Revenge of the Smelly Ducting

For about a month now, we’ve had this odd odor in our house. It started out in our laundry room, on the east end of our main floor. Then we noticed it in our bedroom, on the west end of our 2nd floor. Since then, we’ve noticed it in almost every room in the house on different days, but never all over at once. It’s always one or two rooms, but they could be on opposite ends of the house.

I few weeks ago, I realized that the smell was coming from the furnace vents. After some experimentation, I noticed that running the furnace made the smell go away. It got worse when the temperatures were warmer, and the furnace wasn’t running.

So we called out our local heating/plumber guy. After they were out here three times, they did find a couple of problems with our furnace, but no gas leaks or anything that could explain the smell — which, by the way, three different employees couldn’t identify. They even left an air charting unit with us, which ruled out things like carbon dioxide, carbon monoxide, and particulates. It did helpfully notice that there were odors, but didn’t tell us what they were.

So here we are, still smelling this…. smell. It defies categorization. It mystefies experts. Something like a cross between dirty laundry, mold, and a skunk. And yet…. none of those, either. It’s weird.

I’ve taken to closing the vents in our bedroom before turning in for the night if the smell is bad in the evening.

So, I’m appealing to all the bloggers out there for suggestions, short of selling the place and moving to town :-)

Oh, and this is not the first time we’ve had odor problems here. However, the cause the first time was immediately apparent and thus could be eventually dealt with :-)

250 thoughts on “Revenge of the Smelly Ducting

  1. We are having an on again/off again smell coming from our return air vents. The previous owners had a 3 season sunroom added onto the house and ran the fresh air intake hose under this porch. I can smell this odor under the porch and have put up mice/rat poison and have caught a few mice in traps. It only smells in the fall/spring. I sprayed some cleaner in the the 8″ high crawl space under the porch and I am considering moving the fresh air intake so it is not under there anymore. Any ideas or suggestions?

  2. My wife and I bought a foreclosed house and the furnace stunk like pets and cigarettes. We cleaned the air intake compartment and sprayed bleach in it, changed the air filter, cleaned everything else we could think of that could have been causing the smell and it kept coming back. The smell was only there when the fan or heater was turned off. When we turned it on there was no smell so we knew the ducts to the rooms were okay. I finally looked at this one duct that led to the outside of the house. The duct comes out by our side garage door, where we have a bunch of garlic plants! We removed the plants and the smell in the house went away. The air from these plants was going into the air intake, through the heater, and into the house via the air intake vents! The garlic smell was probably bringing with it the pet and smoke type odors with it through the heater. We now smell nothing and can stop going crazy. Anyway, wanted to share with everyone because it takes some creative investigating to solve some of these issues with odors. Get your CSI hat on and find it! Good luck all.

  3. HI, I too have the same problem after they replaced a crossover duct in my manufactured home. I dont know if you know about this but you can take a digital camera and put it carefully in the vent and take pictures from all sides and turn on zoom and then take some more. I found a bunch of junk and dirt and big wads of god knows what. I dont know if that id what it is but at least I have some proof that they did not use clean materials . even if the smell has nothing to do with it. The smell does smell like a spetic or maybe a dead animal we have problems with mice. But you said they cleaned the vents and atleast with the camera you can check the work they did you might be suprized, Who knows> I sure hope you find out what it is and let me know if the camera helps.

  4. we had a smell that would make you throw-up. i thought something was dead under my home. It was a break in my water pipe under my home ,so every thing that went threw the sink and garbage disposal went right on threw. plumber came fixed it,no more smell

  5. I have similiar problem. Weh we turn the heat on for first time in fall there is a terrible odor, kinda musty. The odor seams to disapate after a while bu t periodically comes back. We have an electric heat pump so it is not fumes or gas we are smelling. Any suggestions?

  6. Have this horrible sewer gas smell but all the traps have water, I monitor them soI know they are full? Did a smoke test from rooftop vent…nothing all venting is tight. Cleaned and sanatized all duct work. Homeowner started to pour Bleach down drains in shower tub and sink as an attempt to sterilzie the drains from possible bateria…Had a smell sniffer from a contractor..searched the usual places around toilet for failing wax ring, shower / tub sink drains, again Nothing…I am so puzzled and have spent far to much for this odor…I am so stumped??? This bathroom and guest room sit above a garage and below another room above, the HVAC is in closet in the Garage below. Anyone have any other ideas? Thanks

  7. Hi, I ended up here because of a similar problem and then discovered what my problem was – we got the carpets professionally cleaned a couple of days earlier and they left them a bit too wet so there is now a gassy/skunky moldy smell – googled this and it is all over the place. Any chance some of you happened to clean carpets before the smell happened?

  8. After a couple of months of obessive searching and cleaning ducts. trying to remove spots that looked like mold, buring scented candles, we turned off the stupid electronic air cleaner and Voila! The smell was gone in 24 hours. THANK YOU!!!!

  9. ok, here goes. I too have a dead animal smell in my house. It’s been a few weeks now. The smell is not always there, not always in the same room. Our house is less than 1 year old. My husband had disconnected every HVAC tube, we have run a snake with a towel thru the other areas, comes out dry and clean. Our house is 2 stories with a finished basement and we have 2 units. The upstairs is on one unit in the atic, the smell is only upstairs. It moves from room to room and down the stairway a little. Is usually strongest in the guest bath or our bedroom if we turn on the AC. I will try running water in all the bathrooms and drains in the basement. I was going to call Sears on Monday to have them come do the vents but after reading some of these posts I feel the $800. would be a real waste. PLEASE anyone, any more ideas. My nose and lungs burn from the smell and it makes me physically sick to go upstairs to my bedroom sometimes. It has even woken me up at 4AM. I love my new house but am afraid to have anyone over afraid the smell will happen. Both guest rooms are upstairs! I even worry that when my pet sitter comes to let our dog out during the day she might smell it by the stairs. This is awful and since we are going into winter, I can’t leave the windows open. PLEASE HELP!!!

  10. For the posters who have looked everywhere for the dead animal but cant find one, and the odor seems to come every time it rains or the humidity spikes…you might have some fiberglass insulation that has been wet at one time (you cannot “dry” fiberglass insulation, you must replace). Residual biological activity reactivates when there is moisture available. Check all insulation in crawl spaces, areas close to the ground, then go for windows and other areas that can leak. Your leak may be so minor that you won’t see it stain an interior wall.

    Also mold testing is usually a waste of money. There is mold everywhere and it is not a significant health hazard to healthy people. Concentrate on finding the source of moisture and eliminating it and keep the relative humidity between 40-60%. EPA has guidelines on mold remediation.

  11. Is the bathroom vented? Does the fan work? Is air coming out the exhaust vent when you operate the fan? How far is the vent duct run and what type of duct is it? Condition of the insulation where the fan is located? Do you have central air or window shakers? Is the condensate pan clean? Try locating a reputable company with a thermal imaging camera who is experienced in detecting leaks.

  12. You may have mildew as well as rodent urine permeation. Spraying a cleaner will not solve the problem. Relocate the air supply; take care not to place it downwind from an exhaust vent (stove, dryer, toilet…)

  13. I also have the same problem. a 5 yr old double wide on a baaement. Never noticed the smell this summer but we also notice the smell goes away when Windows open. My father said it could be the musty smell coming up from the basement. It first started to smell only in the living room but now its in my sons room. We have bad asthma n allergies and I need to get to bottom of this asap. My basement is unfinished. We do not have ac unit but we do have furnace. Help!

  14. ok i have read all your posts but this is what happen to me my sewer backed up and i cleaned the floor used bleach as my basement isnt finished and it is a very old house. when my furnace comes on it smells like poop.. had the vents cleaned didnt help. i dont know what to do? im gonna try and pour hot water with vinger in my sewer drain on the floor in the basement to see if that helps. if anyone else can give me some ideas i would be very thankful

  15. I’ve had identical issues to most in thread…thought it was a dead animal and called exterminators out to investigate. It turned out to be a major crack in a 2″ galvanized pipe that emptied the washing machine and kitchen. In essence, the odor was from moldy, stagnant grey water that’s been pooling in our subfloor area for who knows how long. hope this helps someone!

  16. Please Help! We have a terrible smell coming from our furnace ducts ever time the heat kicks on. We live on a slab and today we noticed you can hear water in the furnace ducts in our livingroom!

  17. ok i have read all your posts but this is what happen to me my sewer backed up and i cleaned the floor used bleach as my basement isnt finished and it is a very old house. when my furnace comes on it smells like poop.. had the vents cleaned didnt help. i dont know what to do? im gonna try and pour hot water with vinger in my sewer drain on the floor in the basement to see if that helps. if anyone else can give me some ideas i would be very thankful. please help someone i cry everyday i cant take this smell

  18. i tried the hot water and vinger no help still smell when i blech the floor the smell goes away only for a couple of days

  19. I am having the same problem. I was told it is called “dirty sock syndrome” and, I did find it on the internet. The a/c repair companies want a minimum of $500 to fix this. I have called several.

  20. We had the problem of a horrendous smell of almost floral/deadanimal burning smell. We called a few air con guys and they said the mouse was in the blower when the heat turned on for the first time. It flipped it into the coil and fried it there. Sure enough. They opened it up and there it was. Tell the heater/air con guy that if you have a horrible smell that smells like something burning too. He hasn’t given us the bill yet, but I’ll let you know how much.

  21. It only cost $49.00!!!! Call Border Air In Weslaco, TX if you are close by!!!! He also said sometimes it gets trapped in the blower and it gets hidden in there when you turn it around to look for it, so don’t give up.

  22. I have been following this web site after searching for an answer for a moldy dirt like smell was coming from only one ceiling vent in my home. It was not constant but when we did smell it – it was pretty bad. I call a reputable HVAC company and after inspection, he determined that when my 10 year old home was built, who ever sprayed the texture on the ceilings did not cover up the vents allowing the texture to harden on the vent boot. There was a blackish brown mold on several other ceiling vents as well bout we could only smell the one vent in the kitchen. I cleaned it with 50/50 water and bleach. I scrubbed the metal inside and gently brushed the actual duct as the mold was only an inch or two above the sheet metal boot. I then sprayed some Lysol mold allergen killer inside as well. I then washed the grill with my mixture and turned the furnace fan switch to on – not on heat or air just the fan itself. I was told by the HVAC guy that this would help dry off the insulation that I had to scrub also (at base of boot as I had to remove the sprayed texture as over the years the condensation settled in the overspray and it turned moldy). We thought all was well. Then it came back in about a day or so. We had decided after looking at this web site that we may simply be doomed! We discussed calling the HVAC back out as maybe I did not get it all. We turned on the heat and I took off the grill and we only smelled fresh air!! As we sat at the table in disbelief, my wife (for whatever reason) put her nose to our wood kitchen table that was right below the vent. Lo and behold she said it’s the table! Long story short, our less than 1 year old wood kitchen table had been permeated by the mold spores (or dirty dust) for the while. The top of the table stunk, the backs and tops of the chairs stunk of the same smell. We used several cleaners to clean the table as not to ruin the finish. The smell is gone and my marriage is intact!! Apparently the junk just kept blowing on the table and was absorbed by the table being somewhat porous. So I would encourage anyone who has cleaned there vent and/or ducts out and the smell returned to smell any furniture whereas the vent could blow the particles on.

  23. We have actual skunks in our neighborhood and YES, when they spray outside the smell can come inside your house very easily!! Some people can smell it better and more easily than others. To me, the skunk smells like very strong garlic odor…. to my neighbor it just smells like the most awful stench possible and to some skunk can smell like smoke from a fire. The skunk spraying outside can create powerful fumes that will make your eyes water and chase you outside for air – it does that to my neighbor. To me, it is just annoying but I think his nose is much more sensitive. If you suspect a real skunk, be sure to check around your property for holes in the ground. They will crawl down there and make nests and will keep returning and do tons of spraying. They generally come out at dusk so you can watch for it then from a window.

  24. I sometimes notice this “paint smell” in my back hallway. Luckily, the back hallway is not used for much really and I keep the kitchen door closed most of the time. I seem to notice it the most during warm weather and it smells like it was just freshly painted, except it was painted over a year ago…. I actually think it’s the PAINT itself. Like there is something wrong with it and it never dried properly, even though it feels dry to the touch. I really can’t think of what else it could be. There are no ducts or anything in my back hallway – its just wood framing with some new fiberglass insulation, that wall stuff (plasterboard or whatever you call it) and paint. Since it smells like paint, I suspect the paint. I bought it at Ace. I also just found a page that confirms my idea, that Ace Latex paint can still smell even a year after painting – its at a site called painttalk – do a search for “Odor dried paint”.

    Did you buy your paint at Ace??? Maybe something you had painted a year or so ago??? My paint odor disappears for months on end and then out of the blue the smell will return! I think the smell is related to the humidity, Maybe a cold air return is near a painted item and is collecting the odor and carrying it around your house. Perhaps try a dehumidifier?

  25. I have pet rodents so I can answer your question. NO, the smell will NOT go away by itself. You need to wash the area, if it is washable that is. Rat and mouse odors can be removed easily with soap and water. Of course, since you are dealing with wild rodents you should wear gloves and a face mask while cleaning the area, to prevent from breathing in any particles they left behind (as they may carry disease). But it needs to be cleaned, otherwise it will keep smelling very ratty-like. Mice actually have a stronger odor so..maybe be glad they were rats.

    Any soap will do. I generally use dish soap to clean the rat cage but use bleach if you like. If you don’t remove the urine it will just keep stinking. And the droppings smell, too, of course. If there are a lot of droppings in hard piles, you may need a scraper to remove the build up. The urine can also be somewhat acidic and rust through metal if left long enough. Be aware they can do nasty damage with chewing and they can have a lot of pinkies (baby rats) at an incredible pace. Rodents always need to chew because their teeth are always growing, so they gnaw on hard objects to keep the teeth trimmed.

    Domestic rats are very prone to colds and drafts. I don’t know if wild ones are but perhaps you could use this somehow to your advantage?

    I realize the attic is not the best place to clean or the easiest. If soap and water is not possible, maybe try the cat spray from Drs Foster & Smith – it is made to break up cat urine smell by using enzymes. Its called CleanAway. You just spray it on the stinky surface and it does the job. It works very well.

  26. We have had the same problem since installilng a new furnace a month ago. We cleaned everything out of the area where it was coming from and there was nothing moldy or musty. I then kept record of when it was most prevalent only to find it was usually when it rained. The furnace salesman told my husband he should move his wood pile so as not to surround the venting – we did make allowance for this and built the woodpile up around the venting. However, it is now considered that possibly the smell was coming from the intake pipe outside wood has a moldy odour and this was being drawn in the intake pipe on rainy days. Since moving the wood pile we HAVE NO PROBLEM. The smell is gone.

  27. I think the pipes or some building materials could stink, especially the ones made in China. Once I bought a pair of brand new shoes at a good store, Ingledews in Canada. These stank up a big area in the house before I found out where the odor was coming from. I kept the shoes outside for the whole summer and in the garage for another half a year before the stench disappeared. I told this to a relative in Hungary and lo and behold she said she threw away a pair if made in China shoes. I just wonder if some building materials, especially nowdays that so many things come from recycled material and very low quality places, might start to let out stink due to heat, moving air and God knows what?

  28. I have been having the same kind of problems in my home with air quality which seems to involve my central air . I have found it to be embarrasing and annoying my home was built in 1928. I have tried setting the vents in different directions but unsuccessful I feel like the air quality that is being produced is poor. My Cenral air unit is only 3 years old which performs fine minus the smell. I have had two plumbers come out to evaluate piping which is fine. Not sure what else to do except for opening all windows and buying an industial fan.

  29. I reside in Brooklyn I have lived in my apartment for 6 years. My apartment is heated by oil. when my heat came on NOV 2011 I thought I would throw up there was a smell emitting out of the steam pouring out of the radiator. I immediately called my landlord who had no idea where the smell was coming from. He turned off the heat because the smell was so bad. The smell went away in 20 mins. Thank god is has been warm in NYC this winter., until last night. The temperature dropped drastically and I needed heat. The landlord turned the heat on and once again I thought I would throw my guts up The smell that came out of the radiators was putrid. My landlord has no idea how to get rid of this smell. Please someone help me so that I can tell my land lord how to help me winter has returned with a vengeance and i need heat.

  30. Did you ever find out what the smell was? I have the same smell coming out of the cold air return. It is upstairs in the Master Bedroom where the cold air return is in 2 from the hallway & master. The furnace is in the laundry room closet across from the master if this helps. There is no basement, only a crawl space. Our house is almost 3 years old.

  31. My favorite product to date is Room Shocker. This stuff really works on everything such as: carpet, furniture, and clothing. It is so interesting because it works like a fumigation bomb, but for bad odors. This stuff kills the odor right at the source and the smell never comes back. You have to get Room Shocker to help you out with your odor problem. Check it out at

  32. Patricia, we are researching the same type of smell in our vents – Did you find solution? We’re considering having our ductwork cleaned, but I’m seeing several comments where it did not help other homeowners. Thanks for your input.

  33. Wow!… after reading entire post of comments I was freaking out last night with that sweet skunky smell.I turned the furnace on /off just running the fan, don’t have an electric air filter so can’t turn that off , but we do have a AprilAire humidifier, so I turned that off and smelled it. That wasn’t it. I went and smelled every vent, finally concluded that the odor was the strongest in basement where furnace is located. Checked to see if pilot light still on and working… yup, everything good there, so I went outside and sure enough I smelled a skunk big time. It was the strongest by my Fresh-Air Intake Vents. Whew! problem solved. Woke up this morning and smell is pretty much gone. Anyway, I would check around your house for a skunk or dead animal(s) first before spending hundreds of $$ on a plumber or HAVAC tech. I live in the Northern Subs of Chicago. We have had a real mild winter so I’m guessing rodents are more active since it was and has been in the 40’s. Hope this helps!

  34. You probably have a dead mouse, rat, or other animal in your vent. I live in a rural community, so I have unfortunately gotten accustom to having to bait for rodents every couple years. The smell will dissipate after time on its own, as the carcass dries out. The fact that the smell goes away when you run the furnace supports this, as it has happened to me. The smell will eminate through the air return through your air ducting when you don’t run your furnace, but disappears when you run it. I usually use scented candles when it happens to me. I always thought that is was better to smell that, than to deal with live rodents running around…

  35. Sour Musty Smell: Sure hope someone can help. We are going crazy and getting sicker in this house…hotels are becoming an expensive and inconvenient alternative. Scratchy sore throats, migraine headaches, brain fog, fatigue. 6 year old house, we’ve been the sole owners. Our smell started in Master Bedrm about 3 months ago. Turned off UV Air Purifier that was attached to air handler (per other posts). Cabinet that houses air purifier had same musty smell, only 10x worse. Air purifier supposedly state of the art, with 6 UV bulbs, a 4″ Merv 13 filter, and a 1″ carbonated metal honeycomb filter. Paper filter is changed 2x yr (even though its supposed to last an entire yr), all 6 bulbs & metal filter are changed every 2 yrs., per Lennox recommendations. Turning this off did not help . Had A.C. checked by 3 different companies: coils are spotless, wall board that lines air handler is clean, ductwork in attic checks out fine, no mold or mildew anywhere. We have AC maintenance 1x year. Had Blower Door Test by utilities company (I highly reccomend this, at $35 you get alot of info on home’s energy usage). Duct work in excellent shape–no duct leaks.
    Concerning Main A.C. return, located in hallway ceiling: Vent was removed during AC guy’s visit, he and I were in garage, AC was running. After about 20 min, walked back into house and upon entering hallway the musty, sour odor just about knocked us out. Reinserted return vent and smell subsided. Smell travels through house, sometimes worse in certain rooms, or non-existent in certain rooms. Smell is definitely in the AC/Return system, but we dont know where its coming from. 2 roofing companies checked roof for leaks and attic for evidence–all dry and fine. Had professional ozone treatment which sucked ozone through whole house. Now house smells like musty, sour ozone. Ran water down both bathtubs as they are rarely used (after reading posts on dried out p-traps), but dont think it helped. Really at a loss at this point. One interesting variable: Approx 3 wks prior to noticing smell, had exterior of house and roof pressure washed. Noticed afterwards on INside of bedrm windowsill (window had been closed) several puddles of water. Pressure must have been intense enough to get through but have to wonder if water somehow got into wall space. (smell did start in this room). We have the E-Walls where its poured concrete with foam in between. If water got into foam maybe we have mold? No dark spots on wall or ceiling though. ANy suggestions by anyone out there would sure be appreciated.

  36. We moved into a new construction home late September 2011. In early October 2011, we started to smell a sewer gas like smell in a 2nd floor bedroom located in the vicinity above the furnace, air conditioner, water heater and 3 external exhaust lines. This smell is also evident at the top of our basement stairs. The smell disappeared entirely in the first week of November (when cold weather stayed). The first week of March (4 months later) the foul sewer gas like odor reappeared after strong winds and warmer temperatures. The builder said that maybe the house is too tight and installed a 6″ fresh air return line from the bottom of the first floor down next to the furnace area in the basement. This did not fix it. One week later we did a peppermint test on the plumbing lines. We detected faint peppermint smells in a few areas right away and stronger peppermint smells in the basement only the following morning. I am at wits end, because the smell is too strong to sleep in the room. We did check all the traps, toilets, ejector pit, etc. too with no success in finding the cause of the smell. I am thinking of doing a smoke test next. What do you think we should do next? One last thing, there is a pvc line that stops in our attic and doesn’t go out the roof. Any correlation to the smell? The pvc line doesn’t smell every time we check the attic, but my gut feeling says this line should be on above the roof.

  37. Just finished inspecting heating system surching for source of foul odor comming from hot air ducts. The ducts are clear of decomposing furry creatures but there is an orange colored deposit on the out side of the heat exchanger. It smells like decomposing organic matter. My furnace guy says there is a manufactures bulliton on my 16 year old furnace heat exchanger and are offering a replacement credit. … how about it?

  38. We have the slight skunk oder in our living room which is on a slab. We have baseboard heating. No septic system. Started about 3 months ago. It isn’t constant.

  39. Hi,
    In March after a good rain we had a sweet skunky smell under our bathroom sink. Had plumber out not sewer or plumbing issue, had exterminator come out not dead animal smell. Finally went away. Now its July after a good rain same smell in guest bedroom REALLY strong. I could smell on wall near baseboard near a electrical socket. Called a different plumber her said not a sewer smell, called HVAC guy he looked everywhere no dead animals nothing can get into walls (we are on foundation). Its been over a week now I have tried everything its driving me crazy! Could Skunk smell last that long and why would it spray that outside wall No vegitation there only rocks! Could Mouse feces smell like that we have had some field mice.

  40. I have this problem with my vents. When I have the heat on, there is no issue. When I have just the fan running, there is no issue. When I have the central air conditioning on, there is no issue until the air is running in a certain mode (like a wind down or low fan mode). My house smells like stinky feet or dirty laundry. Sometimes I turn the temperature down so that it runs more to avoid the smell. I have had the ducts cleaned. I have had 5 different professional people check it out, but no luck. I am at a loss. My only thought and fear is mold of some kind. Hope to find a way to get rid of it without removing all of the ductwork. The smell is throughout the house.

  41. We had this problem…. It turned out that rats had infested our attic and the airborne rat urine which is hazardous was coming through our system because the vents were not sealed off like they should of been in the rooms. All attic air was being pulled down through the sides of our air vents and getting sucked into the return then dropped into all our rooms

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