and vinegar, then later w/hydrogen peroxide. Also, one would think if from bacteria in the pipe the smell wouldn't only be w/AC, but what do I know...

On 2019-11-27 by (mod) -

I don't agree with your HVAC guys that "air doesn't flow up the pipes"

The problem of an air handler creating sufficient negative air pressure to draw sewer gases back up through a condensate drain (that is in turn connected to a sewer line) is so notorious that it led to a requirement for traps on the condensate line as well as prohibition against certain connections of condensate drainage to a sewer line (without an air gap).

If you had that problem, just cleaning the lines wouldn't prevent a recurrence of that backdraft.

On 2019-11-28 by Anonymous

Thank you, Dan. Your comment is interesting and I'm taking note for future exchanges w/my HVAC guys. Meanwhile, just to clarify: We do have a trap full and we also have an air gap.

PS Also, the smell we're smelling is not sewer gas. My wife thinks it smells like a sweaty locker room and everyone else seems to think it smells like wet sawdust.

On 2019-11-28 by (mod) - smells like a sweaty locker room

That's good news

FYI some mold contamination can smell like "sweaty socks"

On 2019-10-14 by Mike - Lennox furnace (natural gas) and air conditioner causing a funny smell.

We have a Lennox furnace (natural gas) and air conditioner that is causing a funny smell. When the AC is running there is no smell.

When the heat starts there is no smell, but once it gets up to temperature there is a "band aid" type smell. It is a medicinal , plastic type of smell. If you are familiar with the product "Bag Balm" it smells almost exactly like that.

We had service come out three times to clean and inspect and they cannot tells us what the smell is from and they say there is no issue wit the furnace. Two of the three service men could smell it the other could not. The one guy said he has been doing this for 25 years and never smelled this before. There is a new air filter and a new pad for the humidifier.

When the heat shuts down we keep the fan running at a lower speed and there is no smell. The smell travels the whole house but seems to be worst closer to the furnace. Looked the best I can into all the ducts and do not see any object or sign or something that might have been spilt.

Could it have anything to do with the Freon from the AC unit getting heated up and then leaking? The service men say we should get the ducts cleaned which I do not have a problem doing but I am not sure this will fix my issue. Any thoughts would be greatly appreciated!

On 2019-10-14 by (mod) - band aid smell

Mike

Puzzling indeed; from your note I suspect something's off-gassing when heated up; that means looking for either a contaminant or something that'd offgas like a plastic or rubber component;

Less likely but possible is that some external odor source is pulled into an air return but doesn't become apparent until the system is hot.

Freon is odorless and colorless.

On 2019-10-18 by Mike

Update on the Lennox furnace smell. We had a third service person come out and when he took off the AC evaporator coil a oily liquid came out. He said it was the oil that is in the refrigerant (HCF-410a).

When he checked the AC the refrigerant was low. So he said there is a leak in the evaporator coil and that needs to be placed. So I am guessing the smell had something to do with the combo of the refrigerant and the oil getting heated up by the furnace? We will see once it get's replaced.

On 2019-10-19 by (mod) -

Thank you so much for that follow up. That's an example of an odor source - refrigerant oil from a refrigerant leak - that I had not considered nor encountered. We'll keep the information in this article series as itwill certainly be helpful to other readers.

On 2019-10-21 by MIke

You are welcome. They changed the AC evaporator coil today and the odor is gone. He let me smell the old evaporator coil tubbing that had the oil in it and it had the same smell that was coming from the vents.

On 2018-02-27 by Carlos Mastran

Have a 1 year old new furnace (natural gas) with new ducts that puts out a sweet smelling odor as the furnace runs longer.

Have run the heater long enough to purge out common"dust" contamination. Installer did an electronic (Inficon) leak test at the cooling coils (with and without the furnace running) since these sit just below the most hot combustion chamber

and the cooling additive POE (a sweet smelling chemical) is in those coils with the R410a refrigerant, but the detector did not register a leak.

Installer is reluctant to suction POE back into the compressor to see if that reduces the odor since he believes residual POE in the coils would still confuse the issue and produce an odor is there still is a leak there not sensor detected.

Pulling the cooling coil cover off during heating you can smell the odor is at that location and it does increase as the furnace gets hotter. Any suggestions to pinpoint the cause or how to stop?

On 2018-02-27 by (mod) - sweet smelling odor as the gas furnace runs

Carlos
and for other readers,

POE that Carlos mentions is a polyol ester (POE) lubricant normally used in refrigerants like HFC-134a .

There could have been a previous leak or spill leaving some lubricant behind even if there's no refrigrant leak (or none detected)

I'd ask the HVAC tech if they think it'd help to clean the cooling coil.

On 2017-12-12 by Danny

I have a heat pump furnace system. Since the weather turned cold, every time the furnace kicks on the is an odor, that only I can smell, that irritates my nasal passages, throat, and lungs.

It makes me cough. Does anyone know what the cause and fix might be?

My email is no1like_3@yahoo.com if anyone can shed some light on this or if anyone knows a company that is versed in this issue. Tia

On 2017-12-16 by (mod) - odors since weather got cold

Danny I can't say by e-text what you're smelling; I can't guess if your heating system is using heating oil, LP gas, natural gas or only electricity.

But if the odor is coming from the actual heating equipment you will want to call a service technician to get some repair help as the system may be unsafe.

On 2015-12-13 by Carrie

We have been in our home for about 10 years. It is a 2 story with attached garage which houses the hot water heater and gas furnace.

I have been researching on what the odor I have been smelling could be and seems like it may be connected to heating system. It is a "fume" type smell that is faint at times and stronger at others. It seems the strongest when we open windows or doors which seems odd because "fresh" air is coming in.

I am concerned the smell may be combustion gases coming back inside.

We have a co2 detector near the garage door inside and it has not alarmed but it concerns me. We will be contacting someone to come inspect and see if there is a problem. Any information you can offer would be great! Thank you!
Carrie

On 2015-12-13 by (mod) - gas fumes and gas odors are dangerous

Carrie:

Watch out: If the fumes are from the heating system and are a gas odor the system is unsafe and should be shut off while you call for repair. Check for fuel gas leaks and for a leaky heat exchanger.

Opening windows usually increases combustion air - your comment that the odor change seems odd is a sensible one.

But sometimes, especially if we are heating a building during cold weather (as one would do), opening a window causes warm air to flow by convection outside - that can create negative pressures elsewhere in the building, particularly moving odors or gases from warmer locations or lower floors towards the open windows.

On 2015-11-17 by Camille

Bought a new heat/air conditioning unit in August. I get a bad smell when its running.

Like oil or a factory. I have now ran my unit with heat on for a few weeks already. Ive had the company come out and look at the unit. I have had them come again and spray oil cleaner on the coils to remove oil. It still smells when I run the heater. What can I do now?

On 2015-11-18 by (mod) - oily smell when heat is on

Look for the odor source, clearly it's not the coil. Maybe a spill in the air handler or in ducts, or near the return air inlet.

On 2015-02-24 0 by J. Halpin - smell gas periodically

Had home ckd. for gas leak, by gas company all ok they say. But I believe I still
smell gas periodically. What to do?

On 2015-02-27 by (mod) - time for further inspection and some tests

Sounds as if you need to ask for further inspection and testing; perhaps an inspection of the heating equipment and chimney can give an idea if there is an intermittent operating problem or a persistent gas leak.

Usually the gas company is quick to respond.

Other sources of help are your heating service company and in an emergency your fire department.

Watch out: as you probably surmise, a gas leak risks an explosion or fire

...

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• DUCT & AIR HANDLER ODORS
• ODORS GASES SMELLS, DIAGNOSIS & CURE - home

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